lot
„ « A to m
efiemistiy sta tu s
P ro c e e d in g s
of
a
P a n e l , V i e n n a , 1 3 -1 7 M a y 1 9 7 5
Xe+RbI
Rbl
I N T E R N A T I O N A L A T O M I C E N E R G Y AGENCY, VIE NN A, 1 975
HOT ATOM CHEMISTRY STATUS REPORT (STI/PUB/393) NOTE On the front cover the date of the Panel should be given as 13-17 May 1974 not 13-17 May 1975. The date is correctly given on the title page and in the Foreword.
HOT A TO M CHEMISTRY STATUS R E PO R T
T h e fo llo w in g S tates are M em bers o f th e In tern ation al A to m ic Energy A g en cy:
AFG H A N ISTAN
H A IT I
PARAG UAY
ALBANIA
HOLY SEE
PERU
ALGERIA
HUNGARY
PHILIPPINES
ARGENTINA
ICELAND
POLAND
AU STR ALIA
IN D IA
PORTUGAL
AU STRIA
INDONESIA
REPUBLIC OF
BANGLADESH
IRAN
BELGIUM
IRAQ
ROM ANIA
BOLIVIA
IRELAND
SAUDI ARABIA
BRAZIL
ISRAEL
SENEGAL
BULGARIA
IT A L Y
SIERRA LEONE
BURMA
IVORY C O A S T
SINGAPORE
BYELORUSSIAN SOVIET
JAM AICA
SOUTH AFRICA
JAPAN
SPAIN
CAM BODIA
JORDAN
SRI LA N K A
CANADA
KENYA
SUDAN
CHILE
KOREA, REPUBLIC OF
SWEDEN
COLOMBIA
K U W A IT
SWITZERLAND
CO S TA RICA
LEBANON
SYRIAN ARAB REPUBLIC
CUBA
LIBERIA,
TH A IL A N D
CYPRUS
LIB YAN ARAB REPUBLIC
TU N ISIA
CZECHOSLOVAKIA
LIECHTENSTEIN
TURKEY
DEMOCRATIC PEOPLE'S
LUXEMBOURG
UG AND A
MAD AG ASC AR
UKRAIN IAN SOVIET SO CIALIST
SOCIALIST REPUBLIC
REPUBLIC OF KOREA
SOUTH V IE T -N A M
DENMARK
M A LA YS IA
D O M IN IC AN REPUBLIC
M ALI
ECUADOR
M AU RITIU S
EGYPT
MEXICO
UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN
REPUBLIC UNION OF SOVIET SOCIALIST
EL SALVADOR
M O NACO
ETHIOPIA
MONGOLIA
FINLAND
MOROCCO
REPUBLICS
IRELAND UNITED REPUBLIC OF
FRANCE
NETHERLANDS
GABON
NEW ZEALAND
UNITED STATES OF AMERICA
GERMAN DEM OCRATIC REPUBLIC
NIGER
URUGUAY
GERM ANY, FEDERAL REPUBLIC OF
NIGERIA
VENEZUELA
GH ANA
NORWAY
YUGOSLAVIA
GREECE
PA K ISTA N PA N A M A
ZAIRE
GU ATEM ALA
CAMEROON
ZA M B IA
Th e A g e n c y ’s Statute was approved on 23 O ctober 1956 by the Conference on the Statute o f the IAEA held at United Nations Headquarters, New York; the A gen cy are situated in Vienna.
it entered into force on 29 July 1957.
Th e Headquarters o f
Its principal o b je c tiv e is "to accelera te and enlarge the contribution o f
atom ic energy to peace, health and prosperity throughout the w orld".
Printed by the IAEA in Austria D ecem ber 1975
P A N E L PROCEEDINGS SERIES
HOT ATOM CHEMISTRY STATUS REPORT PR O C E E D IN G S OF A P A N E L O N T H E ST A T U S OF HOT A T O M C H E M IST R Y A N D ITS R E L A T IO N T O O TH ER F IE L D S OF S C IE N C E A N D T E C H N O L O G Y O R G A N IZ E D B Y THE IN T E R N A T IO N A L A T O M IC E N E R G Y A G E N C Y H E L D IN V IE N N A , 1 3 - 1 7 M A Y 1974
IN T E R N A T IO N A L A T O M IC E N E R G Y A G E N C Y V IE N N A , 197 5
HOT ATOM CHEMISTRY STATUS REPORT IA E A , VIENNA, 1975 STI/PUB/393 ISBN 92-0-141075-1
FOREWORD The chemical effects of nuclear transformations, often described by the term ‘hot atom’ chemistry, involve the reactions of the charged particles, energetic atoms, and molecular fragments formed as a consequence of nuclear reactions in all types of matter. The study of such reactions has both fundamental scientific interest, and practical value for nuclear energy installations. On the fundamental side, the expansion of chemical kinetics to systems not in thermal equilibrium has brought greatly increased understanding of how chemical reactions occur, especially in gaseous systems. The fundamental kinetics of chemical reactions in solids and liquids can also be approached through nuclear-reaction-produced isotopes. On the practical side, all chemical aspects of the handling of radioisotopes must begin with knowledge of the chemical state of the radionuclide, and requires knowledge of the hot atom chemistry of each particular system. These effects are especially important in the formation and utilization of radioactive molecules, e.g. the short-lived radioactivities used as radiopharma ceuticals; in irradiated nuclear fuels; and in the production of isotopes generally. There are thus very practical reasons for extending knowledge of the reaction mechanisms of energetic atoms and ions, since it permits a greater understanding and prediction of the subsequent chemical behaviour that will follow in each kind of material. The Agency has held two Symposia concerned with Chemical Effects of Nuclear Transfor mations, the first in October 1960 in Prague and the second in December 1964 in Vienna. In October 1967 a panel meeting was held dealing with the more specialized subject of the Bio logical Effects of Transmutation and Decay of Incorporated Radioisotopes. To review the status of the field of hot atom chemistry and to define better the Agency’s role in this field, the Agency convened a panel in Vienna from 13 to 17 May 1974. The present book includes the papers presented, the discussions that followed the papers, certain material contributed subsequently to the meeting at the request of the panelists for completeness, and the panel’s recommendations. It is not intended to fill the role of a monograph or manual on hot atom chemistry. Rather, it is an effort to summarize and describe the various areas of possible research and application of hot atom chemistry in chemistry and related fields and in atomic and nuclear energy programmes. The Agency is particularly grateful to Professor F.S. Rowland, of the University of California, who edited the taped transcripts of the discussions. It is hoped that, in the process of presentation for publication, no gross misrepresentation of the participants’ views has taken place.
CONTENTS Accelerators and nuclear reactors as tools in hot atom chemistry (IAEA-PL-615/1)......... L. Lindner Discussion....................................................................................................... Hot atom chemistry in inorganic solids (1AEA-PL-615/2)............................................ G. Harbottle Discussion........................................................................................................ Chemical studies of ion implantation (1AEA-PL-615/3) ............................................. A. G. Maddock Discussion....................................................................................................... Implantation of atoms into materials during reactor operation (IAEA-PL-615/4)............ H. Andresen, W. Lutze, H. Migge Discussion........................................................................................................ Physical methods in hot atom chemistry (IAEA-PL-615/5).......................................... J.P. Adloff Discussion........................................................................................................ Positronium and muonium chemistry (IAEA-PL-615/6).............................................. H.J. Ache Impact of theoretical chemistry on elucidation of hot atom reaction mechanisms (IAEA-PL-615/7)............................................................................................... M.D. Newton Discussion........................................................................................................ Role of atomic beams in hot atom chemistry (IAEA-PL-615/8)................................... Y. Lee Discussion........................................................................................................ Gas phase reactions of atomic tritium, fluorine and chlorine (IAEA-PL-615/9) .............. F.S. Rowland Discussion....................................................................................................... Hot atom chemistry of monovalent atoms in organic condensed phases (IAEA-PL-615/10) ........................................................................................... G. StĂ´cklin Discussion........................................................................................................ Hot atom chemistry of inorganic liquid systems (IAEA-PL-615/11 ) .............................. N. Saito, T. Tominaga Discussion............................................................................................. ; ........ Hot atom chemistry of carbon (IAEA-PL-615/12) .................................................... A. P. Wolf Discussion........................................................................................................ Molecular activation analysis (IAEA-PL-615/13)........................................................ P.M. Grant, F.S. Rowland Discussion........................................................................................................ Role of nuclear techniques in the study of gas-phase ionic reactions (IAEA-PL-615/14) . . . F. Cacace Beta decay and isomeric transition (IAEA-PL-615/1 5 )............................................... T. Shiokawa Discussion........................................................................................................ Inorganic syntheses via beta decay (IAEA-PL-615/16) ............................................... J.P. Adloff
1 10 19 24 33 43 49 55 61 76 81
107 114 123 131 139 149 161 181 191 199 203 209 219 222 229 241 252 261
Fission recoil chemistry (IAEA-PL-615/17)............................................................... S. Amiel, Z.B. Alfassi Applied hot atom chemistry, labelling of compounds, and isotope production (1AEA-PL-615/18) ........................................................................................... A. P. Wolf Discussion....................................................................................................... Biological effects of hot atom reactions (1AEA-PL-615/19) ........................................ L.E. Feinendegen Discussion....................................................................................................... Problems of isolated scientists or research groups (IAEA-PL-615/20)............................ F. W. Lima Discussion.......................................................................................................
265
Conclusions and Recommendations.........................................................................
327
271 275 285 311 317 321
List of Participants ............................................................................................... 329
IAEA-PL-615/1
ACCELERATORS AND NUCLEAR REACTORS AS TOOLS IN HOT ATOM CHEMISTRY L. LINDNER Institute for Nuclear Physics Research (IK O ), Amsterdam, The Netherlands
Abstract A C C E L E R A T O R S A N D N U C L E A R R E A C T O R S A S TO O L S IN H O T A T O M C H E M IS T R Y . The characteristics o f accelerators and o f nuclear reactors — the latter to a lesser extent — are discussed in view o f their present and future use in h ot atom chemistry research and its applications.
IN T R O D U C T IO N The physical pro cess of the nuclear transform ation is still a mandatory n ecessity fo r the hot atom chemist in his re se a rc h efforts. This paper deals with the m ajor tools required, notably a c celerato rs and nuclear reactors. Em phasis is placed on the fo rm e r since the applicability of re a c to rs is alread y much m ore w idely recognized and established. The use of rese a rc h re a c to rs in hot atom chem istry was review ed at a previous IA E A Panel Meeting [ 1] and is adequately illustrated in many contributions to two p r e vious IA E A Symposia on hot atom chem istry [ 2]. Another reason to focus upon a c c e le ra to rs is the rapidly expanding field of the application of short lived accelerato r-p ro d u ced radionuclides in various other disciplines such as nuclear medicine. Because of the num erous p aram eters — such as type and kinetic energy of the bom barding particles — that can be optimized, ac c e le ra to rs are essential fo r gaining access to the isotopes of choice, since radionuclides are not seldom out of reach of the nuclear reactor. Furth er, it has been shown that certain re c o il lab ellin g techniques — m ore p articu larly in a num ber of cases where a c c elerato rs are utilized — are indispensable in the fast synthesis of sh o rt-liv ed radiopharm aceuticals [ 3] and further contributions of basic hot atom chem istry re se a rc h should be envisaged. This report is not exhaustive, it m e re ly touches upon a variety of problem s which confront the hot atom chemist alm ost daily. ACCELERATORS An accelerator is a device which im parts kinetic energy to a particle by virtue of its charge. T h erefo re, and also because they often operate in a pulsed mode, a c c e le ra to rs are b a sic a lly different from nuclear reacto rs which essen tially produce (uncharged) neutrons in a continuous process. A c c e le ra to rs are constructed in a great variety of designs, operating on different principles, and the lim its set to the type of particle to be revved up and the energy to be attained seem m ore a m atter of funds, or lack of them, than human ingenuity. Kinetic en ergies per nucleon (proton) of hundreds of G eV are obtained and ions as heavy as xenon are used as target
1
TABLE
SPE C IFIC A TIO N S
Scanditronix (Sweden).
Thompson-CSF (France).
The Cyclotron Corporation (USA).
I.
OF
CU RRENTLY
A V A IL A B L E
COM M ERCIAL
CYCLO TRO NS
L IN D N E R
IA E A -P L -6 1 5 / 1
3
p r o je c t ile s to induce n u clea r re a c tio n s . Thus, in p rin c ip le , a c c e le r a to r s o ffe r the hot atom ch em ist the p o s s ib ility o f p rodu cin g just about any r a d io n u clide he ch o o ses to in v e s tig a te — m o re p a r tic u la r ly n e u tro n -d e fic ie n t ones which often have d e s ira b le d ecay p r o p e r tie s — c o n tra ry to the lim ite d set of ra d io n u c lid e s attainable with r e a c to r s . In addition, p a r tic le a c c e le r a to r s re n d e r valu ab le sec o n d a ry beam s such as fa s t neu tron s, h ig h -e n e rg y photons in the ca se o f e le c tr o n a c c e le r a to r s [4 ], and even pions and muons at v e r y high p r im a r y e n e r g ie s (> 200 M e V ). L o w - e n e r g y m ach in es (< a fe w M e V ) such as C o c k c ro ft-W a lto n s and s m a ll Van De G ra a ffs (e le c t r o s t a t ic d e v ic e s ) d e r iv e th e ir valu e p r im a r ily fr o m th e ir u sefu ln ess as neutron g e n e ra to rs [ 5] as w ill be d iscu ssed la te r. T h e e n e r g y ra n ge fr o m a fe w M eV to tens o r even hundreds o f M eV is c o v e r e d by c ir c u la r m ach in es (c y c lo tr o n s fo r ion s, b eta tron s and m ic r o tro n s fo r e le c tr o n s ) and lin e a r a c c e le r a to r s (f o r p roton s, h e a v y ion s, e le c tr o n s ). F o r the G eV re g io n v a rio u s typ es o f e x t r e m e ly e x p en sive p roton sy n ch rotron s a re in op eration . T h e m a jo r grou p is fo r m e d by c y c lo tro n s in the m ediu m e n e rg y ran ge — a c c e le r a tin g 1 H, 2H, 3He and 4He p a r tic le s — w hich have p ro v e d to be e x t r e m e ly v e r s a t ile m achines. The o r ig in a l 'sta n d a rd ' c y c lo tro n is lim ite d in its e n e rg y ( < 2 5 M eV f o r deu teron s) b ecau se o f lim ita tio n s of a r e la t iv is t ic nature. T h e s e lim ita tio n s w e re f ir s t o v e r c o m e b y m odulating the o s c illa to r freq u en cy . In th ese s o - c a lle d s y n c h ro c y c lo tro n s the ion s a re a ccep ted into p h a s e -s ta b iliz e d o rb its on ly during a lim ite d fr a c tio n o f the F M c y c le . C on sequ en tly the p a r tic le b eam has a (m a c r o - ) stru ctu re o f sh ort bu rsts la s tin g te n th s o f m ic ro s e c o n d s and a duty c y c le o f on ly a fe w p e r cent. T h e r e fo r e , in te rn a l beam cu rren ts a re lo w e r than in standard c y c lo tro n s , w h e re a s e x tra c tio n o f the beam is a lso m o re d iffic u lt. T h e m o re m odern s e c to r -fo c u s e d c y c lo tro n s do not have th ese draw backs w h ile copin g with r e la t iv is t ic e ffe c ts b y an in tric a te shape o f the m a gn etic fie ld . If a p p ro p ria te c o r r e c tin g c o ils a re p ro v id e d , th ese s e c to r -fo c u s e d (o r iso ch ro n o u s) c y c lo tro n s o ffe r the p o s s ib ility of e n e rg y v a ria tio n , adding s t ill another d im en sion to the v e r s a t ilit y o f th ese m achines. T a b le I lis ts c u r r e n tly a v a ila b le c o m m e r c ia l c y c lo tro n s . T h e y ra n ge in p r ic e fr o m s 0.5 X I O 6 d o lla rs to s e v e r a l m illio n d o lla rs . W h ils t io n - a c c e le r a to r s a re the m o st w id e ly used m ach in es, e le c tr o n lin e a r a c c e le r a t o r s a lso d e s e r v e attention. T h e s e a re m ost u sefu l f o r in itia tin g p h o to -n u clea r re a c tio n s , a fte r c o n v e rs io n of the k in etic e n e rg y o f the e le c tro n s into B re m s s tra h lu n g w ith a continuous sp ectru m extending up to a m axim u m e n e r g y equ al to that o f the in ciden t e le c tr o n s [ 6 ]. The c o n v e rs io n is a ra th e r e ffic ie n t p ro c e s s f o r h ig h -e n e r g y e le c tro n s w h ile tr a v e r s in g high Z m a te r ia ls such as tantalum . W ith thin c o n v e rto rs the e m e rg in g h ig h -e n e r g y photon b eam is s tro n g ly peaked in the fo r w a r d d ire c tio n . T o p reven t h ittin g the actu al ta r g e t m a te r ia l, re s id u a l e le c tro n s can be stopped d ir e c t ly behind the c o n v e rto r in lo w Z m a te r ia l such as gra p h ite c o o le d w ith w a te r, in o r d e r to m in im iz e attenuation o f the photon beam . T o th is end the e le c tr o n s can a ls o be sw ept out o f the photon beam b y m eans o f a "s w e e p in g " m agnet. A m a tte r o f con cern that tends to be o v e rlo o k e d , o r at le a s t u n d er estim a te d , is the stru ctu re as a function o f tim e o f b ea m s p rodu ced by a c c e le r a to r s . T h is r e la t e s to the ra te with w hich a ra d ia tio n dose is d e liv e r e d to a ta r g e t p rob e and m a y have in h eren t c h e m ic a l con sequ en ces fo r the sy s te m under con sid era tio n . A s has a lr e a d y been p r e v io u s ly stated, s y n c h ro c y c lo -
4
L IN D N E R
tro n s o p era te w ith a duty c y c le of on ly s e v e r a l p er cent, d e liv e r in g the a c c e le r a te d p a r tic le s in te rm itte n tly on a m illis e c o n d b asis. C on sequ en tly, the actual in ten sity during th ese bu rsts is an o r d e r of m agnitude h ig h e r than the a v e ra g e d cu rren ts. E ven m o re so this is the case fo r the giant a c c e le r a to r s o f the s y n c h ro c y c lo tro n type which d e liv e r th e ir b u rsts with tim e in te r v a ls of around a second. But on top of th ese m a c ro s tru c tu re s the a c c e le r a to r beam s have a m ic ro s tru c tu re . F o r a ll typ es of c y c lo tro n s th ese su b-bu rsts a re gen era ted as nanosecond p u lses and thus with peak cu rren ts s e v e r a l tim e s that of the m a cro b u rst. A lin e a r e le c tro n a c c e le r a to r such as the 8 5 -M eV m achine of IK O at A m s te rd a m o p e ra te s with a duty c y c le o f 0.1% -pu lses having a burst length o f m ic ro s e c o n d s with a p ico seco n d stru ctu re. E q u a lly im p ortan t in th is connection a re s iz e and stru ctu re of the beam spot as th ey e m e rg e fr o m a c c e le r a to r s . G e n e r a lly this is around 1 cm 2 o r even le s s .
N E U T R O N SOURCES F o r our pu rpose in g e n e ra l, the use o f neutrons — both fa st and slow — to t r ig g e r a n u clea r re a c tio n has an advantage o v e r the use o f ch a rged p a r t ic le s , in that the z e r o ch arge state o f the neutron causes le s s ra d ia tio n dam age to the s y stem under in v e s tig a tio n . (A notable ex cep tio n is the case o f hydrogen eou s sy s te m s in w hich ch a rged knock-on proton s a re fo r m e d .) T h e r e fo r e , it s eem s ju s tifie d to pay attention to d iffe re n t typ es of neutron so u rc e s . F o r m o re e x te n s iv e in fo rm a tio n the in te re s te d r e a d e r is r e f e r r e d to re c e n t r e p o r ts d ealin g with the ap p lication o f neutrons in b io m e d ic a l in v e s tig a tio n s [7 ]. F o r c o m p leten ess sake w e m ention "p o r ta b le " neutron s o u rc e s such as the w ell-k n o w n R a -B e sou rce. Out o f a num ber o f ( « - e m i t t e r , B e )- s o u r c e s a 242C m -B e sou rce has, o v e r the R a -B e sou rce, the advantage o f a lo w e r gam m a contam ination, but the disadvantage o f the r e la t iv e ly sh ort h a lf- life (163 d) and, fu r th e rm o re , is of lim ite d valu e becau se of the low so u rce stren gth a v a ila b le (s 10 8 n/s). Much s tro n g e r so u rc e s of spon tan eou sly fis s io n in g 252C f a re now c o m m e r c ia lly a v a ila b le , but the advantages o f a high neutron output (up to 1 0 10n/s) and the lo n g e r h a lf- life (2 .6 5 y r ) a re s lig h tly o ffs e t by the in ten se accom pan yin g g a m m a -r a y e m is s io n and the r e la t iv e ly soft fis s io n neutron spectru m . M ost a ttra c tiv e fr o m the view p o in t o f both neutron e n e rg y , neutron output and the con stan cy o f neutron output, as w e ll as o f the ea se and s a fe ty o f op era tio n — w ith r e s p e c t to the h a za rd o f tr itiu m contam ination — a re the s o - c a lle d s e a le d - o ff neutron tubes. T h e s e d e v ic e s a re s m a ll contained lo w - e n e r g y (100 - 200 kV) e le c tr o s ta tic DC a c c e le r a to r s . T h e u n d erlyin g p r o c e s s is the T (d , n ) 4He re a c tio n on s e lf-r e p le n is h in g tita n iu m tritid e ta r g e ts . B ecau se o f the high valu e of Q = + 17.6 M eV the neutron e n e rg y is about 15 M eV and the e m is s io n n ea r to is o tr o p ic . T h e r e e x is t s e v e r a l c o m m e r c ia l v e r s io n s o f th ese d e v ic e s (K am an N u c le a r-U S A , P h ilip s H olland, E llio t - U K ); s e a le d - o ff tubes with outputs of г 10 11 n/s and l i f e tim e s of s e v e r a l hundreds o f hours a re in o p e ra tio n w h erea s tubes with an output o f s 1012 n/s a re ander con stru ction . M axim u m neutron flu x d en sities a re an o r d e r o f m agnitude lo w e r than the c o rre s p o n d in g 4 ir neutron output. M ost im p orta n t fo r (D ,T ) g e n e ra to rs , the con tribu tion o f g a m m a -r a y s to the d o s e -r a te am ounts to on ly a fe w p er cent.
IA E A -P L -6 1 5 / 1
5
S m a ll d e v ic e s such as V a n -D e -G r a a ffs with a c c e le r a tin g v o lta g e s of a fe w M eV p ro v id e d w ith u n -se a le d though pumped T i - T ta r g e ts , y ie ld in itia l outputs o f s e v e r a l tim e s 1 0 11 n/s ra p id ly d eclin in g, h o w e v e r, to lo w e r va lu es sin ce the ta r g e t is not rep le n is h e d during op eration . W ith ra p id ly ro ta tin g c o o le d ta r g e ts im p ro v e m e n ts a re to be exp ected . O f p a rtic u la r in te r e s t is a p ro p o sed in ten se neutron so u rce of 1015 n/s and w ith a flu x d en sity of > 2 X 1014n/cm 2 • s. T h e c o llis io n a l in te rs e c tio n of a T +-b e a m (1.5 A , 270 k eV ) and of a su p erso n ic je t o f D 2 m o le c u le s con stitu tes the actual neutron so u rce [ 8 ]. In addition to b ein g a m a jo r to o l to set o ff c h a rg e d -p a rtic le -in d u c e d n u clea r re a c tio n s , c y c lo tro n s a re a lso p o w e rfu l d e v ic e s to p ro v id e con v e n ie n tly in ten se b eam s o f h ig h -e n e rg y neutrons. S e v e r a l re a c tio n s can be used: 9B e (d,n ) 10B 7L i (d,n) 8Be 2H (d,n) 3He
Q = 3.8 M eV Q = 15 M eV Q = 3.3 M eV
W ith in c r e a s in g deuteron e n e rg y the neutron y ie ld g ro w s su b stan tially h igh er w h erea s the neutrons a ls o b eco m e s tro n g ly peaked in the fo r w a r d d ire c tio n , thus e ffe c t iv e ly p ro m o tin g h ig h e r lo c a l neutron flu x d en sities. T h e a v e ra g e neutron e n e rg y a lso in c r e a s e s at h igh er bom b ard in g e n e rg ie s . F o r the D + Be re a c tio n the neutron e n e rg y d istrib u tion is G aussian with its m axim u m at about h alf the in cid en t deuteron e n e r g y and with m axim um neutron e n e r g ie s equal to E d + Q. N eu tron y ie ld s p e r (j A - s go up fr o m 10 8 at 1 M eV , v ia 3 X 10 10 at 14 M eV to 1011 at 35 M eV and about 10 12 at 50 M eV deuterons. In com p a riso n the D + L i re a c tio n produ ces about one th ird of the num ber of neutrons but the sp ectru m is h a rd e r becau se of the high Q valu e. L a t e ly the D + D re a c tio n has b eco m e of in te r e s t sin ce h ig h -p re s s u re , th in -w in dow D 2 ta r g e ts have been put into p r a c tic e . T h is technique seem s m ost p ro m is in g becau se h ig h e r in te n s itie s and h igh er a v e ra g e neutron e n e r g ie s can be obtained than with a Be ta rg e t. One should a lso b e a r in m ind that, g iv e n a p a rtic u la r c y c lo tro n which can a c c e le r a te d iffe re n t ions, the m axim u m e n e r g y o f the proton beam is g e n e r a lly about tw ic e that of the deu teron beam . C onsequ ently it is often advantageous to use the proton beam in stea d o f the deuteron beam , m o re p a r tic u la r ly fo r those neutron re a c tio n s which have a high th resh old . F o r in stan ce, the com bined e ffe c t o f a high in ciden t e n e rg y p e r nucleon (p ro to n ) to g e th e r with the high Q valu e f o r the re a c tio n on L i (o r L iD ) v e r y much helps the fo rm a tio n of a highth resh o ld product such as U C fr o m 12C(n, 2n) U C with a th resh o ld o f 20.2 M eV . H ig h -e n e r g y e le c tr o n a c c e le r a to r s a lso can produce a va st num ber o f h igh ly e n e r g e tic neutrons. In s o - c a lle d th ick ta r g e t a rra n g em en ts the seco n d a ry photons o f the B re m s s tra h lu n g sp ectru m in turn produ ce neutrons as a re s u lt o f (т ,п ) and/or ( 7 ,pn) re a c tio n s . In f i r s t a p p roxim a tion the neutron p rodu ction p ro c e e d s in an is o tr o p ic fash ion with an e x tr a con tribu tion in the fo r w a r d d ire c tio n . T o g iv e an exam p le: at 90° at a distan ce o f about 40 cm fr o m a th ick tantalum ta r g e t b om b ard ed w ith 8 5 -M e V e le c tro n s , at our In stitu te we have m ea su red a neutron flu x d en sity of a p p ro x im a te ly 10 9 n/cm 2 • s. 100 ц А e '. N e a r e r the c o n v e rs io n ta rg e t, and depending on the p a rtic u la r a rra n g em en t of th is ta rg e t, much h igh er neutron flu x d en sities a re to be exp ected . R e la t iv e ly lit t le e x p e rim e n ta l w o rk has been r e p o r te d on the su bject [ 9], but a g re e m e n t with th e o r e tic a l p re d ic tio n s is s a tis fa c to ry .
L IN D N E R
r— h oo <b 1-1 +2
nj >, s
Ю Ë
H «
И
д
и g
H u
о o Оч Сц Й < H J U D Й ï*
> <ü 2
я §
■ S« .s a
H >■ !
*
>o ><ü % 2
t o N " I?"
m
Й S
o
m
O O o и
u
Й fe
o Й
o H
o
i
S
S u Q £ 2 я pî g
■s
Cm s h ¿
.. H fc
E -JH O
_
K
e
c- O .2 « «j ^
n О °o
j in i> <1 Q b,N Й 'd
b< O
£P
0) « 'd 2
ï Du
IA E A -P L -6 1 5 / 1
7
T o conclude this to p ic of neutron s o u rc e s we b r ie f ly m ention the n u clea r r e a c to r , s t ill our m ain so u rce o f th e rm a l neutrons. N ow ad ays a lm o s t any r e s e a r c h r e a c t o r [ 10] is p ro v id e d w ith one o r m o re th e r m a l colum n fa c ilit ie s w ith neutron flu x d e n s itie s ra n gin g fr o m 108 - 1010 n/cm 2 • s. O f s p e c ia l in te r e s t a re f a c ilit ie s such as the patient p o rt f a c ilit y o f the B rookh aven M e d ic a l R e s e a r c h R e a c to r with a w e ll- t h e r m a liz e d neutron flu x and a ra th e r lo w (fe w M rad/h ) con com itan t g a m m a -r a y d o s e -r a te as a re s u lt o f e x c e s s iv e sh ield in g. Such la r g e fa c ilit ie s a ls o g iv e e a s y a c c e s s to the a u x ilia r y eq u ip m ent often r e q u ir e d fo r s tr ic t c o n tro l o f ir r a d ia tio n conditions. B eam h o les re a c h in g to the c o re p ro v id e flu x d e n s itie s o f the o r d e r o f 1011 - 1013 n/cm 2 • s. A t th ose p o sitio n s th e re is a high con tribu tion in the o v e r a ll flu x o f u sefu l fa s t fis s io n neutrons. S m a lle r, though h ig h ly v e r s a t ile , a re the push-button lig h t- w a te r s w im m in g -p o o l r e s e a r c h r e a c to r s , such as the T R IG A M A R K III, w hich is a ls o a v a ila b le in a v e r s io n that can be pulsed. A U X I L I A R Y E Q U IP M E N T It is without doubt that the e x p e r im e n te r should have adequate a c c e s s to the actu al s ite o f bom bardm en t to enable h im to in s ta ll h is ta r g e ts s a fe ly and p ro p e rly . He n eeds am ple space f o r equipm ent such as la r g e ta rg e t am p ou les, h ig h -p re s s u re v e s s e ls , D e w a r 's tu rn in g w h eels. T h is im p lie s a lm o s t a lw a ys the a v a ila b ility o f d e fle c te d beam s e m e r g in g fr o m a c c e le r a to r s w ith su ffic ie n t in te n s ity (1 - 10/uA- h often s u ffic e s ) o r la r g e th e rm a l colum ns in the ca se o f n u c le a r r e a c to r s . P n eu m a tic s y s te m s w ith s u ffic ie n tly la r g e ra b b its — f o r in stan ce, to c o o l sa m p le s w ith d ry ic e during ir r a d ia tio n — a re v e r y u sefu l both with a c c e le r a t o r s and r e a c to r s . L iq u id n itro g e n fa c ilit ie s in r e a c to r s a re a lso m o r e com m on now adays. The use o f in te rn a l beam s o f a c c e le r a to r s fo r hot atom c h e m is tr y is e s s e n tia lly r e s t r ic t e d to b om b ard m en ts on B e - ta r g e ts m ade to ex ecu te in te rn a l fa st neutron ir r a d ia tio n s [ 5]. T H E E X P E R IM E N T T h e ite m s o f in te r e s t to the hot atom ch em ist, who has d ecid ed to in v e s tig a te the c h e m ic a l e ffe c ts a s s o c ia te d w ith r e c o i l atom s o f any p a rtic u la r ele m e n t, a re g o v e rn e d b y co n sid era tio n s such as: (1 ) W h ich is o to p e is the ra d io n u clid e o f ch oice on the b a sis o f d ecay c h a r a c te r is tic s such as h a lf- life , ra d ia tio n s in v ie w o f d etection tech n iqu es, etc. (2 ) W hat n u c le a r re a c tio n s a re a p p ro p ria te on the b a s is o f a v a ila b le beam s and the c r o s s - s e c tio n f o r the n u c le a r p ro c e s s ? (3 ) How a re the beam s m on itored ? (4 ) W hat a r e the ir r a d ia tio n con dition s with r e g a r d to flu x d en sity (d is trib u tio n ), b eam s tru c tu re , te m p e ra tu re , etc.? (5 ) L a s t but not le a s t, how much e n e r g y is d issip a ted into the ta rg e t by the beam i t s e lf and b y con com itan t ra d ia tio n s and what a re the p h y s ic a l and c h e m ic a l consequ en ces? W e attem pt to a s s e s s th ese qu estions as fo llo w s , ad
(1 )
Should th e re be a ch oice b etw een h a lf- liv e s , the s h o rte r one often p r e v a ils i f w e want to m in im iz e ir r a d ia tio n tim e s and con sequ en tly p o s s ib le ra d ia tio n dam age.
8
ad
L IN D N E R
(2)
H avin g d ecid ed upon the ra d ion u clid e to be used, one must s e le c t an a p p ro p ria te n u clear tra n s fo rm a tio n fo r its production. T o this end com p ila tio n s of Q -v a lu e s [ 11] and ex c ita tio n functions fo r c h a r g e d -p a r tic le re a c tio n s [ 12, 13], as w e ll as o f neutron c r o s s secio n s [ 14], a re o f g r e a t help. A sim p le s ta c k e d -fo il e x p e rim e n t u su a lly su pplies this kind of in fo rm a tio n w ith su ffic ie n t a c cu ra cy in th ose ca ses w h ere the in fo rm a tio n on e x c ita tio n functions is not r e a d ily a v a ila b le . On the b a sis o f stopping p o w e rs f o r ch a rged p a r t ic le s [ 15] one can then s e le c t the optim um e n e r g y t r a je c t o r y to be used so that m axim u m a c tiv ity is induced at a m in im u m dose d e liv e r e d to the sam ple. One should b e a r in m ind, h o w e v e r, that in the ca se o f c h a rg e d -p a rtic le -in d u c e d re a c tio n s the use o f protons in g e n e r a l w ill p r e v a il b ecau se o f the r e la t iv e ly lon g ran ge and low lin e a r e n e rg y tr a n s fe r fo r th ese p a r tic le s . S im ila r in fo rm a tio n r e g a r d in g p h o to -n u clea r re a c tio n s is much m o re r e s t r ic t e d and le s s a c c e s s ib le [1 6 ]. ad (3) A p ro b le m fre q u e n tly en cou n tered with c h a r g e d -p a r tic le re a c tio n s is the m ea su rem en t o f the amount o f b eam a ctu a lly s trik in g the ta rg e t. T h is is often fu rth e r co m p lic a te d b y the odd ta r g e t a r r a n g e m ents used. E r r o r s m a y fin d th e ir o rig in , am ong oth er so u rces, in beam m isa lig n m en t and fa u lty re a d in g s o f ta r g e t cu rren ts. The use of m o n ito r fo ils w ith a c r o s s - s e c tio n a l shape m atching that of the actu al ta r g e t and p la ced in fro n t o f the ta r g e t is a convenient technique [ 17]. The amount of r a d io a c tiv ity induced in such a thin a ctiva tio n f o il is a m ea su re fo r the in te g ra te d b eam cu rren t, ad (4 ) T y p ic a l ex a m p les of the e ffe c t o f dose and d o s e -r a te r e c e iv e d and (5) by a ta r g e t s y s te m can be found in studies on the r e c o il c h e m is try o f carbon. An evalu ation [1 8 ] o f the v a rio u s p o s s ib ilitie s o f p r o ducing ra d io c a rb o n fo r r e c o il c h e m is try studies is g iven in T a b le II. T he use of the s h o r t-liv e d is o to p e 1]C is g e n e r a lly p r e fe r r e d to the use o f the lo n g - liv e d 14C becau se of the dan ger o f e x c e s s iv e rad iation dam age during b om bardm en ts in the case o f the la tte r iso to p e. The use o f the re a c tio n 12C(n, 2n) X1C is r e s t r ic t e d by the need f o r ra th e r stron g, h ig h -e n e rg y neutron s o u rces becau se o f the v e r y high th resh old . T h e re a c tio n 14N(p,ci) П С, h o w e v e r, is p a r tic u la r ly advantageous becau se high le v e ls o f a c tiv ity can be g e n e ra te d w h erea s the ra d ia tion dose and d o s e -r a te can be r e a d ily co n tro lle d . T h e e x p e rim e n ts have shown that ir r a d ia tio n con dition s, as g o v e rn e d by the p h y s ic a l p a ra m e te rs o f the a c c e le r a to r s used, and a lso to a g re a t extent the in tr in s ic c h e m ic a l p a ra m e te rs o f the s y stem under con sid era tio n , can have an o v e rw h e lm in g in flu en ce on the e x p e rim e n ta l re s u lts [1 9 ]. A d d itio n a lly , in the case o f neutron irr a d ia tio n s in n u clea r r e a c to r s it has fre q u e n tly been shown, both in o rg a n ic and in o rg a n ic s y s te m s , that r e c o il product d istrib u tio n s a re often g r e a t ly a ffe c te d by fa c to r s such as neutron flu x d en sity and concom itant gam m a ra d ia tio n as w e ll as b y the am bient te m p e ra tu re [2 ]. C O N C LU S IO N T h e id e a l to o l to enable the hot atom ch em ist to m ake his s c ie n tific e x p e rim e n ts , has y e t to be invented. A so u rce o f h igh ly e n e r g e tic r a d io a c tiv e atom s (io n s ), not h avin g the disadvantage of in t e r fe r in g ra d ia tio n
I A E A -P L -6 1 5 / 1
9
fie ld s , is s t ill d iffic u lt to r e a liz e . A s this situ ation is s t ill fa r fr o m the g e n e r a l a p p lic a b ility o f s o - c a lle d c h e m ic a l a c c e le r a to r s , fo r the tim e b ein g we have to s e ttle fo r the n u clea r p r o c e s s as a v e h ic le fo r p rob in g h igh e n e r g y c h e m ic a l re a c tio n s . F o rtu n a te ly , in m any in sta n ces it has been shown that n u clea r re a c tio n s indeed o ffe r an a ccep ta b le though b y no m eans id e a l p o s s ib ility to a c h ie v e our go a ls. C h a r g e d -p a r tic le b eam s and (s e c o n d a ry ) fa s t neutron beam s — i f the la tte r be o f s u ffic ie n t k in etic e n e r g y and in ten s ity — s e e m to o ffe r m any im p orta n t and a ttra c tiv e a lte r n a tiv e s f o r r e a c to r neutrons to t r ig g e r the n u c le a r r e c o il p ro c e s s . In addition, m e d iu m -s iz e c y c lo tro n s se e m p a r tic u la r ly advantageous in this r e s p e c t becau se o f th e ir u sefu l s e c o n d a ry fa s t neutron b eam s. H o w e v e r, h ig h e r e n e r g y a c c e le r a to r s , if a v a ila b le , a re o f e q u a lly g re a t in te r e s t, as th e y g iv e a c c e s s to e x o tic though im p o rta n t r e c o il s p e c ie s such as, fo r in stan ce, 3SS ( t i = 3 h) b y 40A r (p, 3p) 38S o r 40A r ( 7 , 2p) 38S. A p a rt fr o m p r o ducing in a c c e s s ib le ra d io n u clid es o f in te r e s t in a d ir e c t w ay, h ig h -e n e rg y m ach in es can a ls o supply p a ren t-d a u gh ter p a irs that a re o th e rw is e d iffic u lt to m ake, thus en ab lin g one to in v e s tig a te the c h e m is tr y a s s o c ia te d w ith r a d io a c tiv e d eca y in new s y s te m s o f im p o rta n ce. E x a m p les a re 32Si (ti. = 300 y r ) d eca yin g b y pure b eta e m is s io n to 32P ( t i = 14 d) and the a b o ve-m en tio n ed 38S, w hich decays by b e ta -g a m m a e m is s io n to 38C l (t i = 37 m in). Such s y s te m s have the g r e a t a sset of p ro v id in g v ir t u a lly ra d ia tio n le s s en viron m en ts w h ile, in addition, fa c ilita tin g the in te rc o m p a ris o n w ith r e c o il re a c tio n s induced b y n u clea r re a c tio n s .
ACKNOW LEDGEM ENT T h is w o rk is p a rt of the r e s e a r c h p ro g ra m m e o f the Institu te fo r N u c le a r P h y s ic s R e s e a r c h (IK O ), m ade p o s s ib le by fin a n c ia l support fr o m the F ou nda tion f o r Fundam ental R e s e a rc h on M a tte r (F O M ) and the N eth erla n d s O rg a n iza tio n f o r the A d va n cem en t of P u re R e s e a r c h (Z W O ).
REFERENCES [13 IAEA, Chemistry Research and Chemical Techniques Based on Research Reactors, Tech. Rep. Series No. 17, IAEA. Vienna (1963). [2] (a) IAEA, Chemical Effects of Nuclear Transformations (Proc. Symp. Prague, 19(j0) 1, IAEA, Vienna (1961); (b) IAEA, Chemical Effects of Nuclear Transformations (Proc. Symp.Vienna, 1964) 2, IAEA, Vienna (1965). [3] IAEA, Radiopharmaceuticals and Labelled Compounds (Proc. Symp. Copenhagen, 1973) 1 and 2, IAEA, Vienna (1973). [4] USAEC, Photonuclear Reactions and Applications, Proc. Asilomar Conf.,USAEC, OakRidge (1973). [5] EURATOM, Accelerator Targets Designed for the Production of Neutrons, Proc.LiègeConf., Euratom, Brussels (1972). [6] HOSTE, J., et a l., Instrumental and Radiochemical Activation Analysis, Butterworth & Co. Ltd., London (1971). [7] EURATOM, First Symposium on Neutron Dosimetry in Biology and Medicine, I I , Proc. Neuherberg Conf., Euratom, Luxembourg (1972). [8] Los Alamos Rep. LA-UR-73-625, Univ. of Calif. [9] Von EYSS, H.J., LÜHRS, G., Z. Phys. 262 (1973) 393, and Refs therein. [10] IAEA, "Directory of Nuclear Reactors", 2-6 and 8, IAEA, Vienna. [11] Landolt-Bornstein — New Series Group I, 5, Part (a) "Q-values of Nuclear Reactions" (KELLER, K.A., LANGE, J._, MÜNZEL, H., Eds) Springer-Verlag, Berlin (1973).
10
L ÏN D N E R
[12] Ibid, Part (b) "Excitation Functions of Nuclear Reactions". [13] "Charged-particle Reaction List 1948 - 1971" (McGOWAN, F.K., MILNER, W.T., Eds) Academic Press, New York —London (1973). [14]. "Neutron Cross-Sections", BNL 325 and Supplements (2nd Ed.), Brookhaven Nat. Laboratories Report. [15] WILLIAMSON, C.F., BOUJOT, J., PICARD, J., Rapport CEA-R 3042, Comm, à l'Energie Atomique, Paris, 1966. [16] (а) КАТО, T., J. Radioanalyt. Chem. ПЗ (1973) 307; (b) "Photonuclear Reaction Data, 1973", Natl. Bureau of Standards, Special Publication 380, Washington. [17] CUMMING, J.B., Ann.Rev. Nucl. Sciences 13 (1963) 261. [18] WOLF, A. P., Adv. Phys. Org. Chem. 2 (1964) 201. [19] ACHE, H., WOLF, A.P., J. Phys. Chem. 72 (1968) 1988.
DISCUSSION G. S T O C K L IN : It is lik e ly that v e r y in ten se 1 4 -M e V neutron so u rc e s w ill be a v a ila b le in the la te 1970s b ecau se o f the in te r e s t in the p o s s ib ility o f p ow er fr o m fu sion r e a c to r s . T h r e e typ es o f so u rc e s a re a va ila b le: one has a m o re o r le s s c la s s ic a l ro ta tin g ta rg e t, p ro b a b ly p e rm ittin g flu x d en sities in the ran ge o f 1 0 13 n/cm 2 • s; a second, som ew hat m o re ex p e n s iv e and fu rth e r in the fu tu re, in v o lv e s a je t d e v ic e fo r the ta r g e t to a vo id the p ro b le m s of in ten se p o w er a b sorp tion plus d is s o c ia tio n o f the ta r g e t h yd rid e. In this apparatus, tr itiu m ion s w ill be shot into a d eu teriu m je t, w ith exp ectation o f flu x d e n s itie s as high as 1014 n /cm 2 • s. T h e th ird type in v o lv e s the c la s s ic a l deu teron breakup, a c c e le r a tin g deuterons o n to lith iu m o r b e r y l liu m , w ith about h a lf the e n e r g y goin g in to the neutrons. T h is d e v ic e should p ro v id e a r e la t iv e ly b ro a d neutron sp ectru m , w ith a flu x o f 1012-1013п/cm 2 • s fr o m h ig h -in te n s ity -b e a m lin e a r a c c e le r a to r s . A l l th ese tech n iqu es a re b ein g c u r r e n tly and in te n s iv e ly pursued in the U n ited States o f A m e r ic a . I f th ey a re su c c e s s fu l, then th ey w ill a ls o open up the p o s s ib ility o f isotop e p rodu ction w ith the (n, 2n) re a c tio n , and the r e la te d p o s s ib ility o f la b e llin g fo r a p p ro p ria te ta r g e t m o le c u le s . L . L IN D N E R : I have been a w a re of the je t technique, but did not m ention it b ecau se I have no id e a o f the p resen t state o f its d evelopm en t. G. S T Ô C K L IN : Just plans. T h e je ts a re at a v e r y e a r ly stage. L . L IN D N E R : I w ould gu ess then that th ey w ill not be a v a ila b le fo r 5 o r 10 y e a r s , p ro b a b ly much lo n g e r. J. D ANO N: O ur e x p e rie n c e in B r a z il w ith e le c tr o n lin e a r a c c e le r a to r s m a y be of in te r e s t to the v a rio u s d e velo p in g cou n tries. Since 1960 we have built s m a ll, 2 -M e V tr a v e llin g - w a v e lin e a r a c c e le r a to r s w ith ra d ia tio n in ten s ity c o m p a ra b le to that fr o m s m a ll cobalt so u rces. W e have bu ilt fiv e of th ese, and then a la r g e r 2 8 -M e V a c c e le r a to r fo r n u clea r p u rp oses a fte r we gain ed e x p e rie n c e w ith the s m a lle r ones. T h e te ch n o lo g y is not v e r y d iffic u lt — th re e e n g in e e rs w e r e able to bu ild th ese a c c e le r a to r s with not such la r g e am ounts o f m oney. L . L IN D N E R : T h e disadvan tage f o r this type o f m achine is the con tinuous, b ro a d photon sp ectru m . M any o f the photons do not produce the is o to p e d e s ir e d , but do produ ce ra d ia tio n dam age. T h e r e is v e r y lit t le in fo r m ation a v a ila b le about e ith e r the u tility o r the d an gers in v o lv e d in using e le c tr o n a c c e le r a t o r s f o r hot atom c h e m is try . W e have done som e e x p e r i m ents o f th is kind in A m s te rd a m , and our f i r s t e x p e rim e n ts w e re not v e r y fa v o u ra b le b ecau se o f the ra d ia tio n dam age.
I A E A -P L -6 1 5 / 1
11
F .S . R O W L A N D : O ur e x p e rie n c e at Ir v in e with the 1 4 -M e V sea led -tu b e neutron g e n e r a to r is that its p e rfo rm a n c e has been u n b e lie v a b ly good — you push the s ta rt button, and you get fa s t neu tron s, and the 19F (n, 2n) 18F re a c tio n . W e have had ou rs sin ce 1968, and a re on our secon d tube now. T h e f i r s t one g a ve 440 h o f op era tio n , and the second is a lr e a d y above 250 h, w ith v e r y lit t le in the w a y of o p e ra tio n a l p ro b le m s . T h is in stru m en t has the disadvan tage o f w o rk in g so w e ll that a student can w o rk w ith it f o r a lo n g tim e , and n e v e r le a r n anything about a c c e le r a t o r s o r the p ro b le m s of produ cin g ra d io is o to p e s . T h e y ju st push the button and the 1SF c o m es out, and th ey m ay not r e a liz e that oth er m a ch in es a re d iffe re n t. L . L IN D N E R : A r e n 't you s t ill s tr o n g ly h a m p ered in you r r e s e a r c h by the lo w flu x ? A s I understand it, a re n 't you e s s e n tia lly con fin ed to w o rk in g in s y s te m s that in v o lv e o n ly th e rm a l re a c tio n s o f 18F? F .S . R O W L A N D : N o , we a re on ly con fin ed to w o rk in g with s y s te m s that have 19F in the ta r g e t m o le c u le s . W e cannot put in 1% o f som e flu o r in e con tain in g com pound and 99% h yd ro ca rb o n o r oth er n o n -flu o rin a te d m a te r ia l and have much 1SF r a d io a c tiv ity . But we can do both hot and th e rm a l 18F r e a c tio n stu d ies w ith the flu o rin a te d su b strates. D.J. M A L C O L M E -L A W E S : How much does this w o rk out in cost p e r hour f o r o p e ra tin g the fa s t neutron g e n e ra to r? F .S. R O W L A N D : T h e cost o f re p la c e m e n t fo r a new tube is about $3 000 and the tube la s ts fo r m o re than 300 h, so the ir r a d ia tio n cost w ork s out to som eth in g around $10/h. D.J. M A L C O L M E -L A W E S : T h a t's v e r y cheap. D .M . R IC H M A N : T h e m ech a n ica l a b ility to produ ce ra d io is o to p e s s eem s to be w e ll in hand and im p ro v e m e n ts w ill no doubt continue, but the d iscu ssio n d oes not s e e m to have is o la te d produ ction as a lim itin g fa c to r — the r e a l f a c ilit ie s and o th e r e x p e rim e n ta l d iffic u ltie s se e m to lie e ls e w h e re . A s I r e c a ll fr o m e a r lie r d iscu ssio n s, one im p o rta n t p ro b le m s e e m s to be the in a b ility to c a r r y out e x p e rim e n ts on th ese a c tiv a te d s p e c ie s du ring th e ir life t im e to c o lle c t in fo rm a tio n on that tim e - s c a le . Is th is the m ost im p orta n t lim ita tio n at p resen t on e x p e rim e n ta l hot atom c h e m is try ? L . L IN D N E R : In m y opinion, th e re is a g r e a t n eed fo r h ig h -in te n s ity , v e r y high e n e r g y neutron flu x e s to produ ce ra d io is o to p e s without so much o f the e v e r la s tin g p ro b le m o f ra d ia tio n dam age to the sa m p les. P ro to n s a r e a ll rig h t, but th ey do produce a v e r y dense io n iza tio n tra ck . D eu terons a re w o rs e , and alpha p a r tic le s a re h o r r ib le in the d en sity o f the a ccom p an y in g io n iza tio n . U n fortu n ately, not a ll produ ction p ro b le m s can be s o lv e d w ith h ig h -in te n s ity fa s t neutron beam s. A .P . W O L F : Hot atom c h em ists have been in trig u e d f o r a lon g tim e by the p o s s ib ility o f an in -s itu m ethod f o r fo cu sin g on the r e a c tiv e s p e c ie s during its e x t r e m e ly sh ort life tim e . W e have h ea rd about the M ôssb a u er and the P A C (P e r tu r b e d A n g u la r C o r r e la tio n ) s y s te m s , of c o u rs e , but in m any situ ations one is in te r e s te d in studying the p r im a r y even t it s e lf. W e m ade a ca lcu la tio n b a sed on the B rook h aven L IN A C in je c to r f o r the AGS a c c e le r a to r — the in je c to r is a 2 0 0 -M eV m achine with a d esign c u rren t o f 200 ц А , and a r e a l cu rren t o f 150 /uA when the phase o f the m oon is rig h t and the p r o p e r c e r e m o n ia l dances have been p e rfo rm e d . A t th ese le v e ls o f in te n sity , enough hot atom s a re a c tu a lly g e n e ra te d so that th ey could be e x tra c te d through a b ea m tube, and the stru ctu re o f the s p e c ie s exam ined. T h e ca lcu la tio n lo o k ed in te re s tin g u n til we e s tim a te d the cost o f the e x t r a c to r and the attached m ass s p e c tro m e te r. It would c e r ta in ly cost around
12
L IN D N E R
$500 000 to do th is e x p e rim e n t, and one r e a lly has to ask w h eth er the in fo r m ation that m igh t be obtained is w orth that much tim e and that much expen ditu re o f m oney. In p rin c ip le and quite p o s s ib ly in p r a c tic e , one could g e n e ra te enough hot atom s with th ese v e r y h ig h -e n e rg y , h ig h -in te n s ity a c c e le r a to r s to produ ce a m a c ro s y s te m fo r study in p la ce of the p resen t m ic r o s y stem s. F . H A R B O T T L E : A lo n g som ew hat the sam e lin e s , but ra th e r m o re ch eaply, is the p o s s ib ility of com bin in g a p u lsed m ach in e with flo w s y s te m s to e x tra c t in fo rm a tio n about hot atom re a c tio n s , f o r in stan ce, in solution. M anny H illm a n and I at B rook h aven a ctu a lly went so fa r as to set up a flo w s y s te m in fro n t o f a fa s t neutron m achine, and in v e s tig a te d som e o f the re a c tio n s o f a lk y l h a lid es. W e didn't a ctu a lly c a r r y out the ex p e rim e n t, but th e re is no re a s o n why it could not be done. If one has a pulse of ra d ia tio n , one m ight be able to see the d istrib u tion s o f atom s in an o rg a n ic h alid e s y s te m w ithin m illis e c o n d s a fte r the ra d ia tio n is turned off. K. R O S S LE R : Som e e x p e rim e n ts have been done with h ig h -e n e rg y e le c tr o n s and can be used as a m o d el fo r c o n s id e ra tio n of hot atom e x p e r i m ents. One can produ ce a la r g e amount of ra d ia tio n dam age, and then look at the m a c r o s c o p ic a lly dam aged syste m f o r m ech a n istic ob serva tio n s. T h is can be done in situ in the s o lid state, o b s e rv in g on lin e , fo r exa m p le, b y a b sorp tion s p e c tro s c o p y . L e v y has done this at B rookh aven , and we have done this at Jü lich , fr e e z in g a ll the states at 4K. T h is lo o k s lik e a v e r y p ro m is in g approach to m a c ro s c o p ic s o lid state hot atom c h e m is try . J. D ANO N: W e r e th ese e x p e rim e n ts p e r fo r m e d w ith the b eam on? K. R O S S LE R : T h e s e e x p e rim e n ts w e re done w ith the b eam on, but as fa r as I know, th ey have on ly been done w ith e le c tro n s . G. S T O C K L IN : H o w e v e r, th is is not hot atom c h e m is try . T h is is so lid state ra d ia tio n c h e m is tr y — o f c o u rse, th e re a re som e s im ila r it ie s to hot atom c h e m is tr y so that som e o f the in fo rm a tio n is quite useful. A s fa r as H a rb o ttle 's su ggestion f o r o n -lin e e x p e rim e n ts is con cern ed , I'm a lit t le bit p e s s im is tic . F ir s t , w ith the c la s s ic a l 1 4 -M e V neutron s o u rc e s , you w on 't be able to see anything at all. Th en , if you have enough in ten sity, you w ill a lw a ys produce a much la r g e r nu m ber o f ra d ic a ls p er r e c o i l atom than you have r a d io a c tiv e ly la b e lle d . A s M r. L in d n e r has a lre a d y pointed out, you w ill then be o b s e rv in g the re a c tio n s o f the sec o n d a ry ra d ic a ls and not o f the p r im a r y hot atom . G. H A R B O T T L E : L e t m e c la r ify a bit m o re the kind o f e x p e rim e n t w hich I had in mind. The c la s s ic a l technique in hot atom c h e m is try is s im p ly to put som eth in g in fro n t of an a c c e le r a to r o r into a r e a c to r , ir r a d ia te it, and then take it back to the la b o r a to r y to an alyse it by vap ou r phase ch ro m a to gra p h y o r som e oth er technique. O ur id e a is s im p ly that i f the f ir s t stage o f the a n a ly sis could be c a r r ie d out in a fe w m illis e c o n d s , som e new in fo rm a tio n m ight e m e r g e fr o m an a lte ra tio n in the y ie ld pattern. T h e ex p e rim e n t w hich we a ctu a lly did was not dependent upon pu lsin g — it was in fa c t a flo w s y s te m in w hich p ro p y l b ro m id e flo w e d at v e r y high speed past a beam o f neutrons and e n te re d im m e d ia te ly into a m ix tu re with the usual e x tra c tin g agents. It w as just a m o d ifia c tio n o f the c la s s ic a l L ib b y e x p e rim e n t using p ro p y l b ro m id e , hyposulphite and w a ter. O ur intention w as to d eterm in e w h eth er the reten tio n w as the sam e in a fa st flo w e x p e r i m ent with e x tra c tio n in 50 to 100 m s a fte r the bom bardm en t as it w as in the c la s s ic a l tim e - s c a le . A c tu a lly , L ib b y h im s e lf did an e x p e rim e n t that was s im ila r in p rin c ip le , but c a r r ie d out in a s lig h tly d iffe re n t way. In c o m bination with a pu lsin g s y stem , one m igh t be able to in c r e a s e the neutron
IA E A -P L -6 1 5 / 1
13
b eam in ten sity , andthus d e te rm in e w h eth er th e re a re s h o r t-te r m e ffe c ts con tribu tin g to the y ie ld p attern d e te rm in e d in the usual e x p erim en t. F .S . R O W L A N D : B a s ic a lly you w ould be lo o k in g fo r s p e c ie s which a re v e r y h ig h ly r e a c tiv e and w hich have life t im e s o f m illis e c o n d s ? G. H A R B O T T L E : Y e s , th a t's c o r r e c t. F .S. R O W L A N D : I have a sep a ra te com m ent to m ake, plus a question fo r A l W o lf. T h e r e a re s e v e r a l h ig h -e n e r g y proton a c c e le r a to r s , tw o in the U nited States o f A m e r ic a , w hich a re p o te n tia lly la r g e s o u rc e s f o r the produ ction of ra d io is o to p e s , and th e r e fo r e a lso o f hot atom s. One of th ese is the m eson fa c ilit y at L o s A la m o s (L A M P F ) w hich is e x p ected to have m illia m p e r e cu rren ts at 800 M eV . A l l this beam in ten sity, i f e v e n tu a lly dumped into a b ea m stop and ca lcu la tio n s in d ica te fa s t n eu tron flu x e s in the ran ge of 1013 n / cm 2 • s in the v ic in it y o f the beam stop. T h e y have n an oam pere c u rre n ts now, a re ex p e c tin g m ic r o a m p e r e c u rre n ts in the autumn o f 1974, and m illia m p e r e cu rre n ts late in 1975. A t p resen t, the proton s a re not b ein g d ir e c te d on to the fin a l beam stop, and we have not yet c a r r ie d out any e x p e rim e n ts . H o w e v e r, we ex p ect to b egin som e e x p e rim e n ts in the autumn of th is y e a r. T h e oth er p resen t U nited States so u rce is the B L I P fa c ilit y at B r o o k haven, and I w on d er i f A l W o lf has any com m en ts to m ake about the B L IP fa c ilit y o r about such so u rc e s in g e n e ra l? A .P . W O L F : I'm not quite su re just what you a re asking. T h o se a c c e le r a to r s a re c e r ta in ly a v a ila b le , but th e re is a p r a c tic a l, ra th e r than a th e o r e tic a l, p ro b le m in using them . The p r a c tic a l p ro b le m is that th ese m ach in es a re c o n tro lle d by the h ig h -e n e r g y p h ysics com m unity, and you a re alw ays at th e ir m e r c y in the sch edu lin g o f e x p e rim e n ts . It is a c la s s ic a l ex a m p le o f the ch em ist b ein g at the end of the lin e. In p r in c ip le , th e re is no re a s o n w hy you can 't do hot atom c h e m is try , e ith e r at the b eam stop, o r at B L IP w ith the c h e m is tr y d ep a rtm en t's own o n -lin e fa c ilit y in that proton beam . H o w e v e r, the amount o f tim e one can get on that o n -lin e fa c ilit y is quite s m a ll. W h eth er o r not it is p r a c tic a l to do hot atom c h e m is tr y depends on fa c to r s w hich a re not under our con trol. T h e r e is another point that has been m ade b e fo r e , as L ou k L in d n e r has e m p h a sized s e v e r a l tim e s , and that is just this: th ese e x tr a o r d in a r ily high proton cu rren ts do fa n ta s tic ra d ia tio n dam age to ta r g e t s y s te m s , and one has a com pounded p ro b le m o f tr y in g to sep a ra te r a d io ly tic re a c tio n s fr o m hot atom re a c tio n s . Just c o n s id e r the p ro b le m of the slo w in g down o f 1 0 0 -M eV proton s at 200 ц А , fo r e x a m p le, in a m o d e r a te ly th ick ta r g e t, and it v e r y q u ick ly b e c o m e s apparent that you have a s e v e r e p ro b lem . F .S . R O W L A N D : T h e r e is no qu estion th e re . T h e ra d ia tio n le v e ls w ill be high, and one w ill o b s e r v e a com bin ation of ra d ia tio n c h e m is tr y and hot atom c h e m is try , but w ith v e r y high y ie ld s of to ta l r a d io a c tiv ity . A .P . W O L F : W e 'v e su ggested s e v e r a l tim e s that one should do hot atom c h e m is tr y in the B L I P fa c ilit y , and I suppose that som e day w e 'l l a ctu a lly t r y it. It w ould c e r ta in ly be in te r e s tin g becau se of the high y ie ld s . L . L IN D N E R : I think what m a y be e ven m o re im p o rta n t fr o m such m ach in es w ill be the fa s t neutrons, e s p e c ia lly fo r fundam ental studies. F .S . R O W L A N D : T h a t's what w e a re in te r e s te d in at L o s A la m o s — the fa s t neutrons. But the fa s t neutrons a re goin g to be a ccom p an ied by ra d ia tio n dam age, too.
14
L IN D N E R
A .P . W O L F : A g a in , in p rin c ip le , th a t's g re a t. But in p ra c tic e , even with a c y c lo tro n , w o rk in g w ith fa s t neutrons is som eth in g u su a lly to be avoid ed becau se the ra d ia tio n in te n s ity in the c y c lo tro n vau lt and the r a d io a c tiv ity in you r ta r g e ts a re both e x t r e m e ly high. Y ou spend a lo t m o re tim e w o r r y in g about how to get at the ta r g e ts , and in taking p recau tion s about ra d ia tio n s a fe ty than you do in a c tu a lly doing the ex p e rim e n ts . T h e f i r s t n C e x p e rim e n ts which w e e v e r did w e re done with fa s t neutrons p rodu ced by the deu teron b om bardm en t o f lith iu m . T h e s e w e re alw ays sch edu led f o r F r id a y a ftern oon , and the vau lt got so hot that you cou ldn't w alk in th e re . You had to run in, p u ll out you r ta r g e t, and a llo w the w eekend fo r the r a d io a c tiv ity in the ta r g e t to d eca y b e fo r e you could use it again. A gain , th ese a re g r e a t to o ls , and the p o s s ib ilitie s a re v e r y in te re s tin g , but th ere a re r e a l p r a c tic a l p ro b le m s — m on ey p ro b le m s . You could autom ate such e x p e rim e n ts , and sh ield them , e tc ., but that m akes it v e r y ex p en sive. D.J. M A L C O L M E -L A W E S : T h e r e is a s e v e r e lim ita tio n in con cen tratin g, as one tends to do, on the (n, 2n) re a c tio n . R ow land m en tion ed this e a r lie r , and I'd lik e to e m p h a size it in re la tio n to n itro g e n atom c h e m is try . T o study the c h e m is tr y o f 13N , you a re fo r c e d to have a la r g e quantity o f 14N in the re a c tio n m ix tu re. U n fortu n ately, 13N r e a c ts v e r y e a s ily with anything containing 14N to fo r m 13N 14N in la r g e e x c e s s . In th is w ay, one is p reven ted fr o m studying such re a c tio n s as 13N plus CH 4, sin ce the c h e m ic a l re a c tio n c r o s s - s e c tio n of 13N in re a c tio n w ith C H 4 is m inute in c o m p a riso n to the c r o s s - s e c tio n f o r re a c tio n with any n itro g e n -c o n ta in in g s p e c ie s to fo r m 13N 14N . I suspect that the sam e thing happens with 18F to som e extent. L . L IN D N E R : I have a solu tion to this kind o f p ro b le m — f o r in stan ce, that o f doing П С c h e m is tr y in a n on -carb on containing system . In addition to th ese h ig h -e n e r g y proton s and neutrons, th ese la r g e a c c e le r a to r s a lso produ ce c o n s id e ra b le pion and oth er b eam s. Since our d etection techniques a re so re fin e d , you could w o rk w ith the pions and muons and avoid the ta r g e tin g p ro b le m in som e ca ses. F .S . R O W L A N D : I want to com m ent in an sw er to M a lc o lm e -L a w e s . In the p a rtic u la r ca se of 18F fo r m e d fr o m 19F (n , 2n)18F re a c tio n in flu o rin a te d su b stra tes, th e re a re com pounds — th ose with satu rated v a le n c e s h e lls — f o r w hich the hot y ie ld s a re v e r y low , and th e re a re no th e rm a l re a c tio n s at a ll. Com pounds such as SFg, N F 3, e tc ., show y ie ld s o f 1% fo r SF 5 18F , 2% fo r N F 218F , perh aps m in o r y ie ld s o f oth er produ cts such as N F H 1SF , and the r e s t o f the 18F atom s a re fr e e to r e a c t with w h a te v e r oth er m o le c u le s a re p re s e n t in the system . D.J. M A L C O L M E -L A W E S : Y e s , but th e y la r g e ly b eco m e th e rm a liz e d , and then you have the danger that th ey w ill r e a c t w ith SF6 to fo r m a v e r y high y ie ld o f F 18F — lik e a v e r y b ig shadowing e ffe c t. F .S . R O W L A N D : I don't think that th e re is v e r y much fo rm a tio n o f F 18F , c e r ta in ly not by th e rm a l 18F atom s, becau se the a b s tra c tio n o f F fr o m SF 6 is en d o th erm ie fo r th e rm a l flu o rin e atom s. W hen w e put v e r y s m a ll am ounts o f a c e ty le n e and H I in to gaseou s Sig , we fin d as much as 85% of the 18F in the fo r m o f flu o r o v in y l r a d ic a l — m ea su red as C H 2=C H 18F . Some of the m is s in g 18F a c tiv ity p ro b a b ly g o es into h yd rogen a b stra ctio n fr o m a c e ty le n e , so that we don't b e lie v e that flu o rin e atom a b stra ctio n fr o m SF 6 is a v e r y im p orta n t p r o c e s s , i.e . 15%. It r e a lly b eh aves as though SF 6 is a v e r y in e r t s y s te m to w a rd th e r m a l 18F atom s. T o g iv e you som e id e a how in e rt the SF6 is , w e have put in compounds f o r w hich w e a re unable to m ea su re the vap ou r p re s s u re o f the added m o le
I A E A -P L -6 1 5 / 1
15
cu le — trip h e n y lm e th y l-tin , fo r exa m p le. You open one v a lv e on the vacuum lin e , and another one in to the evacu ated sam p le bulb, and a ll the gau ges show no sign o f any gas b ein g p resen t. N e v e r th e le s s , you add s e v e r a l a tm o s p h e re s o f SF 6 to the s y stem , s e a l o ff the sa m p le, ir r a d ia te it, and get p rod u cts that com e fr o m th e rm a l 18F re a c tio n w ith that compound: C H 318F and C6H 518F . T h is m eans that the SFg m o le c u le m ust be e x t r e m e ly in e rt, and the 18F atom s keep bouncing around u n til th ey c o llid e with and r e a c t w ith the su bstrate s p e c ie s that is p resen t. J .P . A D L O F F : I have a qu estion con cern in g the instantaneous r e c o il e n e r g y a r is in g fr o m the (n, 2n) re a c tio n . I r e m e m b e r the w o rk o f A n b a r and N e ta who show ed that the r e c o i l e n e r g y o f 18F m igh t not be as high as ca lcu la ted . A s I r e c a ll, th ey assu m ed that the (n, 2 n) r e a c tio n a ctu a lly o c c u r r e d in tw o steps — the flu o rin e atom w ould captu re a neutron, and then be in an e x c ite d state fr o m w hich s p a lla tio n -lik e p r o c e s s e s would e v a p o ra te tw o neutrons le a v in g 18F . In that ca se, the r e c o i l e n e r g y is much lo w e r than the amount you ca lcu la te ju st fr o m the Q valu e o f the re a ctio n . G. S T O C K L IN : W ith what e n e r g y neutrons w as th is re a c tio n done ? J .P . A D L O F F : I do not re m e m b e r , oth er than that it was above the th resh old . G. S T O C K L IN : It 's v e r y im p orta n t w h eth er you go through a compound nucleus o r a d ir e c t p r o c e s s , and that depends v e r y much on the e n e r g y of the neutrons. J .P . A D L O F F : I b e lie v e that th e y w e r e 1 4 -M e V neutrons. L . L IN D N E R : I think th is qu estion is a lit t le bit a ca d em ic, becau se m o st o f th ese re a c tio n s have f a i r l y high th re s h o ld s , and have a ra th e r high r e c o i l e n e r g y anyway. The on ly dan ger is that the k in etic e n e rg y is too low , f o r then you m a y not have su ffic ie n t tim e f o r the e le c tr o n captu re p r o c e s s n e c e s s a r y to c o n v e rt ion s into the n eu tra l s p e c ie s , as pointed out th is w eek b y N ew ton. But I r e a lly think that in this ca se th is is an a ca d e m ic question. If y o u r atom s a re s o m ew h ere up in the 10- t o l0 0 - k W r e g io n in r e c o il e n e rg y , then you a re p e r fe c t ly safe. F .S . R O W L A N D : And you a re not in that re g io n w ith 18F , but c o n s id e ra b ly h igh er. I think the an sw er to the qu estion r a is e d b y P r o fe s s o r A d lo ff is s im p ly that the e x p e rim e n ts w hich le d to the su ggestio n by A n b ar and N e ta a re w ron g. I b e lie v e that bond ru ptu re did o c c u r in th e ir s y stem , and that the re s u lts a re in c o r r e c t. J .P . A D L O F F : T h e e x p e rim e n ts a re w ron g? F .S . R O W L A N D : Y e s , I think that is the explan ation — the o b s e rv e d re s u lts w e re not cau sed b y fa ilu r e to o b s e r v e bond ru ptu re. In ou r own e x p e rim e n ts on the gas phase bom bardm en t of SF6 , o n ly 1% o f the 18F is found as SF 518F a fte rw a rd s , and 99% is found so m e w h e re e ls e . T h is m eans that bond ru ptu re o f the S - F bond is at le a s t 99%, and I assum e that it is 100%. J .P . A D L O F F : T h e r e c o il e n e r g y is s u r e ly above the bond e n e rg y , but the in itia l r e c o il e n e r g y a fte r the (n, 2n) re a c tio n — I think that th is e n e rg y has n e v e r b een m e a s u re d e x p e rim e n ta lly . F .S . R O W L A N D : I a g r e e with that — I'm su re that the r e c o il e n e r g y has not been m easu red . K. R O S S LE R : T h e r e a re not o n ly lo w - e n e r g y (n, 2n) re a c tio n s , but a lso fa s t (n, y ) re a c tio n s , but with r e c o i l e n e r g ie s a lit t le h ig h e r than th ose fr o m th e r m a l (n, 7 ) re a c tio n s . W e have studied the e m is s io n of rh en iu m atom s fr o m a s u rfa c e o f m e ta llic rheniu m , and have m e a s u re d the y ie ld of l85Re
16
L IN D N E R
(n, 7 ) 186R e shot out fr o m the su rfa ce. It would not be p o s s ib le f o r them to e m e r g e i f th ey had on ly the 3 0 -eV r e c o il e n e rg y fr o m the th e rm a l (n, 7 ) rea ctio n . L . L IN D N E R : I have tw o m o re com m en ts on the p o s s ib le u ses of th ese gian t m ach in es. One is a v e r y e x p en sive ex p e rim e n t. W ith v e r y high e n e rg y e le c tr o n s , you can g e t,a v e r y high e n e r g y b rem sstra h lu n g sp ectru m , and this is then a m eans of ending up w ith m o n o e n e rg e tic proton s. T h is could then b ec o m e v e r y in te re s tin g , e s p e c ia lly when you r e c a ll our discu ssion o f the ( 7 , 7 ') e x p e rim e n ts with c o n tro lle d g a m m a -r a y e n e r g ie s to im p a rt m e a s u re d am ounts o f r e c o il e n e r g y to c e rta in r e c o il sp ec ie s . A n o th er p o s s ib ility e x is ts w hich is not ex p e n s iv e at a ll, as long as th ese m ach in es a re a lre a d y running. T h e y a re a p oten tia l v e r y high a c tiv ity so u rce of e x o tic ra d io n u clid es; f o r in stan ce, 38S. T h e in te r e s t h e re lie s in the d eca y to 38C1. W ith a v e r y high e n e rg y p a r tic le s o u rce, one can p r o duce the paren t o f a p a ren t-d a u gh ter s y stem , and then w o rk in an e x tr e m e ly clea n en viro n m en t w ith r e s p e c t to the accom p an yin g ra d ia tio n dam age. I can g iv e you another exa m p le w hich has in trig u e d m e fo r a lon g tim e — I r e m e m b e r that R ow land and I d iscu ssed it in 1958 at B rookhaven. If we on ly had a stro n g so u rce of 32Si — which decays b y pure b eta d ecay to 32P — w e could do som e in te re s tin g 32P e x p e rim e n ts in a clean en viron m en t. T h e r e is no e a s y w a y to m ake 32Si — it has a 3 0 0 -yea r h a lf- life . W ith h ig h -e n e r g y m ach in es such as L A M P F and B L IP , and with the 500-M eV e le c tr o n a c c e le r a to r in A m s te rd a m , you should be able to m ake p len ty of it — p len ty fo r hot atom ch em ists. T h e r e a re a lso oth er p aren t-d au gh ter p a ir s w hich w ould be h ig h ly in te re s tin g to our fie ld . S. A M IE L ; If w e a re w illin g to c o n s id e r th ese e x tr e m e conditions fo r e x p e rim e n ts , one can a ls o quote the use of o n -lin e iso to p e s e p a ra to rs con n ected to n u clea r re a c tio n s y stem s. In the one at Jü lich and in the n ew er one b ein g put into o p e ra tio n at G ren o b le, you can s e le c t a ll kinds o f s h o rt liv e d is o to p e s prod u ced in fis s io n , and put them into a re a c tio n v e s s e l with s o lid s u rfa c e s , o r with a gaseous ta r g e t — with a w ide ran ge of ch oice fo r d iffe re n t ele m e n ts and is o to p e s , h a lf- liv e s and c o n tro lle d e n e rg ie s . L . L IN D N E R : It should be w e ll u n derstood that w e a re not su ggestin g that th ese m ach in es should be run just to produ ce one iso to p e. V e r y often th ese e x p e rim e n ts can be run in a p a ra s itic w a y without in te r fe r in g with the p r im a r y use o f the m achine. G. S T Ô C K L IN : T h e 38S -38C l s y s te m m en tion ed by L in d n er would open the w ay to have 38C1+ a v a ila b le fo r study, and th is is p ertin en t in connection w ith the C acace kind of ex p e rim e n t. That is , th ey could be used to produce d eca y ion s f o r the study o f e le c tr o p h ilic substitution. A s you know, C acace has done th is f o r a lon g tim e w ith T 2 gas to m ake H e T +. T h is kind o f e x p e r i m ent has r e c e n tly been extended in oth er la b o r a to r ie s to studies of the re a c tio n s o f B r + by is o m e r ic tra n sitio n ; of I+ fr o m the e le c tr o n capture d ecay o f xenon. T h e s e e x p e rim e n ts could then be exten ded to C l+, and with the p r o p e r en viro n m en t — such as a noble gas w ith the p ro p e r io n iza tio n p o ten tia l — y o u 'll fin a lly end up with the +1 s p e c ie s , and can study the e le c tr o p h ilic g a s -p h a s e substitution. W h ile such re a c tio n s can a lso be stu died b y m ass s p e c tr o m e tr ic tech n iqu es, the n u clea r r e c o il technique adds a d d ition al in fo rm a tio n about the stru ctu re o f the products — ortho/ m eta / p a ra r a tio s , etc. T h e s e d eca y ion s a re r e a lly a new to o l f o r the o rg a n ic ch em ist to study e le c tr o p h ilic substitution. F .S. R O W L A N D : T h is qu estion is d ir e c te d to M a lc o lm e -L a w e s . H ave you m ade any e x te n s iv e s e a rc h f o r p o s s ib le in e rt, gaseou s, n itro g e n -
I A E A -P L -6 1 5 / 1
17
containing m o le c u le s w ith which 13N m ight not re a c t? I 'm not m aking any su ggestion s — just asking f o r in fo rm a tio n . D.J. M A L C O L M E -L A W E S ; I w ouldn't say that w e 'v e m ade an ex te n s iv e s e a rc h — we t r ie d e v e ry th in g we could fin d ly in g around in the la b o ra to ry , and th e re w as no in stan ce in which we did not fin d m o re than th r e e -q u a r te r s o f the induced 13N a c tiv ity in the fo r m of 13N 14N product. T h e o n ly tim e that w e found a r e a lly good y ie ld o f any oth er product w ith 13N w as when HCN w as i t s e lf the so u rce o f the 13N. In that ca se, w e did fin d about 30% la b e lle d HC13N as a product. A .G . M AD D O C K : Did you t r y N F 3? D.J. M A L C O L M E -L A W E S : N o. W e would have had a lot o f I8 p a c tiv ity in that case. N . S A ITO : I have a v e r y m in o r point f o r D r. L in d n er. In you r p ap er, you s a y that 252C f has a v e r y long h a lf- life , 2.65 y e a r s . I think that th is is r e a lly a ra th e r sh ort h a lf- life , and is a d efin ite disadvan tage fo r 252C f in use. Do you have any com m ent? F .S . R O W L A N D : T h e a n sw er depends on w h eth er you bought the 252Cf, o r w h eth er it w as g iv e n to you. T . SH IO K A W A : W e a re now p rodu cin g som e n e u tro n -d e fic ie n t n u clides w ith an e le c tr o n lin e a r a c c e le r a to r . T h e m ost s e rio u s p ro b le m is ra d ia tio n dam age fr o m u n con verted e le c tro n s . F o r this pu rp ose, we p la ced a sw eep in g m agnet just behind the c o n v e r te r , and now have a pu re, clean b rem sstra h lu n g beam . L . L IN D N E R : How m uch did it cost? T o do a good job , you r e a lly n eed a v e r y in tr ic a te m agnet. It could be done, but it is exp en sive. A .P . W O L F : It 's not e x p en sive. V a ria n C o rp o ra tio n builds th ese m agnets, and th e re a re m any lin e a r a c c e le r a to r s in the U SA in the lo w - e n e r g y ran ge — used fo r d e te rm in in g fau lts in ca stin gs — which use th is bending technique, and it is quite cheap. F o r som e pu rp oses, th ey cost le s s than $10 000. L . L IN D N E R : T h o s e m agn ets a re fo r sw eep in g out lo w - e n e r g y e le c tro n s . F o r h ig h -e n e r g y e le c tr o n s , you r e a lly need high in te n s ity photon beam s. A .P . W O L F : E ven with 60 to 70 M eV e le c tr o n s th ey a re n 't to o exp en sive. N . S A ITO : I have a qu estion f o r D r. A m ie l about the neutron so u rce m ade som e tim e ago in Is r a e l. If I r e m e m b e r c o r r e c t ly , you r p eop le m ade a s p e c ia l neutron so u rce w ith an a lp h a -e m itte r plus 180 . I should lik e to know w h eth er th e re is any new in fo rm a tio n about this. S. A M IE L : About 12 y e a r s ago, w e studied the re a c tio n o f alpha p a r tic le s w ith 180 — the (y , n) re a c tio n — and found that the neutron output w as about h a lf that found f o r the sam e re a c tio n w ith 9B e, the com m on neutron so u rce ta rg e t. T h e advantage o f this so u rce w as that one could keep it in his pocket, and stop the re a c tio n b y condensing the CO18© , and then s ta rt it again w ith out w o r r y in g about the neutron background. T h e alpha so u rce and the C 0 180 could be kept in sep a ra te a rm s o f the con tain er. It is a v e r y in ten se s o u rc e , but w e h a ven 't done anything c o m m e r c ia l about it, and have le ft it at that stage. J .P . A D L O F F : W as the lim ita tio n on its o p e ra tio n p la ced b y r a d io ly s is o f the carbon dioxid e? S. A M IE L : T h e r e w as no lim ita tio n to the so u rce. It w as c o v e r e d w ith a v e r y thin la y e r o f g o ld — 100 Mg o f gold.
I A E A -P L -6 1 5/2
HOT ATOM CHEMISTRY IN INORGANIC SOLIDS G. H A R B O T T L E Chemistry Department, Brookhaven National Laboratory, Upton, Long Island, N.Y., United States of America
Abstract HOT ATOM CHEMISTRY IN INORGANIC SOLIDS. This paper examines the present status and notes possible future directions in the hot atom chemistry of solids. This area of activity is briefly defined and the theoretical and experimental knowledge that at present exists, or is lacking, is discussed. Suggestions for future work are offered.
T h e study o f the c h e m is try o f hot ( i . e . b ein g tra n s la tio n a lly e n e rg e tic in a ra n g e ly in g ab ove the M a x w ellia n , o r p o s s e s s in g unusual, h igh ly r e a c t iv e ch a rge s ta te s ) atom s prod u ced by n u c le a r tra n s fo rm a tio n s goes back to the e a r ly days o f ra d io c h e m is try , w h ere it was a s s o c ia te d with nam es lik e F e r m i and S z ila r d . T h is p a p e r exa m in es the p re s e n t status, and notes p o s s ib le fu tu re d ire c tio n s , in the hot atom c h e m is tr y o f s o lid s . T h e w o rk o f this P a n e l should be v ie w e d not only in the con text o f the tw o p re v io u s A g e n c y S ym p osia in H ot A to m C h e m is try 1 but a ls o in the lin e o f continuing A g e n c y in te r e s t in R e s e a r c h R e a c to r U tiliz a tio n , as ty p ifie d by, fo r ex a m p le, a P a n e l in V ien n a2 and the IA E A Study Group m e e tin g in M an ila (17-20 M a rc h 1969). A lth ou gh th e re a r e m any w ays o f ge n e ra tin g p a r tic u la r hot a to m s that do not in v o lv e r e a c to r s (fo r exam p le, fa s t neutron o r proton beam s fr o m c y c lo tro n s , beta o r p o s itro n d eca ys le a d in g to r a d io a c tiv e daughters, e t c . ), in m any la b o r a to r ie s , e s p e c ia lly in d e v e lo p in g co u n tries, the r e a c to r cen tre re m a in s the focu s of p re s e n t and p oten tia l hot atom r e s e a r c h . In such a r e a s , the M a n ila Study Group p oin ted out, p ro g ra m m e s in H ot A to m C h e m is try could be ex p ected to inclu de to p ic s such as (1) (2) (3)
(4)
D evelo p m en t o f m ethods fo r the p re p a ra tio n o f c e rta in h igh ly en ric h e d ra d io is o to p e s and la b e lle d com pounds; A ccu m u la tio n o f p r a c tic a l data useful f o r actu al production; Fu ndam ental r e s e a r c h to understand m ech an ism o f hot atom re a c tio n s u sefu l f o r is o to p e produ ction and p re p a ra tio n o f la b e lle d com pounds; T r a in in g o f ra d io c h e m is ts .
1 IAEA, Chemical Effects of Nuclear Transformations-1960 (Proc. Symp. Prague, 1960) 1 , 2 and 3, IAEA, Vienna(1961); and Chemical Effects of Nuclear Transformations - 1964 (Proc. Symp., Vienna, 1964)1 and 2, IAEA, Vienna (1965). 2 IAEA, Chemistry Research and Chemical Techniques Based on Research Reactors ( Proc. Panel, Vienna, 1963), Tech. Rep. Ser.No. 17, IAEA, Vienna (1963).
19
20
H ARBOTTLE
T h e em p h asis in actu al hot atom r e s e a rc h , it w as fe lt in M anila, should be alon g the fo llo w in g lin es: (1)
D evelo p m en t o f new m ethods f o r the prod u ction o f c e rta in r a d io is o to p e s and la b e lle d com pounds; ( 2 ) L o w -te m p e r a tu r e studies and studies on an n ealin g p r o c e s s e s ; (3) R ela tio n sh ip b etw een p h y s ic o c h e m ic a l p r o p e r tie s and annealing beh aviou r; (4) Id e n tific a tio n o f ch arged s ta te s , ch e m ic a l s p e c ie s and ch em ica l fo r m s . It is in te r e s tin g that the p ro g ra m m e p ro p o sed by the Study Group s tro n g ly em p h a sized the p r a c tic a l ap p lica tion s o f hot atom c h e m is try , and the fundam ental r e s e a r c h n e c e s s a r y to p re p a re the b a sis f o r p ra c tic a l stu d ies. S e v e r a l y e a r s ago, as p a rt o f the IA E A p u b lication "R a d io is o to p e P ro d u c tio n and Q u a lity C o n tr o l" (T e c h . R ep . S e rie s N o. 128, 1971) the author (w ith M. H illm a n ) p re s e n te d a ch apter on "S z ila r d - C h a lm e r s P r o c e s s e s f o r Iso to p e P r o d u c tio n ". W hen a "h o t a to m " is g en era ted in a s o lid , by w h a tever m eans, i f it has an in itia l k in e tic e n e rg y g r e a te r than about 30-50 eV, it w ill on the a v e r a g e esca p e fr o m its o r ig in a l c r y s ta l la ttic e site, m ove o ff into the la ttic e , le a v in g behind a vacan cy, and lo s e e n e rg y through c o llis io n s until i t again co m es to r e s t. D u rin g this flig h t it can knock oth er atom s out of th e ir la ttic e s ite s , i f s u ffic ie n t in itia l e n e rg y is a v a ila b le . It can also, in c r y s ta ls m ade up o f m o le c u le s o r co m p lex ions, le a v e behind a t r a il o f r a d ic a ls , oth er m o le c u la r fra g m e n ts , o r lig a n d -d e fic ie n t c o m p le x e s . When the fa s t atom c o m e s to r e s t, if the c r y s ta l is an alysed c h e m ic a lly , o r exam in ed by n o n -c h e m ic a l (in -s itu ) techniques, it is found that the (r a d io a c tiv e ) r e c o il atom m ay be in the o r ig in a l, o r in an a lte re d , c h e m ic a l state: fo r exa m p le, in c r y s ta llin e p ota ssiu m ch rom a te (К 2С Г О 4) ir r a d ia te d with th e rm a l neutrons, the "h o t a to m " 51C r , when the c r y s ta l is d is s o lv e d and an alysed , m ay be found as C rV I, (ch ro m a te) o r in m o n o m e ric , d im e r ic o r p o ly m e r ic C r III fo r m s . But the s to r y does not end th e re . I f the К 2С Ю 4 c r y s ta l containing 51C r atom s is h eated ("t h e r m a lly a n n ea led ") b e fo r e a n a ly s is , i t w ill be found that even though no g ro s s c h e m ic a l change has o c c u rre d the r e la t iv e p e rc e n ta g e s o f a c tiv ity in the d iffe r e n t c h e m ic a l fo r m s m en tion ed e a r lie r w ill have a lte r e d in a c o m p le x p a ttern . T h e s e p e rc e n ta g e s , o r " y ie ld s " , in d iffe r e n t hot atom s y s te m s have a ls o been found to be a ffe c te d by the a p p lica tio n o f ligh t, p r e s s u r e , o r u ltra s o n ic e n e rg y , by the le v e ls o f tra c e im p u ritie s p resen t in the c r y s ta l, type o f a tm o sp h ere su rrou n din g the c r y s ta l du ring annealing, con dition s o f a n a ly s is , e tc. It is obvious that th e re is a broad fie ld fo r r e s e a r c h h e re , the v e r y c o m p le x ity o f the s y s te m s e r v in g as a ch allen ge to s c ie n tis ts . T o s im p lify m a tte rs w e m a y distin gu ish s e v e r a l lo g ic a l te m p o ra l s u b d ivision s in the study o f so lid state hot atom c h e m is try , as fo llo w s : (1)
(2)
T h e p e rio d o f tim e in itia te d by the n u c le a r even t le a d in g to the fo rm a tio n of the hot atom at the s ta rtin g point o f its c r y s ta l t r a je c t o r y . T h is is 10' 14 s o r le s s . T h e p e r io d of flig h t through the c r y s ta l it s e lf, until the atom is e s s e n tia lly at r e s t . T h is is about 10" 13 to 10' 12 s.
I A E A -P L -6 1 5/2
21
(3) A p e r io d o f tim e in clu d in g the fo r m e r but extending onw ards fr o m the stopping o f the fa s t atom , fo r about 10"6 to 10"7 seconds, during w hich the e le c tr o n ic and th e rm a l e x c ita tio n o f the hot atom and its su rrou n din g n eigh bou rs subside. (4) T h e lo n g - te r m p e rio d , fr o m a fe w seconds upw ards, du ring which la b o r a to r y e x p e rim e n ts in v o lv in g an nealing o r the a p p lica tio n o f o th e r s t r e s s e s , p h y s ic a l m ea su rem en ts o r c h e m ic a l a n a ly s is , m ay be ap p lied . U s in g this as a fr a m e w o r k , w e m ay now exam in e the th e o r e tic a l and e x p e rim e n ta l kn ow ledge which w e at p re s e n t p o s s e s s , d efin e what is lack in g, and how e x p e rim e n ta l and th e o r e tic a l r e s e a r c h can be brought to b e a r to f i l l in the gaps. T h e sam e n u m berin g used ab ove is fo llo w e d h e r e . (1) In the p e r io d around the n u clea r even t w e a re d ea lin g w ith n u clea r p h y s ic s . T h e e n e rg y o f the in c o m in g nucleon, the type and s p e c tr a l d is t r i bution and an gu lar c o r r e la tio n s o f the gam m a ra y s o r e m itted p a r tic le s such as e le c tr o n s , o th e r nucleons e t c . , d e te rm in e the e n e rg y and in itia l p h y s ic a l state o f the hot atom . The p ro b a b ility of in te rn a l c o n v e rs io n o f gam m a ra d ia tio n e m itte d d u rin g the n u clea r event is a datum o f g re a t im p o rta n c e , e s p e c ia lly sin ce the in te rn a l c o n v e rs io n p r o c e s s (a) lea d s to h ig h ly ch arged , and con sequ en tly v io le n tly r e a c t iv e atom s, and (b) m ay be d ela y ed to tim e s exten din g p ast the slo w in g-d o w n p e rio d â&#x20AC;&#x201D; in o th e r w o rd s, m a y constitu te a kind of "s e c o n d a c tiv a tio n ". G iv e n the re le v a n t n u c le a r data, th e re is no th e o r e tic a l p ro b le m in c a lc u la tin g the in itia l e n e rg y and ch arge d istrib u tio n s o f the hot a tom s. W hat is n eeded is m o re c o m p lete data on neutron captu re g a m m a -ra y s p e c tra , and on the d iffic u lt p ro b le m o f e s tim a tin g the p r o b a b ility and t im e - s c a le o f the con com itan t in te rn a l c o n v e rs io n even ts. T h e f i r s t o f th ese is under a c tiv e in v e s tig a tio n , w ith su bstan tial prod u ction of new data. T h e secon d p ro b le m has h a rd ly been touched. H e r e a th e o r e tic a l study, f o r exa m p le o f c h a rg e re la x a tio n in d iffe r e n t c r y s ta ls , is a ls o badly needed. (2) T h e b eh a viou r o f the e n e r g e tic hot atom , w h ile slo w in g down in a c r y s ta l, p r o p e r ly is a study in s o lid state p h y s ic s . Som e e x p e rim e n ts , u tiliz in g n u c le a r reson a n ce flu o re s c e n c e , have p e rm itte d us to exam in e the actu al k in em a tics o f d e c e le ra tio n : it is m y v ie w that such e x p e rim e n ts , w hich a re not p a r tic u la r ly d iffic u lt to p e r fo r m , could be m ade to r e v e a l much va lu a b le a d d ition al in fo rm a tio n on th is tim e p e rio d in the lif e o f the hot atom . M o s t r e a c to r c e n tre s have this ca p a b ility . On the th e o r e tic a l sid e, com pu ter studies have dom in ated. In a num ber o f e x c e lle n t p a p e rs , the in te ra c tio n s o f a hot atom , sta rtin g out in v a rio u s d ir e c tio n s , w ith its host c r y s ta l, have been sim u lated: the slo w in g-d ow n t r a je c t o r ie s , prod u ction o f "ch a n n elon s" and "fo c u s in g c o llis io n s " as w e ll as in t e r s t it ia l and va ca n cy s ite s have been studied in d e ta il. M any m o re such com p u ter "e x p e r im e n ts " a r e needed, e s p e c ia lly in m o re co m p le x c r y s ta ls in which m o le c u le s o r co m p le x ion s a re em bedded: the f ir s t attem p ts have r e c e n tly a p p eared , and o ffe r p r o m is e f o r the fu tu re (s e e B ib lio g ra p h y ). But such stu dies a lr e a d y r e q u ir e la r g e com p u ters: exten sion to m o re c o m p lex (and con sequ en tly m o re in te r e s tin g to the hot atom ch em ist) c r y s ta ls would dem and even m o re m e m o ry and h ig h e r speed. A ls o , the com p u ter m o d e llin g should be m ade to be m o re r e a lis t ic to the c h e m is try in v o lv e d .
22
H AR BOTTLE
Q u ite c le a r ly , in th is im p o rta n t tim e p e r io d th e re is g r e a t opportunity fo r new and in te r e s tin g w o rk in both th e o ry and e x p erim en t, in w hich hot atom c h e m is tr y and s o lid state p h ysics must each p la y Ă r o le . Such e ffo r ts h ave the g re a te s t chance o f su ccess if the p r o je c t is in te r d is c ip lin a r y , spanning the tw o s c ie n c e s . T h e re s u lts o f n u c le a r reson a n ce flu o re s c e n c e e x p e rim e n ts can s e r v e as a ch eck on the th e o r ie s . (3) T h e th ird tim e p e rio d , in v o lv in g the ju st-sto p p ed r e c o il atom su rrou n ded and trap p ed by the atom s o f the la ttic e with w hich it m ade its fin a l fe w e n e r g e tic c o llis io n s , b rin g s us s q u a re ly into the r e a lm o f c h e m is try . I f w e r e a liz e that the m ean f r e e path o f the e n e r g e tic atom is sh o rt â&#x20AC;&#x201D; in fact, on ly o f a to m ic s iz e s - then it fo llo w s that the end o f the tra c k m ark s a "d u m p in g" of a su bstantial amount o f e n e rg y , ra p id ly , in to a v e r y r e s t r ic t e d v o lu m e . T h e re s u lt is , o f c o u rs e , a m o m en ta ry p u lse o f high " te m p e r a tu r e ", the s o - c a lle d " th e r m a l s p ik e " of s o lid state p h y s ic s . W hen a d e la y e d in te rn a l c o n v e rs io n even t takes p la c e , with the m o m e n ta ry e je c tio n o f a num ber o f e le c tro n s fr o m the hot atom fo llo w e d by the im m e d ia te re la x a tio n o f the la r g e plus c h a rg e through e le c tr o n d ep letio n o f n eigh b ou rin g atom s, then a p o w e rfu l "s p ik e " o f both a th e rm a l and e le c tr o n ic n atu re m ust be fo r m e d . T h e r e has been lit t le o r no th e o re tic a l w o rk on the p r o p e r tie s o f th ese e le c tr o n -d e p le tio n sp ik es, fo r m e d as m en tion ed ab ove. T h e r e is much th e o r e tic a l d iscu ssio n on the p o s s ib le re s u lts o f the th e rm a l spike, both in s o lid state p h y sics, w h e re it r e la te s to d e fe c t p rod u ction , and in hot atom c h e m is try , w h ere in som e ca s e s it has been p ostu lated as an explan ation f o r the o b s e rv e d d istrib u tio n s o f y ie ld s . Q uite c le a r ly th e re is m o r e w o rk to be done: both c h e m ic a l and p h y s ic a l techniques need to be ap p lied . F o r ex a m p le, the t im e - r e s o lv e d M S ssbau er e ffe c t, e s p e c ia lly w ith s h o r t e r - liv e d s p e c ie s , needs to be r e -in v e s tig a te d , to shed lig h t on this re g io n . A n o th er valu ab le p h y s ic a l technique is t im e - d iffe r e n t ia l an gu lar c o r r e la tio n : both th ese sam p le the c h e m ic a l en viron m en t ra p id ly enough to a llo w us to exam in e s h o r t-te r m hot atom c h e m is try . But, as equipm ent is c o s tly and co m p lex , and e x p e rim e n ts a r e lon g, v e r y lit t le w o rk has been done to date. T h e p u re ly c h e m ic a l in v e s tig a tio n o f hot atom re a c tio n s in the "h o t zo n e " o r th e rm a l spike ought to continue, but w ith v e r y c a re fu l planning, so that the re s u lts can be m o r e d e c is iv e than th ose obtained in the past. A g a in , " e x p e r im e n ts " on the com pu ter, s im u la tin g even ts in c o m p le x -io n o r m o le  c u la r c r y s ta ls , w ould g iv e us a b e tte r id ea o f the s iz e , duration, and te m p e ra tu re h is to r y o f the hot zone o r spike. (4) T h e lo n g - t e r m in v e s tig a tio n o f the hot atom c h e m is try w hich fo llo w s the th e rm a l sp ik e, has accounted fo r m o s t o f the published lite r a tu r e . D is s o lu tio n o f ir r a d ia te d c r y s ta ls , c h e m ic a l sep a ra tio n s, m ea su rem en t of y ie ld s , and the o b s e rv a tio n o f an n ealin g e ffe c ts o f m any kinds has constitu ted the c la s s ic a l ap proach to r e c o il c h e m is try . T h e m a th em a tica l a n a ly s is o f an n ealin g c u rv e s (o f the is o c h ro n a l as w e ll as is o th e rm a l v a r ie t y ) has g iven us so m e in s ig h t in to the a c tiv a tio n e n e r g ie s , and p erh ap s the m ech an ism s, o f m any an n ealin g re a c tio n s . In -s itu m ethods such as p e rtu rb ed an gu lar c o r r e la tio n and (p erh a p s) h a lf- life v a r ia tio n m ay a ls o be a p p lica b le, although i t is not y e t c le a r how c lo s e ly th ese m ethods can id e n tify the c h e m is try o f the r e c o il atom .
I A E A - P L - 6 1 5/2
23
T h e r e is am p le opportu nity and need fo r a c lo s e r th e o r e tic a l exam in ation o f an n ealin g m ech a n ism s. I f a m ech an ism f o r o x id a tiv e an n ealin g has been postu lated , f o r exa m p le the reco m b in a tio n o f an oxygen atom w ith a red u ced fra g m e n t, o r the d iffu sio n o f a h ole on to a tra p p in g s ite , it ought to be p o s s ib le to m ake a t le a s t a rough calcu lation o f the a c tiv a tio n e n e rg y fr o m f i r s t p r in c ip le s , and co m p a re this w ith the e x p e rim e n t. T h e sam e ought to be tru e o f lig a n d -fra g m e n t reco m b in a tio n s. A n o th e r typ e o f e x p e rim e n t c a r r ie s out hot atom re a c tio n s in host c r y s t a ls d e lib e r a te ly "d o p e d " with, fo r in stan ce, lo w le v e ls o f fo r e ig n atom s w hich a r e e le c tr o n a c c e p to r s (tra p s ) - an exam p le w ould be s ilv e r ion s in a lk a li h alid e la ttic e s . O th er types o f doping can le a d to oth er la ttic e d e fe c ts â&#x20AC;&#x201D; an in te r e s tin g one is the a lio v a le n t dopant which r e q u ir e s cation o r anion v a c a n c ie s to m aintain c h a rg e b alan ce. In a ll th ese e x p e rim e n ts the c o lla b o r a  tion o f a s o lid state p h y s ic is t is v e r y d e s ir a b le . A n o th e r fie ld w hich shows p r o m is e o f y ie ld in g som e in fo rm a tio n on an n ealin g m ech a n ism s is the study o f " t r a n s fe r a n n ea lin g ", i. e . exchange re a c tio n s in the s o lid sta te. H e r e again, th e re is lit t le o r no background o f th e o ry a v a ila b le . In con clu sion , it can be said that through c a r e fu l planning o f e x p e rim e n ts , w o rk in the c la s s ic , p o s t- ir r a d ia tio n p e r io d o f hot atom c h e m is try can s t ill p rod u ce in te r e s tin g data and needs to be exten ded. P a r t ic u la r ly d e s ir a b le a r e s in g le - c r y s t a l in v e s tig a tio n s in which th e rm a l o r photo annealing a re c a r r ie d out with c a r e fu l te m p e ra tu re o r w a velen gth c o n tro l so as to r e s o lv e m o s t c le a r ly the p a r tic u la r a b so rp tio n peaks o r a c tiv a tio n e n e rg y groups that m ay be p re s e n t. A v e r y p o w e rfu l com bin ation is the c o lla b o r a tiv e r e s e a r c h betw een a hot atom c h e m is t and a s o lid state p h y s ic is t who is equipped to m ake c o lo r - c e n t r e , th erm o lu m in e s c e n c e , o r ESR m ea su rem en ts. In this w a y it is s o m e tim e s p o s s ib le to id e n tify quite d ir e c t ly the m ech an ism s in v o lv e d in hot atom an n ealin g and to v e r i f y the th e o ry . A lth ou gh the c la s s ic e r a o f hot atom c h e m is try is c le a r ly behind us, it d oes s e e m p o s s ib le that th ese in v e s tig a tio n s could p ro v id e a new point o f d e p a rtu re into the study o f a fa r m o r e c o m p le x ran ge of c r y s ta l d e fe c t s tru c tu re s than those at p re s e n t known. A n d it is s u re ly tru e that, ju st as th ese m ea su rem en ts can help us b e tte r understand the s ta tic s o f c r y s ta l d e fe c t stru c tu re s , the p h y s ic a l tech n iqu es o f t im e - d iffe r e n t ia l an gu lar c o r r e la tio n , n u c le a r reso n a n ce flu o r e s c e n c e and p erh ap s M ftssbau er e ffe c t can shape ou r understanding o f the dyn am ics o f th e ir p rodu ction .
BIBLIOGRAPHY INTERNATIONAL ATOMIC ENERGY AGENCY, Chemical Effects of Nuclear Transformations-1960 (Proc. Symp. Prague. 1960) 1 , 2 and 3, IAEA, Vienna (1961); and Chemical Effects of Nuclear Transformations-1964 (Proc. Symp. Vienna, 1964) 1 and 2, IAEA, Vienna (1965). MTPInt. Rev. of Science, Radiochemistry (MADDOCK, A.G., Ed.) 8, Butterworths, London(1972). Radiochemistry 1., The Chemical Society, London(1972). NELSON, R. S., Observation of Atomic Collisions in Crystalline Solids 1, North-Holland Publishing Co., Amsterdam (1968). Atomic Collision Phenomena in Solids (PALMER, D.W ., THOMPSON, M.W., TOWNSEND, P.D., Eds), North-Holland Publishing Co., Amsterdam; and American Elsevier, New York (1970).
24
H ARBOTTLE
TORRENS, I . М., CHADDERTON, L .T .. Phys. Rev. 159 (1967) 671. The Interaction of Radiation with Solids. Interscience Publishers, New York (1964). BARTHOLOMEW, G.A., GROSHEV, L.V., Nuclear Data ЗА, (1967) 367; Nuclear Data 5A (1968) 1; Nuclear Data 5A (1968); Nuclear Data 5A (1968) 243. ADLOFF, J.P., Radiochim. Acta _15 (1971) 135. SEITZ, F., KOEHLER, J.S., Solid State Physics, 3, Academic Press, New York (1956) 307. TRIFTSHAEUSER, W., CRAIG. P.P., Phys. Rev. 162 (1967) 274. PERLOW, G.J., PERLOW, M.R., Chemical Effects of Nuclear Transformations-1964 (Proc. Symp. Vienna, 1964) 2, IAEA, Vienna (1965) 443. ROSSLER, K ., ROBINSON, M .T., Proc. Vint. Conf. Atomic Collisions in Solids, Gatlinburg Sept. 24-28, 1973. In press. ROBINSON, M .T., ROSSLER, K., TORRENS, I . М.. J. Chem. Phys. 60(1974) 680.
DISCUSSION G. H A R B O T T L E : T h e future o f s o lid -s ta te hot atom c h e m is tr y lie s in c lo s e c o lla b o ra tio n w ith s o lid state p h y s ic s . It can g iv e e m p ir ic a l in fo rm a tio n that s o lid state p h ysics o r d in a r ily does not m ake a v a ila b le . I f the s o lid state p h y s ic is t does not want to attem pt ab in itio ca lcu la tion s beyond K C I, you can c e r ta in ly see why he is n 't v e r y in te re s te d in m aking ca lcu la tio n s f o r K 2I r C l 6. N e v e r th e le s s , you can o b s e r v e the ra d ic a ls that a r e p re s e n t through ESR, and o b s e r v e the lig a n d -d e fic ie n t s p e c ie s that a re p re s e n t, and o b s e r v e the reco m b in a tio n s even i f you cannot ca lcu la te them . Y ou can a ls o d e te rm in e the g e o m e try o f the d iffe r e n t s p e c ie s that m ay be p re s e n t. T h is is the b a sic in te r d is c ip lin a r y nature o f hot atom c h e m is try — s o lid state p h y s ic s , n u clea r ph ysics and hot atom c h e m is try a ll com e to g e th e r. T h e r e is a future to s o lid -s ta te hot atom c h e m is try . But the past is past, and we need to have new w ays o f thinking about hot atom c h e m is try . M . N E W T O N : Y ou have m entioned that, given the n u clea r data, you can at le a s t ca lcu la te the in itia l ch a rge state. T h e ca lcu la tion of a ch arge state at e q u ilib riu m fo r an atom p a ssin g through a m a te r ia l is , o f cou rse, a v e r y d iffe r e n t ca lcu la tio n . A s w e know, th e re a r e v a rio u s ru le s o f thumb that a r e used in gas phase hot a tom c h e m is tr y - ru le s o f thumb fo r e le c tr o n ic s ta te s . Som e o f th ese ru le s o f thumb a r e now b ein g taken as a x io m s , and not in v e s tig a te d fu rth e r. H o w e v e r, w e have an in d ica tion that the protons ch a rg in g through p o ly a to m ic g a ses re m a in as b a re p roton s fo r much lo n g e r than the c la s s ic a l w o rk o f A llis o n would in d ic a te . T h is co m es fr o m w o rk in which w e have been c a lc u la tin g the ch arge states o f 5 0 -k eV p roton s fro m the e le c tr o n ic stopping p o w e rs . In R o s s l e r ’ s study o f the h ex a ch lo ro rh en a tes, what did he assu m e about the ch a rge state o f the ch lo rin e? D id he ju st assum e Cl" ? G . S T O C K L IN : H e assu m ed that the ch a rge state w as -1 . It is not r e a lly im p o rta n t at high e n e r g ie s what the ch a rge state is . L . L IN D N E R : I have trou b le v is u a liz in g a ch a rged s p e c ie s m o vin g in a s o lid la ttic e . W hat does one m ean by a ch a rged s p e c ie s m o v in g in a dense la ttic e f ille d w ith ligan d s?
I A E A -P L -6 1 5/2
25
G . H A R B O T T L E : T h e r e is no qu estion that the re la x a tio n o f the v e r y high in itia l c h a rg e s tak es p la c e v e r y ra p id ly . H o w e v e r, when a b ro m in e atom ch a rg in g up in an A u g e r p r o c e s s gets up to +6 o r +8, then a v e r y in te r e s tin g kind o f p la sm a ex c ita tio n tak es p la c e , as e le c tro n s flo w in fr o m the su rrou n d in gs. I would lik e to s e e a th e o re tic ia n in v e s tig a te this in te rn a l c o n v e rs io n in the s o lid sta te. A t p re s e n t it is a g la rin g d e fe c t in hot atom th e o ry . A . G. M A D D O C K : I would lik e to extend the lis t o f in fo rm a tio n w hich w e w ould lik e to obtain fr o m the th e o re tic ia n . F ir s t , th e re a r e the e x p e rim e n ts by Y o s h ih a ra on (т, t ') e x cita tio n . T h e s e e x p e rim e n ts g iv e in fo rm a tio n about the m in im u m e n e rg y n e c e s s a r y to p rod u ce e ffe c t iv e fra g m e n ta tio n on a m o le c u le in the s o lid . B y e ffe c t iv e fra g m en ta tio n , I m ean p rod u cin g a r e s u lt on the m o le c u le which is la t e r r a d io c h e m ic a lly s e p a ra b le . T h e s e e x p e rim e n ts gave a va lu e o f 50 to 60 eV , w hich is som ew h at h ig h e r than the th e o r e tic a l p re d ic tio n s . H e re w e need a d d ition a l th e o r e tic a l c o n sid era tio n , as w e ll as m o re e x p e rim e n ts . Second, th e re a r e the e x p e rim e n ts on qu asi-h om ogen eou s s o lid -s ta te exchange. T h e r e is now p len ty o f ev id e n c e in d ica tin g that the p o s t- ir r a d ia tio n e ffe c ts a r e in the nature o f s o lid -s ta te exch an ge r e a c tio n s . In stead o f s ta rtin g o ff w ith the c o m p le x s y s te m p rodu ced by ra d ia tio n , why d on 't w e go back and lo o k at q u a si-h om og en eou s so lid state exchange in the s im p le s t p o s s ib le s y s te m . F o r exa m p le, exchange in the com pound Т Г Т 1 С Ц , R e c o il s y stem s have su ggested that exch an ge should e x is t in th is s y s te m . L e t us go back and lo o k at the s im p le re a c tio n s , and then p r o g r e s s iv e ly in trod u ce the c o m p lic a tio n s . A g a in , th e re is som e ro o m fo r m o re o r le s s th e o r e tic a l w o rk on the r o le s of d e fe c t through e le c tr o n ic m ech a n ism s, v a c a n c ie s , e x cito n m ech a n ism s, e tc. T h ir d , w e have the k in e tic s o f fa s t p r o c e s s e s . T h e r e is a g re a t deal o f d iffe r e n c e betw een c a r r y in g out a k in etic a n a ly s is w hich p resu p p o ses a c e r ta in m ech an ism , and is c a r r ie d out w ith the intent o f obtaining a d is trib u tio n o f a c tiv a tio n e n e r g ie s , and p ro v in g that th is is r e a lly the m ech a n ism w hich is in v o lv e d . W hy is it that i f you a n a lyse re s u lts w ith the V a n n -P r im a k p ro c e d u re fo r d e te rm in in g a c tiv a tio n e n e r g ie s , you n o r m a lly s e e m to find a ra th e r n a rro w sp ectru m that is on ly a lit t le b ro a d e r than the re s o lu tio n o f the p r o c e s s ? I have not seen a p la u sib le explan ation f o r this n a rro w sp ectru m o f a c tiv a tio n e n e r g ie s . F o u rth , why a re th e re so m any e le c tro n s in s o lid s ? T h e r e is in v a ria b ly an a lm o s t in d e fin ite supply o f e le c tr o n s in r e c o il e x p e rim e n ts , w h ile at the sam e tim e th e re is a c o m p a r a tiv e ly s m a ll supply o f h o le s . T h is s eem s to m e to p ro p o s e c o n s id e ra b le th e o r e tic a l d iffic u lty . N , G E T O F F : T h e r e a r e p o s s ib le advan tages o f studying the s p e c ie s prod u ced in s o lid s a fte r neutron a c tiv a tio n by o b s e r v in g the d ea ctiva tio n o f th ese s p e c ie s by flu o r e s c e n c e m eth ods. T h e s e re s u lts m igh t be obtained e ith e r fr o m the ir r a d ia te d s o lid it s e lf, o r fr o m the solu tion fo r m e d by d isso lu tio n o f th ese c r y s ta ls , o r fr o m fr e e ra d ic a ls fo r m e d in th ese solutions by subsequent r e a c tio n s . In our stu dies, the quantum y ie ld o f flu o re s c e n c e changes as a d d ition al e le c tro n s a r e bein g fo rm e d . T h e e n e rg y can m ove through s e v e r a l m o le c u la r groups and can fin a lly cause e m is s io n o f e le c tro n s fr o m OH o r N H 2 substituents. G. S T O C K L IN : Y ou a r e r e fe r r in g to o rg a n ic c r y s ta ls , and that is quite a d iffe r e n t situ ation fr o m in o rg a n ic c r y s ta ls , In addition , as fa r as the
26
H AR BOTTLE
n u c le a r r e c o il a c tiv a te d s p e c ie s a r e con cern ed , you a r e goin g to have d iffic u lty using th ese p h y s ic a l tech n iqu es f o r d etection . I think the s o lid -s ta te exchange stu dies o ffe r an e x tr e m e ly im p orta n t a r e a . W e should lo o k not only f o r the p r im a r y re a c tio n s w ith in -s itu m ethods, but w e should a ls o c o n s id e r the r e c o il o r im p lan tation m ethods as m eth ods fo r in trod u cin g r e a c tiv e s p e c ie s in to s o lid s , and then c o n s id e r the re a c tio n s which fo llo w fr o m the v ie w p o in t of th e rm a l s o lid state c h e m is try . Just fo r g e t about the p r im a r y even ts, and study the subsequent re a c tio n s . N . G E T O F F : W e a r e a ls o studying m ix ed com pounds such as sodium p henyl phosphate - w ith s p e c ia l m o d el su bstances to get s p e c ia l e ffe c ts . G. S T Ă&#x201D; C K L IN : F o r such e x p e rim e n ts , h o w e v e r, you do not need n u c le a r m ethods. A . G. M A D D O C K : W e have published e x p e rim e n ts on e n e rg y tr a n s fe r in m ix e d c r y s ta ls o f alu m ino and fe r r io x a la t e . H o w e v e r, it is n e c e s s a r y f o r us to know a g r e a t d ea l m o r e about e n e rg y tr a n s fe r p r o c e s s e s in o r d e r to understand p r o c e s s e s such as ra d ia tio n annealing. F . S . R O W L A N D : I want to r a is e a qu estion about H a r b o ttle 's a d vo ca cy o f c o m p le x ity as an advantage in these e x p e rim e n ts . It is p o s s ib le to c a r r y out v e r y co m p lic a te d e x p e rim e n ts w hich a r e h ard to understand, and which lea d to a lis t o f things n eeded to be understood in o r d e r to understand the e n tire e x p e rim e n t. It is a ls o p o s s ib le that the e x p e rim e n t it s e lf is a v e r y p o o r w a y to obtain in fo rm a tio n about any o f th ese in d ivid u a l step s. T o w hich o f th ese qu estion s a r e n u clea r s o lid -s ta te e x p e rim e n ts the m ethod o f c h o ice f o r obtaining in fo rm a tio n ? A r e th ese e x p e rim e n ts in stead ju st a s o u rce f o r p rod u cin g a v e r y co m p le x s y s te m w hich w ill even tu a lly be u n derstood on ly through s im p le e x p e rim e n ts c a r r ie d out by oth er tech n iqu es? G . H A R B O T T L E : If I understand the question, you a r e ask in g how should w e go about understanding the qu estion o f the stru ctu re o f d e fe c ts in c o m p le x s o lid s . F . S . R O W L A N D : A w h ole s e t o f p ro b le m s has been r a is e d under the c la s s ific a tio n o f p r o c e s s e s which you w ould lik e the th e o r is t to explain . T h e qu estion is s im p ly to what exten t a r e the n u clea r m ethods o f hot s o lid -s ta te , h o t-a to m c h e m is tr y lik e ly to p ro v id e cle a n -c u t in fo rm a tio n fo r the th e o r is t w o rk in g on th ese p ro b le m s ? G . H A R B O T T L E : I think that one can get in fo rm a tio n fr o m s o lid -s ta te hot atom c h e m is try that w ill b e a r on the a n s w e rs to th ese qu estion s, but w hich w ill not a n s w e r them e n tir e ly . I think that w e w ill need p a r a lle l o r c o n trib u to ry in fo rm a tio n . F o r in stan ce, a spin reson a n ce study w ill often id e n tify u n eq u ivo ca lly the stru ctu re o f the s p e c ie s w hich is re s o n a tin g in the apparatu s. O f c o u rs e , this is not n e c e s s a r ily the sam e s p e c ie s which is g e n era ted in a hot atom c h e m is try e x p e rim e n t. In m any c a s e s , one can guess that th e re w ill be a p a r a lle lis m betw een the p rod u cts fo r m e d in hot atom re a c tio n s and th ose fo r m e d by ra d ia tio n c h e m is tr y . In som e ca ses, when you c a r r y out an n ealin g re a c tio n s , and you s e e the d isa p p ea ra n ce o f a c e rta in s p e c ie s as m ea su red ra d io c h e m ic a lly , and at the sam e tim e the d isa p p ea ra n ce o f a r a d ic a l as m ea su red by E SR , you can have som e hope that th ese p r o c e s s e s a r e r e la te d , and that you have id e n tifie d the s p e c ie s w hich is b e in g ann ealed. In the c o m p le x c r y s ta ls , th e re is no hope f o r m akin g ab in itio c a lc u la  tion s, and even in KC1 the p h y s ic is ts a r e a lm o s t at the end o f th e ir te th e r,
I A E A -P L -6 1 5/2
27
and y e t th ey cannot c a lc u la te f o r anything m o re d iffic u lt than an F c en tre. It is u n re a lis tic to th row out m o le c u le s , such as the c h lo ro rh e n a te s o r som e o f the c o m p le x o rg a n ic m o le c u le s , as ex a m p les f o r w hich th e o r e tic a l ca lcu la tio n s a r e d e s ir e d . A . G. M A D D O C K : L e t m e g iv e an e x a m p le o f a situ ation in which ex p e rim e n ts in v e r y co m p le x s y s te m s can, i f p r o p e r ly chosen, be v e r y p r o fita b le , b y d ra w in g attention to A m a r N a th 's d e fin itiv e stu dies d em on  s tra tin g s o lid -s ta te exch an ge w ith a cob alt co m p le x fr o m w hich any th e o re tic ia n w ould r e c o il with h o r r o r . F . S . R O W L A N D : O f co u rse th e re a re som e c o m p le x e x p e rim e n ts w hich a r e in te r e s tin g . But th ere a r e a ls o so m e c o m p lex e x p e rim e n ts w hich a r e on ly c o m p lex . A . G. M A D D O C K : T h e Nath e x p e rim e n ts w e r e in te re s tin g , and a ls o le d to a d e fin ite con clu sion . F . S . R O W L A N D : T h a t is b a s ic a lly m y point. Can w e say that n u clea r m ethods a r e lik e ly to be the b est m ethods fo r in v e s tig a tin g the kinds of qu estion s which have e a r lie r been su ggested as a p p ro p ria te fo r th e o r e tic a l trea tm en t? In s p e c ific in sta n ces, I am su re that they a r e . But one im p r e s s io n w hich I r e c e iv e d fr o m H a r b o ttle 's p a p e r w as that the s o lid -s ta te in o rg a n ic s y s te m s can be g e n e r a lly d iv id e d in to even ts o c c u r r in g in fo u r d iffe r e n t tim e zon es, on ly one o f w hich can be ap p roach ed at a ll through the usual kind o f s o lid state r a d io c h e m is tr y e x p e rim e n t. In the o th e r th re e tim e zon es, f o r w hich H a rb o ttle r a is e d a lo t o f in te r e s tin g s c ie n tific qu estion s, it sounded as though you a r e goin g to have to c a r r y out som e o th e r kinds o f e x p e rim e n ts . G . H A R B O T T L E : I did not intend to g iv e that im p r e s s io n e n tir e ly . C e r ta in ly M Q ssbauer and P e r tu r b e d A n g u la r C o r r e la tio n tech n iqu es a re v e r y u sefu l in id e n tify in g s p e c ie s - they p ro v id e c h e m ic a l in fo rm a tio n about the ex a ct state o f a ffa ir s w ith in the c r y s ta l. T h e s e m ethods p ro v id e som e o f ou r b est in -s itu in fo rm a tio n . T h e s e c o m p lex s o lid s y s te m s c r y out fo r som e kind o f atten tion and understanding, and it m ust be la r g e ly e m p ir ic a l, I ' m a fra id . F . S . R O W L A N D : One w ould at le a s t hope that if you in v e s tig a te one c o m p le x s y s te m that it w ould p ro v id e in fo rm a tio n u sefu l f o r the study of the n ext system , in stead o f fo r c in g you to s ta rt a ll o v e r again w ith the next in v e s tig a tio n . M . N E W T O N : A lth ou gh the h ex a ch lo ro rh en a te s y s te m is a p p a ren tly com p lex, it w as tre a te d w ith a s im p le d y n a m ica l k in e tic m o d el which accounted f o r the p rod u cts. In a sen se, it w as not a co m p le x s y s te m a ft e r a ll. G. H A R B O T T L E : T h e s im p lic ity o f the R o s s le r -R o b in s o n s y s te m is d e c e p tiv e . T o get the co m p u ter p r o g r a m to run at a ll, th ey had to in trod u ce so m e r e a lly d ra s tic assu m ption s w hich R o s s le r is the f i r s t to a d m it w e r e not r e a lis t ic - fo r in stan ce, the assu m ption that the captu re radiu s w as a constant. T h a t is , the d ista n ce w ithin w hich you in e v ita b ly have recom b in a tion , and ou tside o f w hich it n e v e r o c c u rs . E v e r y o n e knows that this captu re radiu s depends on g e o m e try , on te m p e ra tu re , on the p re s e n c e o f n ea rb y groups, on how p e r fe c t the c r y s ta l is at that p a r tic u la r point. T h e assu m ption o f a constant va lu e w as a w ild s im p lific a tio n , but it w as n e c e s s a r y in o r d e r fo r the s y s te m to run on the com p u ter. G. S T O C K L IN : I think N ew ton w as ta lk in g not so much about the com p u ter sim u la tio n w hich contains d ra s tic assu m ption s, but ra th e r about the e a r lie r
28
H ARBOTTLE
c h e m ic a l e x p e rim e n ts in w hich the sam e statem en ts now obtained fr o m the com p u ter sim u la tion w e r e m ade fr o m the ra d io c h e m ic a l a n a ly sis plus a s im p le k in e tic in te rp re ta tio n . The s im p lic ity o f th is p a rtic u la r s y s te m c h e m ic a lly a r is e s fr o m the fa c t that w e used ligan d r e c o il, and not c e n tra l atom r e c o il. W ith ligan d r e c o il, you have on ly tw o p rodu cts, Cl" and the la b e lle d p a ren t compound. In addition, th is s y s te m happens to be e x tr e m e ly in s e n s itiv e to ra d ia tio n dam age. R e fe r r in g back to R o w la n d 's question - i f you have a c r y s ta l such as KC1 it is d iffic u lt to fin d any ch e m ic a l e ffe c t at a ll ex cep t the F c e n tre s . C h em ists a re a lw a ys d ea lin g with co m p le x com pounds, w ith co o rd in a tion com pounds, and, so fa r , the c la s s ic a l co o rd in a tion c h e m is tr y has not been able to produ ce lig a n d -d e fic ie n t s p e c ie s . F o r this re a s o n you a r e r e a lly able to get, fr o m s o lid state r e c o il o r im p lan tation c h e m is try , in fo rm a tio n about co ord in a tion c h e m is tr y w hich is not a v a ila b le in any oth er w ay. T h is , o f co u rse, r e q u ire s the a p p lica tio n o f p h y s ic a l m ethods o f a n a ly s is such as s p e c tro s c o p y . L . L IN D N E R : S o m etim es you need a c o m p lex s y s te m in o r d e r to do a s im p le e x p e rim e n t. T h e r o le o f k in etic e n e rg y is unsettled in som e of th ese s y s te m s . W e can a g r e e that in m o s t s y s te m s the fin a l re s u lt is independent o f the n u c le a r p r o c e s s used to s ta rt it - that the fin a l produ ct shows no m e m o r y o f how the atom s ta rte d out, and that the e ffe c t of the in itia l n u clea r r e c o il e n e rg y is not im p ortan t, th e r e fo r e , on the fin a l re a c tio n s . H o w e v e r, one can think of e x p e rim e n ts b elo w the b o r d e r lin e of 6 0 -eV in itia l e n e rg y . One needs to in c o rp o ra te a c e n tra l atom , p ro v id e it with v e r y lit t le r e c o il e n e rg y , and see w h eth er o r not it b rea k s lo o s e fr o m its c h e m ic a l su rrou n din gs. T h e obvious e x p e rim e n ta l case is lo w - e n e r g y b eta d eca y. S till, one has the p ro b le m o f a n o n -a d ia b a tic p r o c e s s w hich distu rbs the e le c tr o n cloud. A . G. M A D D O C K : In the g a m m a -g a m m a e x p e rim e n ts , the r e c o il points extended fr o m about 15 to 20 eV , up to about 150 eV . T h e r e w e re th re e o r fo u r poin ts w e ll b elo w 50 eV , a steep r is e betw een 50 and 60, and then constant again ab ove 60 e V . T h a t is why I say it is e x tr e m e ly im p orta n t that th ese e x p e rim e n ts be rep ea ted . G. H A R B O T T L E : Joseph D em a and I have s ta rte d an e x p e rim e n t at B rook h aven w hich a llo w s the d ep osition o f a p r e c is e ly known in c re m e n t of r e c o il e n e rg y into a r e c o il atom - then see what happens. T h is uses a v e r y n ic e fa c ilit y at the B rookh aven h ig h -flu x beam r e a c to r â&#x20AC;&#x201D; a 2 5-kV neutron fa c ilit y , in w hich a beam o f h igh ly filt e r e d , v e r y pure 2 5 -k eV neutrons is a v a ila b le . W e ex p ose the sa m p le in a cadm iu m w rap, o b s e rv in g ju st the fa s t neutron captu re w hich in trod u ces the a d d ition al m om entum o f the fa s t neutron on to the r e c o il atom . You can en v is a g e doing this e x p e rim e n t w ith b ea m s o f o th e r e n e r g ie s as w e ll. T h e e x p e rim e n t is fe a s ib le , w ith enough r a d io a c tiv ity to m ea su re, and w e have exa m in ed K M n 0 4 and a fe w o th e r old standbys as ta r g e ts . So fa r , w e have o b s e rv e d no e ffe c t on the r e c o il c h e m is try , but ad d itio n a l w o rk is s t ill n e c e s s a ry . T h e in c re m e n t o f e n e rg y d ep o sited am ounts to 200 to 300 eV . A . G. M A D D O C K : T h is kind of e x p e rim e n t is lim ite d o f c o u rs e to e n e r g ie s ly in g above the n atu ral (n, y) r e c o il e n e rg y . T h e d iffic u lty w ith the Y o s h ih a ra e x p e rim e n ts is that the e n e rg y re q u ire d s e e m s to be so high co m p a re d w ith ex p ecta tion s - about 50 to 60 eV . L . L IN D N E R : T h e H a rb o ttle -D e m a e x p e rim e n ts a re at too high an e n e rg y to s e ttle this kind o f qu estion . W hat is r e a lly needed is a m ethod fo r s ta rtin g out w ith s t ill lo w e r r e c o il e n e r g ie s .
IA E A -P L -6 1 5 / 2
29
F . S . R O W L A N D : F r o m the standpoint o f the Y o s h ih a ra e x p e rim e n ts , the H a rb o ttle -D e m a e x p e rim e n ts s ta rt out high in e n e rg y and go s t ill h ig h e r, and a r e not lik e ly to fu rn ish much in fo rm a tio n about that p a r tic u la r p ro b le m . O f c o u rse, som e oth er in fo rm a tio n could com e out. G. S T O C K L IN : I understand fr o m m y c o n v e rs a tio n s w ith s o lid -s ta te th e o ris ts that the d is p la c e m e n t th resh o ld in in o rg a n ic c r y s ta ls in g e n e ra l w ould be an in te r e s tin g nu m ber to have a v a ila b le . T h e r e a r e now a couple o f b ea m m ach in es around. Could th ese m ach in es be used to shoot n eu tral a tom s — fo r exam p le, c h lo rin e - on to h ex a ch lo ro rh en a te fr o m a fe w e le c tr o n v o lts to a fe w hundred? I am a w a re of som e o f the d iffic u ltie s - the range o f the b eam is n e g lig ib ly s m a ll, and r a d io a c tiv e atom s m ust be used in o r d e r to o b s e r v e any re s u lts . W ould this be a u sefu l e x p erim en t? J. D A N O N : I think that w e should m ake a d istin ctio n betw een co m p lex e x p e rim e n ts and c o m p le x en viro n m en ts. In M o s s b a u e r e x p e rim e n ts , much in fo rm a tio n can be obtain ed fr o m m a tr ix is o la tio n m eth ods. W ith spin reso n a n ce, too, w e have w o rk ed in co m p le x c h e m ic a l s y s te m s such as ru bidiu m cyan ide dilu ted in p ota ssiu m o r sodium c h lo rid e . W e should d istin gu ish betw een c o m p lex c h e m ic a ls in s im p le en viro n m en ts and c o m p lex s y s te m s . J. P . A D L O F F : W ith r e s p e c t to S to c k lin 's question, the va lu e of the th resh o ld e n e rg y fo r d is p la c e m e n t w ill depend on the d ir e c tio n o f o rie n ta tio n o f the ta r g e t s o lid to the in com in g b eam . Then, how w ill you m ake the e x p e rim e n ta l m ea su rem en t of the ou tcom e o f the ex p e rim e n t? S om e in te re s tin g e x p e rim e n ts have been c a r r ie d out in M e x ic o using th e ir M a rk I I I T R IG A r e a c to r w hich can be pu lsed. W e have m ea su red the reten tio n in K IO 3 f o r (a) a sa m p le ir r a d ia te d in the n o rm a l flu x; (b) in a sa m p le w h ich has been pulsed; and (c) in a sa m p le w hich has been ir r a d ia te d in a tru e th e rm a l neutron beam e x tra c te d fr o m the r e a c to r . A l l th ree re s u lts w e r e a p p ro x im a tp ly the sam e. T h e sam e kind o f e x p e rim e n t has a ls o been c a r r ie d out w ith an o rg a n ic system , eth yl io d id e. A g a in , the re s u lts w e r e s im ila r : the reten tio n w as independent o f the m ethod o f irr a d ia tio n . N . S A IT O : T h e r e is a lw a y s a b ig p ro b le m in s o lid -s ta te h o t-a to m c h e m is tr y b ecau se the sa m p le s m ust be d is s o lv e d b e fo r e the c h e m ic a l p r o c e s s in g can be c a r r ie d out. D o you think that w e t c h e m is tr y is u sefu l in p ro v id in g in fo rm a tio n about the m e ta sta b le o r unstable s p e c ie s in s o lid s? F . S . R O W L A N D : L e t me expand on P r o f e s s o r S a ito 's qu estion b e fo r e you a n sw er it. T h e r e is a sen ten ce in H a r b o ttle 's p a p er that says, "A lth ou gh the c la s s ic e ra o f hot atom c h e m is tr y is c le a r ly behind us. . . " L o o k in g back into th is c la s s ic e ra , what do w e see that the p re s e n t and fu tu re hot atom ch em ists should no lo n g e r do? G. H A R B O T T L E : I think that what is behind us is the is o la te d e x p e r i m ent. T h e p e rs o n who goes into the la b o ra to ry , does a p a r tic u la r hot atom e x p e rim e n t, and w r it e s a p a p er about it. W hat lie s in the fu tu re is that the e x p e rim e n t w ill be done in c o lla b o ra tio n w ith studies m ade by o th e r techniques — w ith M ô ssb a u er; w ith P e r tu r b e d A n g u la r C o r r e la tio n ; with c o lo u r ce n tre re s e a r c h ; with th e rm o lu m in e s c e n c e . O r by any of the o th er w ays of gettin g in fo rm a tio n about the tr a n s ito r y s p e c ie s — som e a d d ition al in fo rm a tio n beyond that a v a ila b le fr o m w et c h e m is tr y alon e. T h e r e a r e to o m any lo o p h o le s — too m any p o s s ib le in te rp re ta tio n s o f the sam e p ie c e s o f in fo rm a tio n - fr o m w et c h e m is tr y by it s e lf. T h is is the w a y the fu tu re lie s and I think w e have b een s e e in g it e v e r sin ce the Vienna m eetin g in 1964.
30
H AR BOTTLE
L . L IN D N E R : One p o s s ib le a n sw er is to do s o lid -s ta te pulsed r a d io  ly s is , but that w ill be d iffic u lt becau se o f the amount o f heat w hich m ust be d is s ip a te d w ithin the c r y s ta ls . The w a velen gth of lig h t you a r e in te re s te d in d e te c tin g ju s t does not com e out o f the c r y s ta l e ith e r - it gets ab sorb ed by it. A . G. M A D D O C K : Y o u r ir r a d ia te d s o lid is a n ice lit t le box in which m any things, although not a ll, a r e p r e s e r v e d . T h e point that M r. G e to ff w as m akin g e a r l i e r w as that in the in te rp re ta tio n o f a lo t o f hot atom e x p e rim e n ts , one needs a ls o to understand what e n titie s a re p r e s e r v e d in this box in te r m s o f the m a c ro s c o p ic constitu ents o f the o r ig in a l la ttic e . G. H A R B O T T L E : I would lik e to m ake a com m en t on the unfortunate d ich o to m y in hot atom c h e m is tr y betw een o rg a n ic and in o rg a n ic s y s te m s . S om etim e ago, one o f the s c ie n tis ts in A l W o lf's la b o ra to ry in v e s tig a te d the p o s s ib ility o f an n ealin g re a c tio n s o c c u rrin g fo r U C in s o lid benzene, and did not o b s e r v e any ch an ges. H o w e v e r, I b e lie v e that a num ber o f o rg a n ic c r y s ta ls should be exam in ed to d e te rm in e w h eth er annealing p r o c e s s e s a re a c tu a lly taking p la ce under conditions o f lo w -te m p e r a tu r e bom bardm en t and lo w -te m p e r a tu r e p r e s e r v a tio n . M any o rg a n ic compounds a r e solu b le in acetone w hich can be taken down to v e r y lo w te m p e ra tu re s and s t ill rem a in liq u id , and the c r y s ta ls can be d is s o lv e d at th ese lo w te m p e ra tu re s . I would lik e to p ro p o s e this as a fie ld which d e s e r v e s a lit t le m o re study than it has r e c e iv e d in the past. F . S . R O W L A N D : W e t r ie d som e e x p e rim e n ts alon g these lin es about 15 y e a r s ago. W e p rodu ced r e c o il tritiu m a tom s in s o lid glu cose, and then d is s o lv e d the m a te r ia l in d iffe r e n t s o lven ts at d iffe r e n t te m p e ra tu re s , and then did a s p e c ific c h e m ic a l re a c tio n w hich a llo w e d the d eterm in a tio n of the s p e c ific r a d io a c tiv ity o f the tritiu m w hich had substituted in to the -C H 2 OH p o s itio n a ft e r d isso lu tio n o f the c r y s ta ls . W e did o b s e r v e d iffe r e n t s p e c ific r a d io a c tiv itie s , but ou r p ro b le m then w as that w e had s ta rte d with a h etero g en eo u s m ix tu re o f glu cose and the lith iu m s a lt used as the ta rg e t s o u rce f o r the tritiu m . Then w e had no e a s y w ay to d e te rm in e w hether the s p e c ific a c tiv ity d iffe r e n c e s w e had found a ro s e fr o m the h e te ro g e n e ity o f the o r ig in a l ir r a d ia tio n o r fr o m d iffe r e n t c h e m ic a l re a c tio n s o c c u rrin g d u rin g the solu tion o f the ir r a d ia te d so lid . S ince the d iffe r e n c e s w e r e not e n o rm o u s ly la r g e , w e d id n 't pursue these e x p e rim e n ts fu rth e r. J. P . A D L O F F : D r. Danon r a is e d a point e a r lie r about m a trix is o la tio n stu dies â&#x20AC;&#x201D; up to now, v e r y few m a trix is o la tio n e x p e rim e n ts have been attem pted in hot atom c h e m is try , and those that have have m a in ly been done in con n ection w ith M Ă´ ssb a u er s p e c tro s c o p y . It m igh t be v e r y in te re s tin g fo r in stan ce to f r e e z e a m a te r ia l into a C O 2 m a trix , ir r a d ia te the system , and to exam in e it by w h a te v e r m eans a r e a v a ila b le . It w ould p ro b a b ly be e a s ie r to do the e x p e rim e n t in a study o f the ch e m ic a l e ffe c ts fo llo w in g n u c le a r d ecay, is o la tin g the r a d io a c tiv e m o le c u le in a s o lid la ttic e . S. A M IE L : In y o u r p ap er, you r e fe r r e d to som e e x o tic e x p e rim e n ts by e le m e n ta ry p a r tic le p h y s ic is ts . Can you e la b o ra te on that com m ent? G. H A R B O T T L E : U n fortu n ately, I c a n 't. T h e n eu tral c u rre n t is one o f the p re s e n t th e o r e tic a l con cepts in beta d ecay. It is thought to have so m e change f o r d e te c tio n i f it in te ra c te d w ith an atom and r a is e d it to a h ig h e r state of e x c ita tio n - a n u c le a r is o m e r . W hat one w ould lo o k f o r w ould be a hot atom re a c tio n o c c u r r in g f o r this e x c ite d m eta sta b le state, w hich would then s e p a ra te i t fr o m the bulk m a te r ia l. T h e e n e rg y is not the 6 to 8 M eV o f neutron captu re, but on ly o f the o r d e r o f hundreds o f k ilo v o lts .
I A E A -P L -6 1 5/2
31
S. A M IE L : And you w ould then have to s e p a ra te it fr o m oth er p r o c e s s e s that w ould p rod u ce the sam e e x c ite d n u clea r state? G. H A R B O T T L E : Y e s , it w ould have to be is o la te d fr o m o th e r so u rc e s o f that state. It w ould be a v e r y d iffic u lt ex p e rim e n t, and has not y e t been c o n s id e re d in d e ta il to see w h eth er it has any fe a s ib ilit y at a ll. A second e x a m p le in v o lv e d the p o s s ib ility o f co n cen tra tin g a p a r tic u la r e x o tic fo r m o f m eson through a S z ila r d - C h a lm e r s m ethod. A g a in , the s u g g estio n has not been thought through fa r enough to c o n s id e r the r e a l fe a s ib ilit y o f an e x p e rim e n t. A . G. M A D D O C K : T h e attention o f so m e th e o ris ts should be brought to the p o s s ib le im p lic a tio n s o f som e o f L a z z a r in i's e x p e rim e n ts w hich se e m to in d ica te that, fo llo w in g the produ ction of a deep vacan cy, th ere s e e m s to be a rea s o n a b le p o s s ib ility fo r the m o le c u le to s u r v iv e any kind o f fr a g m e n ta tion by re c a p tu rin g e le c tr o n s fr o m its su rrou n d in gs. A s h o rt w h ile ago one w ould have thought that the fra g m en ta tio n would take p la ce too ra p id ly fo r the e le c tro n s to m o ve in. It w ould be n ice to have som e ca lcu la tio n s on som e s im p le s y s te m s to see w h eth er e le c tr o n flo w would be m o re ra p id than fra g m en ta tio n . F . S . R O W L A N D : A r e you thinking of this in con n ection w ith the p r o c e s s o f beta d ecay, o r in te rn a l co n versio n , o r what o th e r p ro c e s s ? A . G. M A D D O C K : A n y p ro c e s s which p rod u ces a К s h e ll va ca n cy. The e v id e n c e su ggests that the p r o b a b ility of s u r v iv a l o f the in tact m o le c u le depends upon the a v a ila b ility o f e le c tr o n s fr o m the su rrou n din gs. J . P . A D L O F F : W hat happens to the e n e rg y o f n eu tra liza tio n ? A . G. M A D D O C K : Th at is one o f the m ain p ro b le m s . It s e e m s to m e that the p re s e n t a n s w e rs a re to ta lly in co m p a tib le w ith the e x p e rim e n ta l re s u lts . The t im e - s c a le is down in the su b -p ico seco n d re g io n . The e le c tr o n flo w m ust be v e r y ra p id in o r d e r to p re v e n t any kind of C oulom b ex p losion . G. S T Ô C K L IN : I think this is an im p o rta n t question — the c h e m ic a l con sequ en ces o f A u g e r e le c tr o n e je c tio n . W e on ly have the C a rls o n -W h ite m o d e l f o r the gas phase; w e have the A u g e r e le c tr o n m o d el fr o m W illa r d G e is s le r fo r the liq u id phase; but we r e a lly d on 't know anything about the c h e m ic a l con sequ en ces o f the A u g e r e ffe c t in condensed ph ases, e s p e c ia lly the s o lid . W e do have the evid en ce in the s o lid phase that n e u tra liza tio n o c c u rs b e fo r e the fra g m e n ta tio n fr o m the rep u lsio n of the C oulom b ex p lo sio n can o c c u r. Y ou then have the high n e u tra liza tio n e n e rg y a v a ila b le which, at le a s t in o rg a n ic s y s te m s , lea d s to ex c ita tio n d e c o m p o s i tion. T h e c la s s ic a l C a rls o n -W h ite m o d el is p ro b a b ly not a p p lica b le in s o lid s . A . G. M A D D O C K : T h e e x p e rim e n ta l evid en ce su ggests that this extends as fa r as liq u id s y s te m s . G. H A R B O T T L E : I w ould c e r ta in ly a g r e e that the liq u id s y s te m s a re a ls o w orth study. I d on 't b e lie v e that the W illa r d - G e is s le r m o d el was e v e r ad equ ately w o rk ed out th e o r e tic a lly . T h e m o d el w as p ostu lated on the b a sis o f e x p e rim e n ta l re s u lts which ap p ea red to have som e s im ila r it y to those found du rin g X - r a y r a d io ly s is , but th e re has n e v e r been a good th e o r e tic a l lo o k at the liq u id phase. In the s o lid , you can d is c h a rg e a h igh ly ch a rged c e n tra l atom lo n g b e fo r e any atom s s ta r t to m o ve - th a t's no p ro b le m . The qu estion is - what do you do w ith the hundreds o f k ilo v o lts o f e n e rg y brought in during this d is c h a rg in g p r o c e s s . How do you get r id o f this e n e rg y fr o m th is site? S. A M IE L : T h is goes into the qu estion o f the in su latin g p r o p e r tie s o f con den sed p h a ses. F r o m o th e r d is c ip lin e s , one can le a r n som eth in g about
32
H AR BOTTLE
the re la x a tio n , o r t im e - s c a le s , by w hich e le c t r ic a l cu rren ts respond to d istu rb a n ces. J . P . A D L O F F : A t the Jßlich hot atom m e e tin g la s t S ep tem b er, th ere w as a m e s s a g e fr o m P r o fe s s o r G o l'd a n s k ij who r a is e d the p ro b le m o f the p o s s ib le use of hot atom c h e m is try f o r studying a v e r y ch a llen gin g e x p e r i m en tal p ro b le m - the p re p a ra tio n o f a g a m m a -ra y la s e r . The p ro b le m in v o lv e s the e x c ita tio n of the e x c ite d n u clea r le v e l v e r y ra p id ly , m o re ra p id ly than the d eca y back to the ground state. P r o f e s s o r G ol'd a n sk ij has su ggested that (n, y ) r e c o ils m igh t be used to s e p a ra te the e x c ite d atom s, to be c o lle c te d in a gas s tre a m fa s t enough to produ ce an avalan ch e. A t the n ext M ôssb a u er c o n fe re n c e in F r a n c e in S ep tem b er (1974), th e re w ill be a s e s s io n on 'E x o tic P r o b le m s ', and one o f th ese p ro b le m s w ill be the p re p a ra tio n o f a g a m m a -ra y la s e r . S. A M IE L : It s e em s to m e that the pum ping is a v e r y s e v e r e p ro b le m . J . P . A D L O F F : T h e e x p e rim e n t has not been c a r r ie d beyond the in itia l concept. P r o f e s s o r G ol'd a n sk ij su ggested it to the J lilich c o n fe re n c e as a su b ject f o r fu rth e r thought.
I A E A -P L -6 1 5/3
CHEMICAL STUDIES OF ION IMPLANTATION A.G. M A D D O C K The Chemical Laboratories, Lensfield Road, Cambridge, United Kingdom
Abstract CHEMICAL STUDIES OF ION IMPLANTATION. Various methods used for studying the environment and the chemical and electronic states of ion-implanted atoms are presented and their strengths and limitations discussed. An account is given of the correlation between the behaviour of ion-implanted and nuclear recoil generated atoms in complex solids, and details of experiments to determine differences in behaviour are presented.
1.
IN T R O D U C T IO N
The en viro n m en t and the c h e m ic a l and e le c tr o n ic sta tes o f ion im plan ted atom s have been in v e s tig a te d in a v a r ie t y o f w ays. In p rin c ip le , one w ould p r e fe r in -situ m eth ods, such as e le c tr o n spin re s o n a n ce, angular c o r r e la tio n and M ôssb a u er e m is s io n s p ectra . The low co n cen tra tion s o f im plan ted atom s that can be a ch ie v e d without g ro s s m o d ific a tio n o f the m a tr ix exclu d e a ll but the m o st s e n s itiv e o f such p ro c e d u re s . In p r a c tic e , m o st o f th ese m ethods p re s e n t som e d iffic u ltie s in the in te rp re ta tio n o f the re s u lts . Thus, although the M ôssb a u er e m is s io n s p e c tra o f a 57 C o im plan ted m a trix p ro v id e s u sefu l in fo rm a tio n about the e le c tr o n ic e n v iro n m ent o f the 57F e m it p ro d u ces by o r b ita l e le c tr o n ca p tu re, con clu sion s r e g a r d in g the en viron m en t o f the cob alt r e q u ir e in fo rm a tio n , not at p re s e n t a v a ila b le , about the e ffe c t on the e le c tr o n ic state o f the cob alt o f the e le c tro n captu re even ts. A c ru d e r, but s t ill u sefu l, approach to the p ro b le m is to e x p lo re the state o f oxid ation and c h e m ic a l com bin ation o f the im plan ted atom by a p p ro p ria te con ven tion al c h e m ic a l m ethods. The d iffic u ltie s due to the s m a ll con cen tration s o f the im plan ted s p e c ie s a re r e a d ily m itig a te d by im p lan tin g re a s o n a b ly lo n g - liv e d r a d io a c tiv e s p e c ie s . O b vio u sly , by u sin g suitable etch in g o r se c tio n in g tech n iqu es one can obtain both the g e o m e tr ic a l d is t r i bution and the c h e m ic a l state o f the im p lan ted s p e c ie s . The io n -im p la n te d so lid s can be r e g a r d e d as a p ro b a b ly m eta sta b le s o lid solu tion o f the im p lan ted atom s in the m a tr ix and its beh aviou r should r e fle c t that o f a v e r y dilute ato m ic d is p e rs io n o f the im p lan ted e le m e n t in the m a tr ix m a te r ia l. It is im p orta n t to note that the con cen tra tion s o f the im plan ted e le m e n t in the m a tr ix w ill g e n e r a lly be o f c o m p a ra b le , o r even lo w e r , o r d e r o f m agnitude to those o f the n atu ral ra d ia tio n g en era ted d e fe c ts in the m a trix . It is th e r e fo r e not s u rp ris in g that m o st o f the c h e m ic a l w o rk so fa r published on ion im p lan tation has been c a r r ie d out by ch e m is ts who had been studying the c lo s e ly re la te d s y stem s that can be gen era ted by n u clear
33
34
M ADDOCK
tra n s fo rm a tio n s in s o lid s . In m any n u clear re a c tio n s , in clu din g p a r tic u la r ly the r a d ia tiv e th e rm a l neutron capture re a c tio n , the a ffe c te d atom s u ffe rs a s u ffic ie n t m ech a n ica l r e c o il to b rea k a ll the c h e m ic a l bonds, by w hich it w as p r e v io u s ly bound, and to in je c t it into the su rrou n din g la ttic e . It s eem ed rea s o n a b le to ex p ect that io n -im p la n ted s y s te m s should c lo s e ly r e s e m b le the r e la te d sy s te m s p rodu ced by n u clea r tra n s fo rm a tio n . The la r g e r p a rt o f the e x te n s iv e body o f re s u lts on the beh aviou r o f r a d io a c tiv e atom s, o r perh aps s o m e tim e s m o le c u la r fra g m e n ts , re le a s e d in a c r y s ta llin e m a tr ix by n u clea r r e c o il, r e la te d to atom s p rodu ced by ra d ia tiv e th e rm a l neutron cap tu re. The e ffe c ts could on ly be studied ra d io c h e m ic a lly in m a tr ic e s o f som e c h e m ic a l c o m p le x ity . F o r e x a m p le, a p otassiu m ch rom a te c r y s ta l is about as sim p le a system as can be used. F o r th is re a s o n som e o f the io n -im p la n ta tio n studies have also been c a r r ie d out in such c o m p lex m a tr ic e s . H o w e v e r, d e ta ile d p h y s ic a l m o d e ls fo r such stru ctu red p o la r in su la to rs a re not a v a ila b le and th e re a re c o n sid era b le a ttra c tio n s in ch oosin g a b e tte r u n derstood kind o f m a trix . F o rtu n a te ly th ere is also much r a d io c h e m ic a l in fo rm a tio n on the beh aviou r o f 35 S and 32 P , gen era ted by the (n ,p ) and (n, a ) r e a c tio n s , r e s p e c tiv e ly , in a lk a li c h lo rid e c r y s ta ls . F o r such c r y s ta ls th e re is an adequate fr a m e  w o rk o f th e o r e tic a l know ledge o f the m a tr ix and sin gle c r y s ta ls o f these m a te r ia ls a re r e a d ily a v a ila b le . I sh a ll b egin th e r e fo r e with a b r ie f account o f the c o r r e la tio n betw een the b eh a viou r o f io n -im p la n ted and n u clea r r e c o il gen era ted atom s in co m p le x so lid s . In addition to th ese stu dies, w hich a re d ir e c t ly c o r r e la te d w ith hot atom in te r e s ts , ion im plan tation is a va lu a b le technique fo r b rin g in g about oth er c h e m ic a lly u sefu l phenom ena. (1) The e ffe c t o f the im p lan tation is la r g e ly con fin ed to a s u p e r fic ia l la y e r o f a few m ic ro n s th ick n ess. It can th e r e fo r e lea d to a c o n s id e ra b le m o d ific a tio n o f the c a ta ly tic p r o p e r tie s o f the im plan ted s u rfa c e ; (2) the im p lan ted atom m ay r e a c t with the m a trix to produ ce new and unusual prod u cts; (3) m o re than one c h e m ic a l s p e c ie s m a y be im p lan ted and one can e x p lo re the re a c tio n s both betw een the im plan ted s p e c ie s and w ith the m a tr ix ; and (4) any c h e m ic a l re a c tio n o f the im p lan ted atom s can be studied as a fr a c tio n o f the depth o f pen etration o f the atom . In p a rtic u la r, the beh aviou r o f the ch an n ellin g atom s w hich t r a v e l much fu rth e r into the c r y s ta l can be e x p lo re d .
2.
IO N I M P L A N T A T I O N IN C O M P L E X SOLIDS
I do not want to b eco m e in v o lv e d in a len gth y account o f the e ffe c ts o f neutron captu re in c r y s ta llin e s o lid s ; num erous r e v ie w s a re a v a ila b le on the su b ject [1, 2 ], The ra d io a c tiv e produ ct is is o to p ic w ith one o f the atom s o f the m o le c u le s o f the ta r g e t m a trix . V e r y often one can r e p re s e n t the ta r g e t s p e c ie s as M L n, which m ay be e ith e r a n eu tra l o r io n ic s p e c ie s . The atom M y ie ld s the ra d io a c tiv e * M on neutron cap tu re. The neu tronir r a d ia te d s o lid is su b jected to ra d io c h e m ic a l a n a ly s is to d e te rm in e the state o r sta tes o f c h e m ic a l com bin ation and oxid ation o f the product *M . The o v e rw h e lm in g m a jo r ity o f a n a ly tic a l p ro c e d u re s re q u ir e d issolu tion o f the ir r a d ia te d c r y s ta ls . T h is con fron ts one w ith the m a jo r d iffic u lty o f th is technique o f in v e s tig a tio n : the re la tio n o f c r y s t a l p r e c u r s o r s to the s p e c ie s found a fte r solu tion.
I A E A -P L -6 1 5/3
35
T h e p ro b le m o f re a c tio n s on solution is a g g ra v a te d by the v e r y low con cen tra tio n o f the s p e c ie s * M and by the p o s s ib ility o f re a c tio n s w ith point d e fe c ts at the m om en t o f solution. It is tru e that w ith som e ingenu ity, fo r ex a m p le by the use o f nonaqueous s o lven ts and oth er m ean s, a m o r e r ig o r o u s id e n tific a tio n o f the c r y s ta l p r o c e s s e s m a y be p o s s ib le , but the c h e m is t has been ra th e r slow in obtain in g a solu tion o f th is e s s e n tia lly c h e m ic a l p ro b le m . Som e p ro p o rtio n o f the ra d io a c tiv e produ ct is in v a r ia b ly found in the fo r m o f the m a tr ix s p e c ie s , and fo r this p a rt th e re is h a rd ly any doubt that the c r y s ta l p r e c u r s o r m ust be the sam e c h e m ic a l s p e c ie s . Since solu tion and gas phase stu dies show that the n u clea r even t is a lm o s t alw ays fo llo w e d b y m o le c u la r ru p tu re, any a p p re c ia b le y ie ld o f the ra d io a c tiv e m a tr ix s p e c ie s m ust be due to ra p id r e - fo r m a tio n fo llo w in g the fra gm en ta tio n . T he c h a r a c te r is tic and c h e m ic a lly in te re s tin g a sp ect o f these neutronir r a d ia te d c r y s t a l s y s te m s is that the r e - fo r m a tio n re a c tio n s , y ie ld in g the ra d io a c tiv e m a tr ix com pound, can be induced a fte r the neutron i r r a d i  ation b y h eat, lig h t, io n iz in g ra d ia tio n , p r e s s u r e and oth er m ean s. T h ese re a c tio n s have been shown to be a type o f exchange re a c tio n that can be re p re s e n te d as *M + M L n -* M + * M L n, and w hich a re p r a c t ic a lly unknown in solu tion . The exchange p r o c e s s m u st in v o lv e both tr a n s fe r o f ligan d s and one o r m o r e e le c tr o n ic p r o c e s s e s because the oxid ation state o f * M is g e n e r a lly lo w e r than that o f M in M L n. P e rh a p s becau se o f the re d o x step th ese re a c tio n s a re u su ally v e r y s e n s itiv e to the con cen tration and nature o f the d e fe c ts in the c r y s ta l m a trix . T he k in etic a sp ects o f th ese re a c tio n s a re a lso unusual and ra th e r re m in is c e n t o f the annealing o f oth er kinds o f ra d ia tio n dam age. The p r o c e s s e s a re f ir s t o r d e r , but the e n e rg y o f a ctiva tio n , and, p o s s ib ly , a ls o the fre q u e n c y fa c to r , appear to have a sp ectru m o f va lu e s . A num ber o f io n -im p la n ta tio n e x p e rim e n ts have th e r e fo r e been design ed to e x p lo r e how fa r the im p lan ted s y s te m s r e s e m b le the n e u tro n -irra d ia te d m a te r ia ls . The f ir s t r e p o r t 13] used io n -im p la n te d slC r in a potassiu m ch ro m a te m a tr ix because a p a r tic u la r ly e x te n s iv e ran ge o f data is a v a il able on the beh aviou r o f the n e u tro n -irra d ia te d m a te r ia ls . Im plan tation e n e r g ie s o f fr o m 5-450 keV w e r e used with 51C r + and 51C r 2+ beam s. High s p e c ific a c tiv ity ch ro m ic o x id e w as c o n v e rte d to c h lo rid e in the so u rce 14]. Ir r a d ia tio n s w e r e conducted at ro o m te m p e ra tu re and the im plan ted c r y s ta ls contained fr o m 10"4 to 1 /uCi o f 51C r. About 80% o f the 51C r w as found in the im plan ted c r y s ta ls as ch rom ate c o m p a red with about 60% in a neu tronir r a d ia te d sam p le o f the sam e c r y s ta ls . The r e m a in d e r w as found in the solu tion in the fo r m o f the m o n o m e ric aquated c h ro m ic ion and the v a rio u s r e la te d p o ly m e r ic aquo cation s fo rm e d by C r III. The la tte r in d icated the fo rm a tio n o f C r - O - C r lin k a g es in both sy s te m s . The io n -im p la n ted and the n e u tro n -irra d ia te d m a te r ia l showed q u a lita  t iv e ly , but not q u a n tita tively s im ila r an n ealin g re a c tio n s . The 51C r w as a ls o found as ch rom a te a fte r solu tion fo llo w in g im p lan tation in K C 1 0 4, K 2S 0 4 and K 2B e F 4 , although in d e c r e a s in g y ie ld s . A m o re re c e n t co m p a ris o n o f the annealing o f io n -im p la n ted and r e c o il g e n era ted 75Se in p otassiu m and sodium se le n a te s [5 ] has shown that the fin e stru c tu re s o f the an n ealin g is o c h ro n a ls obtained a re the sam e fo r the two tre a tm e n ts . F r o m the c h e m is ts ' point o f v ie w these a re s t ill re a s o n a b ly sim p le su bstances and although the re a c tio n o f the r e c o ile d 75Se atom w ith its su rrou n din gs to r e - f o r m 7oSeO| is unusual it is by no m eans in c re d ib le .
36
M ADDOCK
T A B L E I. Y IE L D D IS T R IB U T IO N O F 60Co IN G E O M E T R IC IS O M E R E X P E R IM E N T S ION-IMPLANTED (per cent yield) Target compound
Ion energy „ . (keV)
T , „ , Labelled trans
trans [СоепгСЫИОз
20
33.3
1.8
trans [ Coen2CI2] NO3
60
23.3
4.0
cis [ Coen2Cl2]N 03
20
11.7
18.5
cis [Coen2Clz]N03
60
12.1
12.2
Labelled cis
NEUTRON-IRRADIATED a (Per cent yield) After 1 h at 100°C Target compound
Labelled trans
Labelled cis
trans
cis
trans [СоепгСЛгЗЫОз
7.3
0.2
48.0
0.2
cis [Coen2Clz]N03
0.1
3.1
0.1
42.9
a Data from RAUSCHER, H.H., SUTIN, N., MILLER, J.M., J. Inorg. Nucl. Chem. 12 (I960) 378.
But the much m o r e re m a rk a b le annealing re a c tio n s found to fo llo w neutron ir r a d ia tio n o f much m o re c o m p lex substances a re a lso du plicated by ion im p lan ted sy s te m s . C u rio u sly, two independent grou ps have v e r ifie d such co rre s p o n d e n c e with p r a c tic a lly the sam e system [6, 7]. One o f the s u r p r is in g fe a tu re s is the s t e r e o s p e c ific it y o f the r e a c tio n fo llo w in g neutron ir ra d ia tio n . The sam e c h a r a c te r is tic s have been found fo r io n -im p la n ted s y s te m s . Thus io n -im p la n ted 60Co in tra n s Co ( C 2H 4N 2H 4 ) 2C12- N 0 3 b eco m es in c o rp o ra te d as the tra n s c o m p lex , as does the ra d io c o b a lt fo rm e d by neutron cap tu re. W ith the sa lt o f c is co n figu ra tio n the r a d io a c tiv e cobalt, in trodu ced in e ith e r w a y, is p r e fe r e n t ia lly in c o rp o ra te d as the c is com p lex. In a ll c a s e s th e re is som e fo rm a tio n o f the o th e r is o m e r ic fo r m , but the p r e fe r e n c e s o f the tra n s fo r in c o rp o ra tin g in the tra n s fo r m is much m o re m a rk ed than that o f the c is as c is co m p lex . (T a b le I. ) E x p e rim e n ts w ith d iffe r e n t e n e r g ie s o f ion beam and d iffe re n t d oses o f d ep osited kin etic e n e r g y in the m a tr ix showed that the s te r e o s p e c ific it y d e c re a s e d as the e n e r g y d ep osited in c re a s e d . In the sam e p a p er it w as shown that ion im plan tation with ra d io a c tiv e co p p er o f the |3-crystallin e fo r m o f cop p er phthalocyanine gave a much g r e a t e r y ie ld o f the la b e lle d co m p le x than did the а -m o d ific a tio n , an analogous re s u lt to that found on neutron irra d ia tio n .
37
IA E A -P L -6 1 5 / 3
T A B L E II. P E R C E N T A G E Y IE L D S IN O X ID A T IO N S T A T E S O F PH O SPH O R U S
35Cl(n, a)3ZP
70 keV P+ in KCI
V p
43
46
III p
27
20
I p
30
34
Im p lan tation o f 56Mn in C r (C O )6 has shown [8 ] that a s m a ll p ro p o rtio n o f M n (C O )5 and M n (C O )4 r a d ic a ls a re produ ced. The y ie ld s a re o n ly one o r two p e r cent o f the im plan ted a c tiv ity . V e r y p o s s ib ly a m ix e d m anganese ch rom iu m ca rb o n y l is a lso produ ced, but the a n a ly tic a l p ro c e d u re s used m igh t not r e y e a l such a product. T h e s e o b s e rv a tio n s s e r v e to c o n firm that in tru d in g fo r e ig n atom s o r ion s r e a c t in a c r y s t a l m a trix in much the sam e w a y h o w e v e r th ey a re in trod u ced into the la ttic e .
3.
IO N IM P L A N T A T IO N IN S IM P L E P O L A R SOLIDS
The s im p le s t type o f so lid fo r w hich much data e x is te d on the beh aviou r o f fo r e ig n atom s produ ced by n u clea r re a c tio n s w as the a lk a li c h lo rid e s . R e a c to r neutron ir r a d ia tio n p rod u ces both 35 S and 32P in such c r y s ta ls . E x p e rim e n ts m ade m any y e a r s ago showed that both th ese s p e c ie s appeared in a v a r ie t y o f c h e m ic a l fo r m s a fte r solution o f the irr a d ia te d c r y s ta ls , in clu din g th e ir oxyan ion s, e s p e c ia lly sulphate and phosphate. T h ese ob s e rv a tio n s sta rted a c o n tr o v e r s y about the id e n tity o f the c r y s ta l p r e c u r s o r s o f the e n titie s found a fte r solu tion , and, indeed, the an sw er to this question is s t ill in c o m p le te , p a r tic u la r ly in the ca se o f the phosphorus s p e c ie s . The situation is co m p lic a te d by co va len t bond fo rm a tio n b etw een the sulphur o r phosphorus and the c h lo rin e in the la ttic e . B ecau se phosphorus and sulphur a re not v e r y d iffe r e n t in s iz e fr o m c h lo rin e one m igh t ex p ect that the phosphorus and sulphur containing analogu es o f the V c e n tre s , such as, SC I and Р С 1 2, m igh t be fo rm e d , p a r tic u la r ly if h ole donors a re a v a il able in the m a trix . E ven the qu estion w h eth er fo rm a tio n o f som e P -О o r S-O bonds can be fo rm e d in the c r y s t a l m a tr ix needs som e d eta iled c o n sid era tio n . U n less s p e c ia l p reca u tio n s a re taken in the p re p a ra tio n o f the a lk a li c h lo rid e s c r y s ta ls (fu sion under an a tm osp h ere o f HC1 g a s ), th ey w ill contain a s m a ll amount o f O H ", and under the in flu en ce o f the io n iz in g ra d ia tio n , a lm o st in e v ita b ly a ccom p an yin g the neutron ir r a d ia tio n , these ions y ie ld oxygen s p e c ie s w hich m igh t r e a c t w ith the phosphorus o r sulphur atom s o r ions. In addition, a lk a li c h lo rid e c r y s ta ls have been shown to ab sorb o xygen and fo r m s u p eroxid e ion s, C l ' + 0 2 ----- i C l 2 + 0 2, w hich can m ig r a te in the
38
M ADDOCK
T A B L E III. SU LPH U R
P E R C E N T A G E Y IE L D S IN O X ID A T IO N S T A T E S O F
Proportion o f 35 S
Mode of production
S"
CNS
so3
S.04
3SCl(n,p)35S
46.1
9.6
21.5
22.8
Ion-implanted
46.4
26.4
17.7
9.5
T A B L E IV . R A D IA T IO N DOSE E F F E C T O N P E R C E N T A G E Y IE L D S IN O X ID A T IO N S T A T E S O F PH O SPH O R U S [10]
3 5 - ,,
v32 n
Cl(n,a) P
Dose (Mrad)
60 keV 32P in KCI
pv
pill
P1
PV
лале
p1
0
50
15
35
30
16
54
20
19
12
69
13
13
57
100
8
14
78
15
10
75
la ttic e at m o d e r a te ly e le v a te d te m p e ra tu re s . F o rtu n a te ly , both these p o s s ib ilitie s can be avoid ed by the use o f m a te r ia l u se, f i r s t under HC1 gas and then an in e r t gas. One m igh t suppose th e r e fo r e that the fo r e ig n atom s would be p resen t as sim p le m on atom ic an ion ic, n eu tra l o r ca tio n ic s p e c ie s o f v e r y low ch arge o r , p erh a p s, as analogous s p e c ie s to the V c e n tre s found in these c r y s ta ls w ith one c h lo rin e atom r e p la c e d by phosphorus o r sulphur. F o r the pu rpose o f co m p a riso n o f ion im p lan tation and g e n era tio n by n u clea r re a c tio n , data obtained b y the sim p le a n a ly tic a l p ro c e d u re s a re s t ill s ig n ifica n t. The f i r s t re s u lts re p o rte d by A n d e rs e n and S oren sen [10 ] showed that th e re w as c o n s id e ra b le s im ila r it y b etw een the beh aviou r o f io n -im p la n ted and n e u tro n -g e n e ra te d 32P in KC1 (T a b le II). A t about the sam e tim e a s im ila r study o f the d istrib u tio n o f 35 S s p e c ie s in io n -im p la n te d and n e u tro n -irra d ia te d N a C l w as re p o rte d [11] (T a b le III). H a vin g shown that th e re is at le a s t a q u a lita tiv e s im ila r it y b etw een the two kinds o f sy s te m it b e c o m e s u sefu l to exam in e the quantitative d iffe r e n c e s m o r e c lo s e ly .
39
1АЕА -РЬ615/3
TAB LE V. A N N E A L IN G E F F E C T O N P E R C E N T A G E Y IE L D S IN O X ID A T IO N S T A T E S O F S U L P H U R [14]
35S distribution Treatment S
CNS"
SO3
S04
50.8
10.7
27.7
10.7
2. (n,p) annealed 1 h at 205eC
65.0
1.9
10.3
22.7
3. Ion-implanted
68.6
21.3
7.7
2.3
4. Ion-annealed 1 h at 205°C
22.3
57.0
15.9
4.8
1.
(n,p) produced
The lo c a l co n cen tra tion o f sulphur o r phosphorus atom s is g e n e r a lly th re e o r fou r o r d e r s o f m agnitude g r e a t e r in the io n -im p la n ted than in the n e u tro n -irra d ia te d m a te r ia l. In both c a s e s the m a tr ix s u ffe rs ra d io ly tic dam age. The to ta l dose r e c e iv e d is g r e a te r in the ca se o f the neutron ir r a d ia tio n , becau se o f the io n iz in g ra d ia tio n accom p an yin g r e a c to r irr a d ia tio n . The r e c o il o f the n ascen t sulphur o r phosphorus atom in the n e u tro n -irra d ia te d m a te r ia l is o f the sam e o r d e r o f m agnitude as the im p lan tation e n e rg y . Thus, the lo c a l ra d ia tio n dam age to the sy s te m should be p ro p o rtio n a l to the con cen tra tio n o f fo r e ig n atom s and thus be h ig h e r in the io n -im p la n ted m a te r ia l. H o w e v e r, in n e ith e r ca se w ill th ere be a g r e a t d ea l o f o v e r la p o f the tra c k s o f the r e c o ilin g o r im plan ted s p e c ie s so that the lo c a l en viron m en t o f the th e r m a liz e d atom s o r ions should not be v e r y d iffe r e n t fo r the two kinds o f s y s te m s . But the m o s t d e e p ly p en e tra tin g o f the io n -im p la n ted s p e c ie s should re a c h re g io n s o f c o m p a r a tiv e ly low ra d ia tio n dam age and th is w ill be s t ill m o r e m a rk ed fo r those ions p e n e tra tin g the m a tr ix v e r y d e e p ly by a ch an n ellin g p r o c e s s , w ith an a p p r o p r ia te ly o rie n ta te d m a tr ix c r y s ta l. In e ith e r ca se the co n cen tra tion o f the fo r e ig n atom used is le s s than, o r at m o s t co m p a ra b le w ith, that o f the le s s com m on d e fe c ts in the m a tr ix c r y s ta l, such as the anionic v a c a n c ie s . F i r s t the o v e r a ll beh aviou r o f the fo r e ig n atom s w ill be c o n s id e re d and then the ran ge e ffe c ts w ill be exam in ed .
4.
D IF F E R E N C E S IN B E H A V IO U R O F IO N - IM P L A N T E D A N D N E U T R O N -G E N E R A T E D S P E C IE S
F o r both the 35S and 32P , ion im p lan tation p rod u ces a s m a lle r p r o p o rtio n o f the o x id iz e d s p e c ie s than is p rodu ced by neutron ir r a d ia tio n [1 1 -1 5 ]. R e p r o d u c ib ility is v e r y d iffic u lt to a ch ieve w ith d iffe r e n t p rep a ra tio n s o f c r y s ta ls and m o st o f the con clu sion s have been draw n fr o m c o m p a riso n e x p e rim e n ts u sin g m ir r o r - im a g e p ie c e s o f c le a v e d c r y s ta ls .
40
MADDOCK
T A B L E V I. R A D IA T IO N DOSE A N D A N N E A L IN G E F F E C T S O N O X ID A T IO N S T A T E O F S U L P H U R [14]
35S distribution
Treatment
1. Ion-implanted ^S/NaCl crystal 2. Ion-annealed 1 h at 205°C 3. Ion-irradiated 10 Mrad 4. Ion-irradiated and annealed 1 h at 205°C
S
CNS
SOl
S04
55.7
25.9
15.3
3.0
8.2
40.4
46.0
5.6
45.0
32.9
18.2
3.8
7.6
32.4
48.2
11.7
T A B L E V II. E F F E C T S O F 35C l C O N T A M IN A T IO N A N D O F P R E - I M P L A N T E D IONS O N O X ID A T IO N S T A T E O F IM P L A N T E D 35S [14]
35S distribution Treatment
S
CNS"
SO3
SO4
1. ^Cl/^S
1.1 x 102
71.9
12.2
12.4
3.3
2.
5.5 x 102
68.6
15.3
13.7
2.3
3. ^Cl/^S
8
54.5
25.5
16.5
3.3
4. ^Cl/^S
annealed 1 h at 205"C
8.2
40.4
46.0
5.6
45.5
21.2
25.8
7.5
8.9
30.2
46.8
14.2
7. Pre-implanted 160
37.0
46.2
11.7
5.0
8. Pre-implanted 1бО annealed 1 h at 205°C
21.0
64.1
10.3
4.6
9. Pre-implanted 32S
23.4
63.5
10.9
2.0
9.3
75.1
13.1
2.6
^Cl/^S
5. Pre-implanted
6.
10.
X 103
35Cl
Pre-implanted 35Cl annealed 1 h at 205eC
Pre-implanted 32S annealed 1 h at 205°C 35Cl, 160. and 32S 1016 atoms/cm2 crystal 35S 1011 atoms/cm2 crystal
IAEA-PL-615/3
41
P r e - ir r a d ia t io n o f the m a trix c r y s ta l with io n iz in g ra d ia tio n has a much g r e a t e r e ffe c t on the neutron irr a d ia te d than on the io n -im p la n ted c r y s ta ls . (T a b le IV ). . S im ila r re s u lts have been obtained fo r the 35S. The e ffe c t o f h eatin g the 35S im p lan ted c r y s ta ls is quite d iffe r e n t fr o m that on the neu tronir r a d ia te d m a te r ia ls . R e s u lts a re shown in T a b le V fo r N a C l. But both kinds o f 32P im p reg n a ted KC1 seem to behave in the sam e w ay. In both sy s te m s ev id e n c e fo r re a c tio n s b etw een the fo r e ig n atom s and F and V type (e le c tr o n o r h o le-d o n a tin g ) c e n tre s in the m a tr ix can be obtained. Thus, i f the e le c tr o n s trap p ed at F c e n tre s a re r e le a s e d by o p tic a l o r th e rm a l b lea ch in g, the p ro p o rtio n o f 35S o r 32P a p p ea rin g in low oxid ation sta tes (S = and P 1 , h yp op h osp h ite) in c r e a s e s [10, 12-15 ]. A n io n iz in g ir r a d ia tio n o f an io n -im p la n ted 35S in N a C l c r y s ta l p rod u ces a c o m p a r a tiv e ly s m a ll m o d ific a tio n o f the 35 S d istrib u tio n but, on th e rm a l an n ealin g o f such a c r y s ta l, a su bstan tial in c re a s e in the 35SC>3 and 35SO¿ y ie ld o c c u rs (T a b le V I). T h is su ggests that the re a c tio n o f the S and CNS p r e c u r s o r s , p re s u m ably 35S =, 35S" a n d 35S °, w ith the V -ty p e c e n tre s , r e q u ir e s th e rm a l a ctiva tio n . H o w e v e r, the data fo r 32P -im p la n te d c r y s ta ls [1 3 ,1 5 ] s e e m s d iffic u lt to a ccom m od a te w ith this explan ation o f the o r ig in o f the phosphate and sulphate fra c tio n s (v. in fra ). T h e apparatus that w as used fo r the 35S im p lan tation [14] had p r e v io u s ly been used w ith v a rio u s c h lo rin e com pounds and it p ro v e d im p o s s ib le to decon tam in ate it c o m p le te ly fr o m a 35C1 beam . T h is m eant that the e ffe c t o f the 35C1/35S r a tio in the beam used had to be e x p lo re d , w hich su ggested the id ea o f oth er e x p e rim e n ts in which two s p e c ie s w e r e im plan ted. R e s u lts fo r v a rio u s 35C1/35S r a tio s and fo r p re -im p la n ta tio n o f c h lo rin e , oxygen and 32 S b e fo r e the 35 S a re shown in T a b le V II. T h e s e re s u lts show that o n ly a v e r y la r g e e x c e s s o f c h lo rin e atom im p lan tation has much e ffe c t on the 35S d istrib u tio n . W ith such e x c e s s , a g r e a te r p ro p o rtio n o f SOg and SO¿ p r e c u r s o r s fo r m on th e rm a l annealing. N e ith e r p re -im p la n te d 160 o r 32S much a ffe c ts the p ro p o rtio n o f 35 S as the SO| and SO| p r e c u r s o r s , but both tre a tm e n ts in c re a s e the p r o p o rtio n o f 35S° (C N S ' fr a c tio n ) at the exp en se o f oth er 35S =. T h is m ight be due to the 160 and 32S occu p yin g n e a r ly a ll the s ite s w hich can a c c o m m odate 35 S~.
5.
RANGE EFFEC TS
A p a r tic u la r ly in te re s tin g fe a tu re o f th is technique o f in v e s tig a tio n is the p o s s ib ility o f e x p lo r in g the w a y in w hich the d istrib u tio n o f fo r m s o f 35S o r 32P change w ith the d istan ce the im p lan ted ion has p en etra ted the c r y s ta l. R e s u lts have been re p o r te d both fo r 35S and 32P in N a C l and KC1, r e s p e c t iv e ly . The phosphorus w o rk [13] u sed an im p ro v e d etch in gstrip p in g technique [16] to r e m o v e s u c c e s s iv e la y e r s of the ch lo rid e c r y s ta l fo r a n a ly s is . In e s s e n tia l fe a tu re s the re s u lts fo r the tw o sy s te m s a re in a g re e m e n t and th ey r a is e a num ber o f in te re s tin g qu estions.
42
M ADDOCK
It is c le a r that the ra n g es o f 35S and 32P in th ese c r y s ta ls a re con 足 s id e r a b ly la r g e r than those n o r m a lly p re d ic te d . It is found that the p en e足 tra tio n o f the ions (u su ally o f betw een 20 and 60 k e V ) into the c r y s ta l is c o m p lic a te d by both d iffu s iv e e ffe c ts and, p ro b a b ly , m ovem en t along d is lo c a tio n s . Indeed, a s m a ll but e a s ily m ea su ra b le p ro p o rtio n o f im plan ted 35S, u sin g a 4 0 -k e V b eam , p en etra ted a 1 -m m -th ic k KC1 sin g le c r y s ta l [14] (~ 0.2%). T h e g e o m e tr ic a l d istrib u tio n o f im p lan ted phosphorus in KC1 changes w ith s to ra g e tim e , even when the im p lan ted sa m p les a re sto red at liq u id n itro g e n te m p e ra tu re [13]. The depth w ithin w hich the d eep est p e n e tra tin g 40% o f 32P is found at f ir s t in c r e a s e s , then d e c r e a s e s and fin a lly in c r e a s e s again du ring s to ra g e at ro o m te m p e ra tu re . The d istrib u tio n betw een the d iffe re n t s p e c ie s a lso changes w ith the depth o f p en etra tio n o f the ion s. W ith both 35S and 32P th e re ap p ea rs to be an in c re a s e in the p ro p o rtio n o f the m o re o x id iz e d s p e c ie s , the sulphate and phosphate p r e c u r s o r s , fo r the m o s t p en etra tin g ion s. T h e re also ap p ea rs to be som e c o r r e la tio n betw een the m ean p en etra tio n o f the ions and the p ro p o rtio n o f the P v p r e c u r s o r . T h is su ggests that th is s p e c ie s m ust be v e r y m o b ile , even at ra th e r low te m p e ra tu re s , w hich is h a rd ly co m p a tib le with its id e n tific a tio n as a c o v a le n tly bonded V ce n tre type o f s p e c ie s and is p erh ap s m o re in k eep in g w ith an in te r s titia l ca tio n ic phos足 phorus ion.
6.
P R E P A R A T IV E A S P E C T S O F IO N IM P L A N T A T IO N
G. W o lf has shown that m ic r o s c o p ic amounts o f the in te rc a la tio n com pounds o f the a lk a li m e ta ls with grap h ite can be p re p a re d by ion im p lan tation o f the a lk a li m e ta l ion s into the gra p h ite. It s e e m s p rob ab le that ato m ic and m o le c u la r ion s can be in trodu ced in this w ay, which could not be by the usual th e rm a l p re p a ra tio n o f in te rc a la tio n com pounds. R e fe r e n c e has a lre a d y been m ade to the p o s s ib le prod u ction o f h e te ro d in u clea r tra n s itio n m e ta ls ca rb o n yls; th ese have not y e t been p re p a re d by con ven tion al m ethods [8 ]. So lit t le data a re y e t a v a ila b le that the p o s s ib ilitie s o f the technique can h a rd ly be a s s e s s e d .
7.
C A T A L Y T IC E F F E C T S
W e a re a ll a w a re that e a r lie r studies o f the changes in c a ta ly tic a c tiv ity p rodu ced by sim p le ir r a d ia tio n w ith io n iz in g ra d ia tio n p ro v e d v e r y d is 足 appointing. L a r g e d oses w e r e n e c e s s a r y and the changes w e r e v e r y e a s ily th e r m a lly annealed. But gam m a ir r a d ia tio n is c le a r ly an in e ffic ie n t m ethod o f p rod u cin g s u p e r fic ia l changes. Ion im plan tation , on the oth er hand, should be h ig h ly e ffe c t iv e . In addition, w e can in trodu ce not o n ly changes in the p oten tia l e n e r g ie s o f the su rfa c e atom s but w e can io n -im p la n t one ch e m ic a l s p e c ie s into another. T h e r e would appear to be co n s id e ra b le lik e lih o o d o f p rod u cin g in te r e s tin g and p o s s ib ly e c o n o m ic a lly valu ab le e ffe c t s . In fa c t, th e re is a ll too lit t le fa ctu a l data. G. W o lf has shown that ion im p lan tation o f n ic k e l into a cob alt jo in t has a c o n s id e ra b le e ffe c t on its a c tiv ity as a h yd rogen ation c a ta ly s t [1 7 ], but not much e ls e has d evelop ed . A n in d u s tria l r e p o r t d e s c r ib e s changes in the s u rfa ce h ard n ess o f s te e l o w in g to im p lan tation o f n itrogen .
1А Е А -РЬ615/3
43
REFERENCES [1] MÜLLER, H., Angew. Chem. 6 (1967) 133. [2] MADDOCK, A.G., WOLFGANG, R.H., in Nuclear Chemistry 2. (YAFFE, L., Ed.), Academic Press, New York (1968) Chap.8. [3] ANDERSEN, T.. SORENSEN, G., Tians. Faraday Soc. 62 (1966) 3427. [4] ANDERSEN, T.,SÇfRENSEN, G., Nucl. Inst. Methods 38 (1965) 204. [5]COGNEAU, M., DUPLATRE, G.,VARGAS, J.I., J. Inorg. Nucl. Chem. 34 (1972) 3021. [6] ANDERSEN, T., LANGVAD, T.. SÇfRENSEN, G., Nature (London) 218(1968) 1158; Corr. Nature (London) 219 (1968) 544. С7] WOLF, G.K., FRITSCH, T., Radiochim. Acta 11_ (1969) 194. [8] JENKINS, G.M., WILES, D.R., Chem. Comm. No.21 (1972) 1177. [9] MADDOCK, A.G., MAHMOOD. A.J., Inorg. Nucl. Chem. Lett. 9 (1973) 509. [10] ANDERSEN, T., SÇfRENSEN, G., in Interaction of Radiation with Solids (BIFHAY, A., Ed.), Plenum Press, New York (1967) 373. [11] FREEMAN, J.H., KASRAI, М., MADDOCK, A.G., Chem. Comm. (1967) 979. [12] MADDOCK, A.G., MIRSKY, R.M., in Chemical Effects of Nuclear Transformations (Proc. Symp. Vienna, 1964) 2, IAEA, Vienna (1965) 41. [13] ANDERSEN, Т .Г EBBESSEN, A., Trans. Faraday Soc. 67 (1971) 3540. [14] KASRAI, M„ MADDOCK, A.G., FREEMAN, J.H., Ibid., 2108. [15] ANDERSEN, T.. BAPTIST A, J.L., Ibid.. 1213. [16] ANDERSEN, T., EBBESSEN, A„ Radiat. Effects 11_ (1971) 113. [17] WOLF, G.K., Report on 8th Int. Conf. on Low Energy Ion Accelerators and Mass Separators, Sweden 1973.
DISCUSSION J. D A N O N : Ion im p lan tation has been used fo r the study o f h y p erfin e in te ra c tio n s by M ôssb a u er s p e c tro s c o p y o r b y p ertu rb ed angular c o r r e la tio n , but u n til now the situ ation is s t ill not c le a r about the su rrou n din gs o f the im p lan ted a tom s. Although the techniqu es s e e m to be v e r y p ro m is in g , w e do not have a c le a r - c u t id e a o f what kinds o f re s u lts can be obtained w ith them . A . G. M A D D O C K : That is w h y I e m p h a sized the advantages o f d ea lin g w ith the channelled s p e c ie s ra th e r than d e a lin g w ith the ions that re m a in in the bulk o f the m a te r ia l — in this h o r r ib ly ill- d e fin e d zone n ea r the su rfa ce. S. A M IE L : U sin g the se ch an n ellin g e ffe c ts m a y in trod u ce a c e rta in s e le c t iv it y w hose con sequ en ces a re not u n derstood — again an ob scu re situation. A . G. M A D D O C K : You m a y be rig h t, but what you a re su ggestin g is o n ly a h yp oth esis. S. A M IE L : But a v e r y lik e ly one becau se you know v e r y lit t le about the m ech a n ism by w hich the ch an n ellin g is o c c u r r in g — the tra n s p o rt o f th ese s p e c ie s into the channels. It in tro d u ces y e t another unknown fa c to r when you study on ly these lo n g -ra n g e p a r tic le s . A . G. M A D D O C K : H o w e v e r, th ey a re at le a s t a c c e s s ib le to e x p e rim e n t w ithout g r e a t d iffic u lty . G. H A R B O T T L E : I w ish to m ake two com m en ts about io n -im p la n ta tio n e x p e rim e n ts v i s - à - v i s the c h e m is tr y o f the s o lid state. F i r s t , m o s t o f what you s e e , ex c e p t fo r the ch an n elled atom s, is a s u rfa ce e ffe c t. T h e re is a la r g e r p ro b le m than th is, h o w e v e r, fo r the su rfa c e in any im p lan tation
44
M ADDOCK
e x p e rim e n t is su b jected to a ra th e r h ea vy bom bardm en t o f atom s which have nothing to do w ith the e x p e rim e n ta l atom you a re tr y in g to fo llo w . E ven i f you produ ce a beam w ith high s p e c ific r a d io a c tiv ity , you in e v ita b ly c a r r y alon g m any in e r t atom s fo r each r a d io a c tiv e atom , even a fte r m agn etic s e le c tio n , etc. One con cen tra tes an im m en se ra d ia tio n dose in a v e r y s m a ll vo lu m e o f the c r y s ta l. O f c o u rs e , the channelled atom s p en etra te to a much g r e a te r depth in the c r y s t a l w h ere the ra d ia tio n dam age is much le s s . Second, w e have been in v o lv e d in a s e r ie s o f e x p e rim e n ts s im ila r to those o r ig in a lly published by V a r g a s r e la tin g to the p o s s ib ility o f h a lf- life v a r ia tio n s , and th ere a p p ears to be a v e r y good op p ortu n ity h e re to use ion im p lan tation to im bed 7Be atom s in a v a r ie t y o f c r y s ta ls . A . G. M A D D O C K : The m ention o f the h ea vy ra d ia tio n dam age p rom p ts m e to com m en t that it is p e r fe c t ly p o s s ib le w ith ion im p lan tation to look at the m a c r o s c o p ic changes in the m a te r ia ls , and to o b s e r v e re a c tio n s with each oth er o f tw o im plan ted s p e c ie s . W e have been con cern ed in our im plantation stu dies w ith 35S that w e have been u sin g a m ach in e, not our own, that is contam inated w ith c h lo rin e atom s. In v a ria b ly , w e have a substantial beam o f 35C1 +. W e have th e r e fo r e e x p lo re d the beh aviou r o f 35S im plan ted into sodium c h lo rid e w ith v a r y in g amounts o f accom p an yin g 35C1+ ions. F o r good m e a s u re , w e have a lso done the e x p e rim e n ts w ith 160 + ions becau se o f the lo n g -sta n d in g c o n tr o v e r s y about the distan ce oxygen p a sses into an a lk a li h a lid e c r y s ta l. T h is is another type o f e x p e rim e n t w hich is e a s ily a c c e s s ib le to im p lan ted ion techniqu es. S. A M IE L : I have one com m en t and one question. The com m en t has to do w ith im p lan tation into the su rfa ce o f a c r y s ta l. E v e ry o n e who w o rk s w ith an iso to p e s e p a ra to r is a w a re o f the p ro b le m o f the "b lo w -u p " o f the b eam at the s u rfa c e , e s p e c ia lly o f an in su la to r, becau se o f the buildup of a sta tic ch a rg e . T h is lea d s to d e c e le ra tio n o f the beam . The question co n cern s you r d iscu ssio n o f r e c o il into a s u rfa ce. W hen e v e r one d ea ls with the c o lle c tio n o f r e c o ils fr o m a su rfa ce into a gas, one s e e s that you do not c o lle c t in d ivid u a l atom s, but c lu s te r s , say w ith fis s io n fra g m e n ts fr o m C a lifo rn iu m . D o e s n 't this im p ly that a bundle o f atom s w ill be c a r r ie d alon g a c e rta in d ista n ce, one o r two o r th ree a tom ic d ista n ces, through the c r y s ta l? T h e m ech an ism o f r e a c tio n w ould then be even m o re co m p lic a te d becau se you have c a r r ie d alon g these bundles o f atom s. A . G. M A D D O C K : In a ll our e x p e rim e n ts w e have used a scanned beam in which the beam con tin u ally m o v e s o v e r a 10- to 20-m m d istan ce. U nder these con d ition s, w e had no trou b le with the b low -u p o f the beam . The second point is an in te re s tin g one, but th ere a re data on the subject fr o m p h y s ic a l m ea su rem en ts. In th ese e x p e rim e n ts , fo r in stan ce, r a r e ga ses have been im plan ted in m e ta ls and in a lk a li h a lid es. In stu dies o f the k in e tic s o f the subsequent r e m o v a l o f the r a r e gas fr o m the s o lid , it is p o s s ib le to d istin gu ish an a g g re g a tio n stage. T h e r e is a stage in which the "d e s o r p tio n " — i t 's not r e a lly d eso rp tio n — o c c u rs through the a g g re g a tio n o f s e v e r a l atom s m o r e o r le s s into a bubble, and then the bubble m o v e s to the su rfa c e . It w ould appear that th ere is a r e g io n o f dose e x p re s s e d as atom s p e r c m 2 in which the ra d ia tio n dam age does not b rea k down the la ttic e stru ctu re. The breakdow n o f the la ttic e o c c u rs ra th e r s h a rp ly so m ew h ere in the v ic in it y o f 1014 p a r tic le s/ cm 2 , w h ile our e x p e rim e n ts have been con fin ed to the 101:1-1 0 12 p a r tic le s / c m 2 le v e l, w e ll b elow the breakdow n point. Thus, w h ile you produ ce a la r g e num ber o f d e fe c ts , you a re n 't ru in in g the halide la ttic e .
IAEA-PL-615/3
45
S. A M IE L : T h e r e is a p ap er by M a c F a rla n e in w hich he t r ie d to d ep osit m o n o la y e rs o f a b e ta -d e c a y in g iso to p e on a s u rfa c e , and then attem pted to c o lle c t the r e c o ils fr o m the s u rfa c e . W hat he found w as that th e re is no v e lo c it y - s p r e a d in th ese c o lle c te d r e c o ils . The m o n o la y e rs w e r e p rodu ced b y a h eliu m je t tech n iqu e. In oth er w o rd s , the r e c o il e n e rg y has b een tr a n s fe r r e d to the su rfa ce and th e re is no net k in etic r e c o il. The on ly r e c o il o f these s p e c ie s is fr o m ch a rge re p u ls io n , with no d istrib u tio n o f v e lo c it ie s . The in te ra c tio n o f r e c o il p a r tic le s at the s u rfa ce betw een a gas and a s o lid is s t ill in a ra th e r ob scu re situation. Â . G. M A D D O C K : I f our im plan ted m a te r ia ls re m a in e d at o r v e r y n ear the s u rfa ce w e should have obtained quite d iffe r e n t re s u lts in the fo llo w in g e x p e rim e n ts . W e have r e lie d on the ran ge o f the in c o m in g ion s b ein g not one o r two but s e v e r a l la ttic e d ia m e te rs in sid e the s u rfa c e . W ith sulphurim p lan ted c r y s ta ls exp osed to a ir , th e re w as no ev id e n c e w h a ts o e v e r fo r any oxid ation o f the sulphur. I f the sulphur has been stuck in the f ir s t la ttic e unit o r tw o, you could have an ticip ated som e oxidation . S. A M IE L : U n less th ere w as an nealing o f the s u rfa ce on a v e r y sh ort t im e - s c a le , u sin g the e x tr a e n e r g y le ft in the s u rfa ce. G. H A R B O T T L E : A lr e a d y , c o n s id e ra b le pu blished in fo rm a tio n fr o m ion im p lan tation is a v a ila b le . One such e x p e rim e n t is the o b s e rv a tio n that, up to a s u r p r is in g ly high e n e rg y , the atom s ju st bounce back fr o m the su rfa c e w ith no p en etra tio n at a ll. The im p lan tation o f lo w - e n e r g y ions is p ro b a b ly a p h y s ic a l im p o s s ib ility . T h is has been known fr o m the e a r ly days in hot atom e x p e rim e n ts when it w as found that b ro m in e atom s w ith lon g m e a n -fr e e -p a th s w ould not stick to s u rfa c e s . T h e y had to tr e a t the su rfa ce w ith s ilv e r in oth er to hold the b ro m in e atom s w ith a c h e m ic a l re a c tio n . T h is w as the situ ation in the o r ig in a l W e x le r - D a v ie s e x p e rim e n t. G. S T O C K L IN : Som e in fo rm a tio n can a ls o be obtained fr o m sp u tterin g e x p e rim e n ts . H e re it is known that c lu s te rin g o c c u rs , but that it is a r e la t iv e ly r a r e event. I would now lik e to re tu rn to the c a ta ly tic asp ects o f ion im plan tation pointed out in D r. M a d d o ck 's p ap er. Is this ju st ra d ia tio n d am age, o r is the im p lan ted m a te r ia l r e a c tin g , o r is it both? D oes it have anything to do w ith hot atom c h e m is try , o r is it ju st h e a v y ion ra d ia tio n dam age? O f c o u rs e , c a ta ly s is o c c u rs at the s u rfa ce and h e a v y ion ra d ia tio n dam age m igh t be m o re e ffe c t iv e than g a m m a -r a y ra d ia tio n d am age, but is th ere any hot atom c h e m is tr y in v o lv e d in th is at a ll? A . G. M A D D O C K : T h is is no m o r e than a gu ess. P r e v io u s w o rk has shown that changing the d istrib u tio n o f p o ten tia l e n e r g ie s o f the s u rfa ce atom s w i l l change the c a ta ly tic a c tiv ity o f the su rfa c e . H o w e v e r, and this is w h y in d u s tria lly m inded p eop le a re not too in te re s te d , these changes anneal back ra th e r qu ick ly. The annealing m a y not o c c u r on a rap id tim e s c a le fo r la b o r a to r y ch e m is ts — a few days fo r in stan ce — but in d u s tria l h etero g en eo u s c a ta ly s ts m ust go on w o rk in g fo r s e v e r a l m onths. I have a fe e lin g that one should not hope fo r too much fr o m the point o f v ie w o f using the c a ta ly tic e ffe c ts induced by ra d ia tio n c h e m is try . H o w e v e r, th ere should be som e in te r e s tin g c h e m ic a l e ffe c ts fr o m putting one c h e m ic a l ele m e n t into another. G. S T O C K L IN : You have a lre a d y m en tion ed the in te rc a la tio n com pounds studied at H e id e lb e r g by G erh a rd W o lf. T h e re has also been a p r a c tic a l a p p lica tio n in the tra n s p o rt o f fis s io n p rod u cts through gra p h ite. It is im p orta n t in p r a c tic a l r e a c to r tech n ology.
46
MADDOCK
H. M IG G E : The p ro b a b ility o f gettin g good re s u lts is not v e r y high. G rap h ite is a v e r y d iffic u lt m a te r ia l to w o rk w ith, and in te rc a la tio n com pounds w ould be d iffic u lt to handle in th is m a trix . R e tu rn in g to the c a ta ly tic e ffe c t you d e s c rib e d , an e ffe c t which o ccu rs a fte r you have fin ish ed ir r a d ia tin g — what w ould happen w ith c a ta ly tic e ffe c ts du rin g ir r a d ia tio n ? I am in te re s te d in the p ro b le m o f n u clear r e a c t o r s in which im p lan tation o c c u rs du ring o p era tio n . M ight th ere also be .c h e m ic a l re a c tio n s o c c u r r in g at the s u rfa ce o f a m a te r ia l d u rin g the op e ra tio n o f a n u clea r r e a c t o r fr o m this continuous io n -im p la n ta tio n bom b ard m en t, e ffe c ts that would not o c c u r at the sam e s u rfa ce im m e d ia te ly a fte r ir r a d ia tio n stops becau se the c a ta ly tic e ffe c t w ould have been annealed out? A . G. M A D D O C K : The an sw er to this question is that tim e on ion im p lan tation m a ch in es is d iffic u lt to obtain, and in-situ e x p e rim e n ts r e q u ire much m o r e tim e than bom bardm en t w ith c h e m ic a l a n a ly s is a fte rw a rd s at som e oth er lo ca tio n . It is o b vio u s, h o w e v e r, that ion im p lan tation is quite p ertin en t to m any con tain er p ro b le m s in n u clea r r e a c to r s , o r fu sion d e v ic e s , o r anything e ls e in which the m a te r ia l is s u ffe r in g c o n s id e ra b le ra d ia tio n dam age. I f you o b s e r v e e ffe c ts a fte rw a rd s — and th ese have been found — th e re is c e r ta in ly an e x c e lle n t chance o f fin d in g oth er e ffe c ts o c c u r r in g du ring the p r o c e s s o f ir r a d ia tio n . A ft e r - e f f e c t s o f another kind b esid es c a ta ly tic e ffe c ts a re o b s e rv e d . W ith r a r e - e a r t h o x id es and oxygen ions, one fin ds that the o x y g e n -im p la n te d r a r e - e a r t h o x id e s m aintain a high o x id iz in g p o ten tia l fo r a v e r y lo n g tim e , a lm o st in d e fin ite ly a fte rw a rd s . I cannot t e ll you w h eth er these e ffe c ts a re caused b y a r a r e - e a r t h p e ro x id e , o r a h ig h e r oxid e than has so fa r been id e n tifie d , o r what. G. S T Ô C K L IN : It is c e r ta in ly c le a r that the p ro b le m o f p a r tic le in te ra c tio n w ith the w a ll o f a co n ta in er is a v e r y im p o rta n t p ro b le m fo r fu sion r e a c t o r s , and w e w ill no doubt h ear m o re fr o m M r. M ig g e la te r about th is su bject. M any o f the im p o rta n t p a r a m e te r s a re known to the p h y s ic is ts w o rk in g on these p ro b le m s , but th e re a re som e p ro b le m s w hich th ey s im p ly d e s c rib e as "c h e m ic a l s p u tte rin g ", and w hich w e m igh t c a ll io n -im p la n ta tio n hot atom c h e m is try . J .M . P A U L U S : T h e r e a re som e s p e c ia l advan tages o f beam im plan tation m eth ods with o rg a n ic ta r g e ts , e s p e c ia lly when the e n e rg y is red u ced below a few k ilo - e le c t r o n v o lts . A s you red u ce the in itia l e n e r g y o f the ion s, som e in te r e s tin g o b s e rv a tio n s can be m ad e. W ith " t r u e " hot atom re a c tio n s , fo r ex a m p le tritiu m ion s plus s o lid cyclo h ex a n e, the produ ct y ie ld d isa p p ea rs at an e n e r g y c o rre s p o n d in g to the th resh o ld o f the re a c tio n , and at h igh er e n e r g ie s the y ie ld s a re independent above a few tens o f e le c tr o n v o lts . H o w e v e r, when cage re a c tio n s o c c u r, the y ie ld continues to in c re a s e to e n e r g ie s up to a few thousands o f e le c tr o n - v o lts . One can distin gu ish in this c a se b etw een tru e hot re a c tio n s and cage re a c tio n s . G. S T Ô C K L IN : P le a s e ex p la in again how th ese re a c tio n s can d iffe r e n tia te betw een tru e hot re a c tio n s and cage re a c tio n s . J . M . P A U L U S : W hen cage re a c tio n s o c c u r, one cannot d etect the tru e hot re a c tio n s in th e ir p re s e n c e — the y ie ld s continue to in c re a s e w ith in c r e a s in g ion e n e rg y . H o w e v e r, w ith tritiu m ions the M e n z in g e rW o lfg a n g re s u lts show that th e re is no fu rth e r in c r e a s e above about 20 e V , thus c o rre s p o n d in g to the tru e hot y ie ld , w ith v e r y lit t le o r no con tribu tion fr o m cage r e a c tio n s . W e have found in our la b o r a to r y no e ffe c t on the y ie ld s with tritiu m ions fo r k in etic e n e r g ie s b etw een 200 and 6000 e V . W hen
IAEA-PL-615/3
47
ca ge r e a c tio n s o c c u r, the a sy m p to tic y ie ld va lu e w ith an io n -b e a m m achine d o e s n 't re a c h a m axim u m u n til about 10 keV . F .S . R O W L A N D : A r e you su ggestin g that, in the iodin e beam s y s te m s , you cannot get any in fo rm a tio n about the " t r u e " d ir e c t hot atom su bstitu tio n s b ecau se these re s u lts a re a lw a ys ob scu red by the cage e ffe c ts at a ll e n e r g ie s ? J. M . P A U L U S : Y e s . You cannot distin gu ish the tru e hot atom re a c tio n s in the iodin e system . G. S T Ü C K L IN : D r. M addock did not d iscu ss the fie ld o f im p lan tation o f ions into o rg a n ic s y s te m s , but Pau lu s has ju st touched upon it now. How in fo rm a tiv e a re io n -im p la n ta tio n stu dies in o r g a n ic s y s te m s ? W e should c o n s id e r not on ly the e x p e rim e n ts o f P au lu s, but a lso those o f L em m o n , who is im p la n tin g carbon ion s down to ra th e r low e n e r g ie s . A . P . W O L F : I don 't think L e m m o n 's w o rk gets at the p ro b le m s o f ion im plan tation . T h is is one situ ation in w hich o rg a n ic sy s te m s a re r e a lly v e r y much m o r e co m p le x than the in o rg a n ic s y s te m s . In L e m m o n 's w o rk , you n e v e r r e a lly know w h eth er the r e a c tin g s p e c ie s is an ion o r a n eu tra l s p e c ie s . N e a r the end o f the ran ge the ion and the n eu tra l can be ex p ected to r e a c t in such d iffe r e n t w a y s , with so m any prod u cts, that it is e x t r e m e ly d iffic u lt to u n scra m b le the m any p r o c e s s e s goin g on in the ta rg e t. It is h ard to get fundam ental in fo rm a tio n about in d ivid u a l p r o c e s s e s . A . G. M A D D O C K : I am in clin ed to a g r e e . H o w e v e r, even w ith v e r y co m p lic a te d s y s te m s , one can get out the odd fra g m e n t o f in te re s tin g in fo rm a tio n . W e have du plicated m o st o f the things w hich L e m m o n has d e s c rib e d . W e have a lso lo o k ed at the re a c tio n s o f carb on ions w ith v e r y sim p le ta r g e ts such as w a te r. A ft e r lo o k in g at the re a c tio n s w ith benzene and with w a te r in ou r carb on ion m ach in e, w e thought that it m igh t be in te r e s tin g to lo o k at a ta r g e t c o n s is tin g o f an a lk y l-su b stitu ted ethylene analogue — eth ylen e it s e lf w a s too v o la tile fo r our p u rp ose. P e rh a p s I'm a bit c y n ic a l, but I 'v e a lw a ys thought that S k e ll's w o rk w as ju st a lit t le hard to s w a llo w , and we thought that w e m igh t t r y to approach the sam e system through our ion e x p e rim e n ts . W e had a c o n s id e ra b le amount o f trou b le in g ettin g a balan ce b etw een the in c o m in g 14C a c tiv ity and the amount a ctu a lly found in the prod u cts. U ltim a te ly , on the grounds o f m a t e r ia l b ala n ce, we w e r e le d to the con clu sion that som e o f th ese 14C ions w e r e s u rv iv in g u n rea cted on the s u rfa ce o f th is v o la t ile eth y le n ic com pound, and p resu m a b ly went to g e th e r d u rin g d is tilla tio n to fo r m som e kind o f grap h ite nucleus in an in v o la tile fo r m . T h is le d us then to do the sam e e x p e rim e n t — carbon ion bom b ard m en t o f e th y le n ic analogu e, and then w e d is tille d benzene on to the su rfa c e o f the a lre a d y b om barded ta rg e t. W hen we m e lte d this s y stem , w e then found produ cts in the s y stem that a re the sam e as those found fro m the re a c tio n s o f carbon atom s o r a g g re g a te s with ben zen e alon e. O ur con clu sion is that som e o f th ese s p e c ie s s u rv iv e d on the s u rfa ce o f the eth ylen e analogue, and when m e lte d w ith b en zen e, r e a c te d w ith b en zen e. W e don 't see any o th e r p o s s ib le conclusion. G. S T O C K L IN : W hen you a re d ea lin g w ith carbon atom s in p i- e le c t r o n s y s te m s , you can fo r m r e la t iv e ly stable adducts which can u ndergo fu rth e r r e a c tio n . A . P . W O L F : Has th is w o rk been published y et? I'm cu riou s to see the d e ta ils . W ith S k e ll's v a p o u r-d e p o s ite d carb on , the carbon is d ep osited on a s u rfa ce and is p ro b a b ly a g g re g a te d . T h is is an a re a in w hich hot atom c h e m is try is in an e x t r e m e ly fa v o u ra b le p o sitio n becau se in m any w ays
48
MADDOCK
it is e a s ie r to in te r p r e t what happens. A ch e and I did an e x p e rim e n t som e tim e ago in which w e bom barded fr o z e n neon w ith 3 0 -G eV p roton s, fo rm in g n C by s p a lla tio n o f the neon. W e argu ed that in th is cold m a trix the n C would not r e a c t w ith anything, and w e could then fla s h -e v a p o ra te this carbon in an o rg a n ic m a tr ix and perh aps get som e re a c tio n s o f th e rm a liz e d carbon . In e v e r y case a ll w e e v e r got w as n CO. T h is is n 't r e a lly so s u rp ris in g now. T h e re is now a ra th e r good body o f k in etic data in the lite r a tu r e on carbon atom s in the (3P ) state w hich show that the re a c tio n w ith 0 2 is 105 tim e s fa s te r than with anything e ls e you can put in th ere. You can n e v e r get a s y stem with so lit t le oxygen in it to keep the re a c tio n o f n on -a d so rb ed , th e r m a liz e d carbon atom s fr o m o c c u r r in g with oxygen. On a s u rfa c e , o f c o u rs e , the system is quite d iffe re n t. W hat is in te re s tin g h e re is the question o f how the carbon atom adducts it s e lf to the s u rfa ce. One n e v e r fin ds in the vap ou r dep osited carbon re a c tio n s the two c h a r a c te r is tic hot p rodu cts fr o m carbon atom s: eth ylen e and a c ety len e. W hen one does find th ese s p e c ie s , th ey com e fr o m the re a c tio n s o f the m u lti-a to m s p e c ie s , C 2, C 3, e tc. I t 's an in te re s tin g fie ld , but it is s t ill ra th e r ob scu re. F . S . R O W L A N D : The sam e p r e fe r e n c e fo r re a c tio n with m o le c u la r oxygen is found fo r t r ip le t m eth ylen e. In those s y s te m s , it is also not p o s s ib le to s ta rt with a s y stem fr e e o f m o le c u la r oxygen , but you can burn the oxygen out ju st by s ta rtin g the re a c tio n . The f ir s t t r ip le t m eth ylen e s p e c ie s take out the oxygen , and the re m a in d e r r e a c t as t r ip le t m eth ylen e w ith the oth er s p e c ie s in the s y s te m . W hat happens i f you t r y such a burn out e x p e rim e n t with carbon atom s? A . P . W O L F : You c a n 't do the e x p e rim e n t. In the ty p ic a l system , y o u 'r e d ea lin g with one p a rt p e r b illio n o f m o le c u la r o xygen , and on ly lO 10 to 1011 n C atom s. W e have a lso used the burnout technique in som e ra d ia tio n e x p e rim e n ts , but w e could not do it h e re . F .S . R O W L A N D : I w as thinking not about the 11С e x p e rim e n ts , but about those in which the k in etic ra te m ea su rem en ts w e r e m ade. A . P . W O L F : I h a ven 't thought about this p o s s ib ility in d e ta il, and do not know o f a s y stem w h e re it w ould w ork. G. S T O C K L IN : P e rh a p s w e can te n ta tiv e ly conclude that the im p la n ta tion o f ions into o rg a n ic s o lid s , putting it m ild ly , le a v e s som e p ro b le m s open. And that it is qu estion able w hether these sy s te m s can p ro v id e v e r y m any a n sw ers to the b a sic p ro b le m s o f hot atom c h e m is tr y its e lf. N . G E T O F F : M any p eop le a re v e r y in te re s te d in the produ ction o f o rg a n ic s e m i-c o n d u c to rs . P e rh a p s ion im p lan tation on s e m i-c o n d u c to rs m igh t p ro v id e som e in fo rm a tio n about the re a c tio n m ech a n ism s in such sy s te m s . A . G. M A D D O C K : It is p o s s ib le that ion im p lan tation in such m a te r ia ls m igh t avoid the d is to rtio n that is o b s e rv e d w ith the im p lan tin g o f im p u ritie s by m o re standard c h e m ic a l tech n iqu es.
I A E A - P L - 6 1 5/4
IMPLANTATION OF ATOMS INTO MATERIALS DURING REACTOR OPERATION H. A N D R E S E N , W. L U T Z E , H. M I G G E Hahn-Meitner-Institut fur Kemforschung Berlin G m b H , Berlin, Federal Republic of G e rmany
Abstract IMPLANTATION OF ATOMS INTO MATERIALS DURING REACTOR OPERATION. Nuclear reactions which lead to the formation of hydrogen and helium in nuclear reactor materials are discussed in detail. The extent of materials damage resulting from these, as well as other atoms implanted in reactor materials as a result of hot atom reactions, is discussed. The relevance of neutron-induced hot atom reactions in fusion technology is also discussed.
IN T R O D U C T IO N In n u clea r r e a c to r s e n e r g e tic atom s a re p rodu ced by v a rio u s p r o c e s s e s , (n , a ) - and (n ,p )-r e a c tio n s a re , b e s id e s the fis s io n re a c tio n , m ost im p ortan t in n u clea r tech n o lo g y . H eliu m as w e ll as h yd rogen cau ses e m b rittle m e n t o f m e ta llic m a te r ia ls . The in flu en ce o f h eliu m on the s w e llin g o f s tru c  tu ra l m a te r ia ls (fo r m a tio n o f v o id s ) is a s e rio u s m a tte r in the develop m en t o f fa s t b r e e d e r r e a c to r s . M uch r e s e a r c h w o rk has b een p e r fo r m e d with r e a c t o r m a te r ia ls to study bulk e ffe c ts due to (n ,p )- and (n ,a )-r e a c tio n s . L e s s attention has been paid to n e a r -s u r fa c e re g io n s o f these m a te r ia ls w h e re im p lan tation fr o m gaseou s o r liq u id coolan ts, bondings o r m o d e ra to rs , m a y c o n s id e ra b ly in c r e a s e the co n cen tra tion o f im p u rity a tom s. T o our kn ow ledge the e ffe c ts o f im p lan tation have been u n d erestim a ted in the past. Som e to p ic s o f p r a c tic a l s ig n ific a n c e inclu de s u rfa c e e r o s io n o f stru ctu ra l m a te r ia ls o f co n tro lle d th e rm o n u c le a r r e a c to r s , im p lan tation o f bonding gas into UO 2 o f fu el pins, d estru ctio n o f p r o te c tiv e coatin gs, etc.
IM P L A N T A T IO N PR O C E SSE S Som e n u clea r re a c tio n s lea d in g to im p lan tation a re g iv e n in T a b le s I and II. R e c e n tly the authors [1] p e r fo r m e d som e w o rk on io n im p lan tation s im u la tin g e la s tic neutron s c a tte rin g . A sch em a tic d ia g ra m o f this p r o c e s s is shown in F i g . l . T h e ex a ct ca lcu la tio n o f im p lan tation r a te s and the co rre s p o n d in g depth d istrib u tio n s is a r e la t iv e ly sop h istica ted p ro b le m . T h r e e typ es o f r e a c  tions a re to be distin gu ish ed fo r the m a th em a tica l trea tm en t: (1)
R e a ctio n s with th e rm a l neutrons
In the s im p le s t case m o n o e n e rg e tic p a r tic le s a re p rodu ced in a hom ogeneou s m edium , e .g . 6L i (n,o-)T o r 10B (n , a ) 7L i and e n te r a second
49
50
A N D R E S E N e t al.
T A B L E I. N E U T R O N IN T E R A C T IO N S L E A D IN G T O IM P L A N T A T IO N IN T O R E A C T O R C O M P O N E N T S (C L A D D IN G S , C O O LIN G P IP E S , S T R U C T U R A L M A T E R IA L S ) U N D E R T E C H N O L O G IC A L C O N D ITIO N S Medium Helium (coolant for HTGRs, fast breeders, fusion reactors, and bonding gas in fuel elements)
Interaction, process
Implanted particles
He (n, n) He
4 He
H(n,n) H D(n, n)D 0(n,n)0 160(n, a ) n C
H D 16o 4He, 13C
H(n,n)H
H
C02 (coolant in magnox-reactors and AGRs)
C(n.n) С 0 (n, n)0 160(n, a ) 13C
12С 160 4He, 13C
Lithium (coolant, moderator, and tritium-breeding material for fusion reactors)
6Li(n, a) T 7Li (n, a) T Li(n,n)Li
4He. T 4He, T 6Li, 7Li
Li-Be-fluoride (molten salt reactor coolant, moderator and tritium breeding material for fusion reactors)
6Li(n, a)T 7Li(n.n'a)T 9Be(n, 2n) 2He Li(n#n) Li Be (n, n)Be F(n, n) F
4He, T 4He, T 4He 6Li, 7Li 9Be
Al Li-alloy (tritium-breeding material for fusion reactors)
6Li(n, a) T 7Li(n, n’ a) T Li(n, n)Li
4He, T 4He, T 6L i,7Li
Sodium (coolant in fast breeders) Potassium (coolant in fast breeders)
23Na(n. a )20F 39К (n, а ) “ c i
4He 4He
H20 and D20 (coolant and moderator)
Metal hydrides (moderators in reactors and for fusion reactors)
19 F
hom ogeneous m ediu m . D iC o la and M a tzk e [2] have published a solu tion o f th is p ro b le m assu m in g lin e a r e n e rg y lo s s . A m o r e g e n e r a l solution, takin g into account the d iffe re n t ran ge functions fo r the tw o m ed ia, is g iv e n in R e f. [ 1]. (2)
T h re s h o ld re a c tio n s
The secon d type o f re a c tio n is m o re c o m p lica ted b ecau se the re s u ltin g p a r tic le s have e n e r g y d istrib u tio n s re la te d to the neutron sp ectru m and the re a c tio n c r o s s - s e c t io n fu n ction s. B ir s s [3] show ed som e con cen tration p r o file s o f h eliu m in m e ta ls as the re s u lt o f (n ,o )-r e a c tio n s in the coolant. It is , h o w e v e r, not quite c le a r how he ca lcu la ted th ese p r o file s .
51
IA E A -P L -6 1 5 / 4
T A B L E II. I M P L A N T A T IO N IN T O S O LID S A M P L E S D U R IN G R E A C T O R IR R Á D IA T IO N S U N D E R E X P E R IM E N T A L C O N D ITIO N S Sample environment during irradiation Air
Helium (for irradiation experiments at higher temperatures)
H20,
(3 )
d 2o
Interaction process
Implanted particles
14N(n.n)uN 160(n, n)160 160(n, a f 3C
4He, 13C
He(n, n)He
4He
H(n,n)H D(n, n)D 0(n,n)0 160 (n, a )13С
H D 0 4He, 13C
14N 160
Experiment Activation, radiation damage studies Creep tests. radiation damage studies, corrosion studies Corrosion studies, radiation damage studies
E la s t ic neutron s c a tte rin g
In m o st r e a c to r s the an gu lar d is trib u tio n o f the neutrons is alm o st is o tr o p ic . In th ese ca s e s the im p lan tation ra te depends upon m a ss, density, and s c a tte rin g c r o s s - s e c tio n o f the kn ocked-on p a r tic le as w e ll as upon the stopping p o w e r o f the s c a tte rin g m edium , the flux, and the e n e rg y sp ectru m o f the neutrons. In addition, the im p la n ta tion p r o file depends on the stopping p o w e r o f the ta r g e t. T h e eva lu a tion o f an im p lan tation ra te is ra th e r sim p le i f slow in g down in the s c a tte rin g m ediu m can be n e g le c te d and i f the s c a tte rin g p r o c e s s is is o tr o p ic . C alcu lation s w e re p e r fo r m e d under th ese con dition s [4 ]. K n ock ed -on atom s w ith e n e r g ie s le s s than Eo = 30 k eV w e re not taken into account. The im p lan tation ra te R is then: oo
R = c< J
(l
-
Е (Е п)ф ( E n) d E n
( 1)
4b
w h ere r¡0 = E 0 /( 1 - ce), a = (A - 1 )/ (A + l ) 2 , A = m a ss num ber o f im plan ted p a r tic le , £ = m a c r o s c o p ic s c a tte rin g c r o s s - s e c tio n , and<i>(En) dE n is the neutron flu x in d E n around E n. T h e constant c ' is a g e o m e tr ic fa c to r . T h e im p lan tation p r o file fo r th ese con dition s is a lso d e s c rib e d in R e f. [4 ]. I f slo w in g down in the s c a tte rin g m edium and a n iso tro p y o f s c a tte rin g a re taken into account, the im p la n ta tion ra te is g iv e n by E q .(2 ):
R =£ En
Фне( Еп)
(2)
52
ANDRESEN et al.
Wq U
C oolant
WaLL
y / '" \
\
X
С)
n
FIG. 1. A particle, e.g. reactor coolant atom, knocked-on by a neutron loses its kinetic energy along its path x' +x and comes to rest at the distance y beneath the surface of the structural material.
фHe was d e r iv e d [5] and is g iv e n by
фНе ( Е п) =
4 °N ( Е п) фм ( E n) N Lp M _1h x
(3)
Emax x = ;-------- -------t-max Y
p (E ) A E
У
¿_,
x (E ) p(E ) Д Е
(4)
°
о T h e fo rm u la is g iv e n in th is fo r m to use neutron flu x data fo r s p e c ia l r e a c t o r c o n fig u ra tio n s, e .g . a fu sion r e a c to r blanket. T h e data a re p ro v id e d by neutron tra n s p o rt cod es, such as the code A N IS N [6 ]. UN ( E n)
= e la s tic s c a tte rin g c r o s s - s e c t io n o f neutrons o f e n e rg y E n fo r heliu m (fr o m E N D F -ta p e ) [7] фы ( E n) = neutron flu x in the e n e rg y group n o f A N IS N com p u ter code Nl = A v o g r a d o 's num ber, p = den sity, and M = m o le c u la r w eigh t o f He E, E max = (m a x im u m ) k in etic e n e rg y t r a n s fe r r e d fr o m a neutron to helium x (E ) = obtained fr o m the ra n ge ta b le s o f N o r th c liffe and S ch illin g [8] x = w eigh ted m ean ran ge o f kn ocked-on h eliu m s p(E ) = p r o b a b ility to tr a n s fe r the e n e rg y E by knock-on. It is not a constant due to the a n iso tro p y o f s c a tte rin g h = is o tr o p ic abundance
IM P L A N T A T IO N O F IN E R T A T O M S H eliu m w as ir r a d ia te d w ith neutrons in a r e a c t o r [4 ]. The knockedon h eliu m atom s w e r e im p lan ted fr o m the gas phase into s ilv e r fo ils . T h e m e a s u re d valu e o f the im p la n ta tion ra te R w as about h a lf the value
53
IA E A -P L -6 1 5 / 4
T A B L E III. D E U T E R IU M C O N T E N T O F S IL V E R DISC O F 99.999 P U R IT Y A F T E R N E U T R O N IR R A D IA T IO N S IN D E U T E R IU M O F 400 to r r . N eu tron d ose (E > 0.1 M e V ): 4 X 1017 c m '2 . T h e va lu es c o rresp o n d in g to 99.9 s ilv e r a re g iv e n in b ra c k e ts . F r o m R e f . [4 ]. Sample distance (cm)
Stacked on one another
0.1
Deuterium atoms (per cm2 of sample surface)
3 x 1014 (3 X lO1*)
8 x 1014 (6 X 1015)
0.5 4 X 1015 -
ca lcu la ted fr o m E q . ( l ) . T h e d iffe r e n c e w as p a rtly attrib u ted to b a ck s c a tte rin g , p r o c e s s e s lik e spu tterin g o r d iffu sio n b ein g c o n s id e re d to be n e g lig ib le . T h is w as in fe r r e d fr o m the fact that the m ea su red helium con cen tra tio n p r o file in the s o lid a g re e d w e ll w ith c a lcu la tio n s. C o n sid era b le qu an tities o f h eliu m w ill be im plan ted into the w a lls o f h eliu m c o o lin g p ip es, e .g . in C T R blan k ets. The v e r y high neutron flu x es n ea r the f ir s t w a ll (s o m e 1015 n / cm 2 • s, E > 0.1 M e V ) o f such d e v ic e s lea d s to im p lan tation ra te s o f about 1011 h eliu m a to m s / c m 2 • s a c c o rd in g to E q .(2 ) [5 ]. In the ca se o f niobium as a w a ll m a te r ia l h eliu m is im plan ted to a m axim u m depth o f 30 (Jm. A t high te m p e ra tu re s the a g g lo m e ra tio n o f h eliu m le a d s to s u rfa ce dam age (b lis te r in g e tc .) s im ila r to that a lre a d y d iscu ssed fo r the w a lls fa c in g the p la sm a o f a C T R d e v ic e . W ith in the im p la n ted la y e r h eliu m g e n e ra tio n by n ,a -re a c tio n s is n e g lig ib le com p a red w ith im p la n ta tion [5 ].
I M P L A N T A T IO N OF C H E M IC A L L Y A C T IV E A T O M S T h e im p la n ta tion o f c h e m ic a lly a c tiv e atom s is a fre q u e n tly o c c u rrin g p r o c e s s in r e a c to r s . A l l is o to p e s o f h yd rogen a re in v o lv e d . In the blanket o f a fu sion r e a c t o r im p lan tation o f tr itiu m in to s tru c tu ra l m a te r ia l is caused by both the 7L i(n .n 'c c jT and 6L i(n ,a )T - r e a c t io n s . The m ean implant; tio n ra te o f tritiu m can be estim a te d to be 4 X 10 9 cm "2 • s"1 in ca se o f a 5-G W (th ) fu sion r e a c t o r w hich is e x p ected to produ ce 820 g tritiu m / d a y in the blanket. The tr itiu m im p lan tation ra te is subject to lo c a l v a ria tio n s w ithin the blanket and m ay in c r e a s e m o r e than one o r d e r o f m agnitude. A lm o s t nothing is known about im p lan tation o f c h e m ic a lly a c tiv e atom s a fte r neutron re a c tio n s . N eu tron ir r a d ia tio n s o f d eu teriu m at 80°C le d to take-u p o f gas in s ilv e r d is c s [4 ]. N o take-u p w as found without ir r a d ia tio n . H o w e v e r, in con tra st to the re s u lts obtained in the case o f A g - H e e x p e rim e n ts (s e e above) the amount o f d eu teriu m w as th re e o r d e r s o f m agnitude h igh er than that ca lcu la ted fr o m E q . ( l ) . A s can be seen fr o m T a b le III the d eu teriu m content in c re a s e d p r o p o rtio n a lly to the th ickn ess o f the d eu teriu m g a s - la y e r ab ove the sam p le s u rfa c e (= in c r e a s in g sam p le d is ta n c e ). T h is dependence on the g e o m e tr ic a l a rra n g e m e n t o f the sam p le and hence on the im p la n ta tio n ra te show s that the im p in gin g p a r tic le s induce a seco n d a ry p r o c e s s , w hich lea d s to a take-u p o f about 103 - 104 D atom s p e r in ciden t D ion . N e ith e r the gam m a n or the neutron flux is changed
54
A N D R E S E N e t al.
s ig n ific a n tly with the g e o m e tr ic a l a rra n g em en t. R a d ia tion dam age is exclu ded as an exp lan ation fo r the in c re a s e d take-u p. A c o m p a riso n based on the ca lcu la tio n s o f K u lcin sk y et al. [9] shows that the d isp lacem en t d en sity is m a in ly g iv e n by the neutron ir r a d ia tio n . T h e im plan ted p a r tic le s in c r e a s e the d isp la cem en t d en sity on ly s lig h tly . T h e r e fo r e , it is con cluded that c h e m ic a l e ffe c ts a re in v o lv e d in the p r o c e s s . K lu eh and M u llin s [10] o b s e rv e d a re a c tio n - in the absence o f a ra d ia tio n fie ld — o f h yd rogen w ith o xygen d is s o lv e d in s ilv e r at 800°C and w a te r w as p r e cip ita ted in the so lid . Som e re la tio n o f th is p r o c e s s to the take-u p o f d eu teriu m cannot be exclu ded, though the ir r a d ia tio n te m p e ra tu re was r e la t iv e ly lo w (80°C ). T h e w o rk on the c h e m ic a l e ffe c ts o f ion im p lan tation is n eith er ex te n s iv e n or c le a r as fa r as m e ta ls a re con cern ed, but th e re is evid en ce that unusual ch e m ic a l re a c tio n s o c c u r during im plan tation . In 1957 Suzuki found that e ven noble m e ta ls lik e s ilv e r and platinum o x id iz e during cathodic sp u tterin g [1 1 ]. It should be noted that he r e p o rte d that the oxid ation depends on the o xygen p re s s u re m o re than on spu tterin g tim e , a fa ct w hich in d ic a te s c a ta ly s is o f the c h e m ic a l re a c tio n . T r i l l a t and c o - w o r k e r s o b s e rv e d ra p id re a c tio n s o f o x ygen - and n itro g e n -io n s w ith thin m e ta l fo ils [12] lea d in g to ox id es and n itrid e s . K e lly and L a m [13] scanned the w o rk re la te d to oxide spu tterin g, and a ch ap ter is d ed icated to changes in s to ic h io m e try o f ox id es as a re s u lt o f io n b om b ard m en t (p r e fe r e n t ia l spu tterin g o f o x y g e n ). T h e re s u lts o f th e ir w o rk show that in m any c a s e s it is d iffic u lt to distin gu ish the ch e m ic a l e ffe c ts fr o m the e ffe c ts o f im p act. A c o m p lica tio n , com m on to w o rk on c h e m ic a l e ffe c ts o f ion im p la n ta tion, is the fa ct that a s o rt o f steady state is rea ch ed betw een spu tterin g of a la y e r and its grow th b y the c h e m ic a l re a c tio n [1 4 ]. In the case o f the h e a v ie r ion s lis te d in T a b le s I and II th is c o m p lic a tio n has to be taken into account, not m a in ly b ecau se o f spu tterin g o f the im p lan ted p a r tic le s w hich is n e g lig ib le fo r the lig h t e r ones - but b ecau se o f neutron spu tterin g. I f the la tte r e ffe c t is as im p orta n t as, fo r ex a m p le, K a m in sk y [15] cla im s , it is u n lik e ly that n eu tron-indu ced im p lan tation o f h e a v ie r ions w ill cause any re m a rk a b le e ffe c ts . A t the p re s e n t state o f know ledge o f the c h e m ic a l e ffe c ts it seem s im p o s s ib le to d ecid e w h eth er o r not im p lan tation fr o m the coolan ts w ill a ffe c t c o r r o s io n b eh a vio u r o f r e a c t o r m a te r ia ls . Rough e s tim a te s based on E q . ( l ) show that the ca lcu la ted im p lan tation r a te s fr o m the g a s e s O 2, N 2 , and C O 2 a re about one o r d e r o f m agnitude lo w e r than th ose fo r helium and d eu teriu m (about 1010 a to m s / c m 2 • s taking the neutron spectru m o f the D F R ). H o w e v e r, as can be seen in the ca se o f deu teriu m , the take-u p o f c h e m ic a lly a c tiv e g a s e s m ay be much h igh er than the calcu lated im p la n ta tion ra te .
REFERENCES [ 1]
ALTENHE1N, F. K. , ANDRESEN, H. , LUTZE, W. , MALOW, G. , MIGGE, H. , J. Nucl. Mat., in press. [2] Di COLA, G., MATZKE, Hj. , Nucl. lnstr. Methods 57 (1961) 341. [3] BIRSS, J.R. , I. Nucl. Mat. 34 (1970) 241. [4] GAUS. H.. MIGGE, H., MIRUS. K.-D.. Radiat. Effects 18 (1973) 79.
I A E A -P L -6 1 5/4
55
[5] ANDRESEN, H., GRUBER, J.. LUTZE, W ., MIGGE, H., Proc. 8th Symp. Fusion Technology 1974, in press. [6] ENGLE, W.W., Jr.. Rep. No. K-1693 (March 1967). [7] HONECK, H. C., BNL Rep. 50066 (1966). [8] NORTHCUFFE, L. C., SCHILLING, R. F., Nucl. Data Tables A 7 (1970) 233. [9] KULC1NSKY, G.L., BREMHALL, J.L.. KISSINGER, H.E., in Proc. Radiation InducedVoids in Metals, Albany, New York, 1971. [10] KLUEH, R.L., MULLINS. W.W.. Acta Met. 17 (1969) 59. [11] SUZUKI. T., Z. Naturforsch. 12a (1957) 497. [12] TRILLAT, J. J., Le Bombardement Ionique, Theories et applications, éditions du CRNS,Paris (1961). [13] KELLY, R., LAM, N.Q., Radiat. Effects 19 (1973) 39. [14] Ion Implantation (AMELINCKX, S.. GEVERS, R., NIHOUL, J., Eds), North-Holland Publishing Co. (1973). [15] KAMINSKY, М., DAS, S. К ., in Proc. 5th Symp. on Engineering Problems of Fusion Research, Princeton, 1973.
DISCUSSION G. H A R B O T T L E : A ft e r h ea rin g you r p a p er, none o f us can have any doubt w h a te v e r that th e re is , and w ill be, an enorm ou s amount o f hot atom c h e m is try in the study o f the s u rfa c e s that a re goin g to be in v o lv e d in the v a rio u s kinds o f r e a c to r s , and p a r tic u la r ly in fu sion r e a c to r s . T h e s e data a re lit t le sh ort o f aston ish in g to m e. I had no id ea that th e re w e r e th ese kinds o f e ffe c ts , o r that th is kind o f study w as b ein g m ade at a ll. M . N E W T O N : I want to be su re that I understand what ty p ic a l h eliu m atom k in etic e n e r g ie s a re a fte r th ese c o llis io n s w ith neutrons. Did you say 300 keV ? H. M IG G E : T h e e la s tic neutron s c a tte rin g o f h eliu m has a v e r y b ig peak in the ran ge o f about 1 -M e V neutron e n e rg y , and the m ean tr a n s fe r o f e n e rg y f o r this neutron e n e rg y is about 300 k eV fo r h eliu m . M . N E W T O N : A m I rig h t in assu m in g that ab ove 200- to 300-keV heliu m is m o s tly in the +2 o r m aybe the +1 ch a rge state? In oth er w ord s, a fte r it has undergone one o r tw o c o llis io n s , is it g e n e r a lly b e lie v e d that the e q u ilib riu m ch a rge state is the io n iz e d state? H. M IG G E : W e h aven 't c o n s id e re d such e ffe c ts . T h e calcu lation s w e r e done u sin g ran ge ta b les o f N o r th c liffe and S ch illin g , and we fitte d the va lu es to .... M . N E W T O N : O kay. T h is is e m p ir ic a l. F .S . R O W L A N D : T h is is h eliu m in heliu m , o f c o u rse, so that it is hard f o r it to tr a n s fe r e le c tr o n s . M . N E W T O N : W e ll, le t us say w e a re in te r e s te d in what happens when it gets to the w a lls . What I am in te r e s te d in is the slo w in g down, o r the ra n g e - how much is le ft in the w a ll it s e lf. T h e r e the ra n ge w ill depend upon what the ch arge state o f the h eliu m w as when it en tered . G. H A R B O T T L E : Th at is tru e, but that w as a ll w ork ed out lon g ago. T h e r e a r e ran ge ta b le s , and a ll th ese va lu es a re tabulated. In g e n e ra l you can say that the atom w ill re m a in strip p ed so lon g as the v e lo c it y o f the atom ex c e e d s the v e lo c it y o f the o r b ita l e le c tr o n s . M . N E W T O N : Okay. That is what I wanted to know - is th is a ll w e ll known? One would get a d iffe re n t ra n ge in the gas phase f o r d iffe re n t ch a rge sta tes. H o w e v e r, I see that you r point o f v ie w is s tr ic t ly e m p ir ic a l.
56
A N D R E S E N et al.
H. M IG G E : A n o th er im p orta n t point is this: w e took the lo w est e n e rg y o f the h eliu m as 30 k eV in the ca lcu la tion s. The s c a tte rin g c r o s s sectio n s a r e a ll so high - la r g e r than one barn - that you w ill a lso get h eliu m atom s w hich have e n e r g ie s b etw een th e rm a l and 30 k eV . W e have n e g le c te d th ese h eliu m atom s becau se th ey w ill not be im plan ted v e r y d eep ly, but th ere w ill d e fin ite ly be an e ffe c t at the s u rfa c e . F o r exam p le, th e re m a y be som e sp u tterin g at the s u rfa ce. Sputtering tak es p la ce with lo w - e n e r g y atom s, but the ca lcu la tion s a re v e r y d iffic u lt. Y ou have to take account the ca sca d es, which w ill build up in the gas phase. F .S . R O W L A N D : A lon g tim e ago — 20 y e a r s ago — W olfga n g and I did som e t r a c e r e x p e rim e n ts with h eliu m in w hich the t r a c e r was 6He p rodu ced in sid e p la tes o f s ilv e r , lead, p o lyeth ylen e and gra p h ite. T h ese w e r e done at ro o m te m p e ra tu re , and we o b s e rv e d d iffu sio n o f these 6He atom s fr o m th ese p la tes to depths that went as fa r as 2500 A with lead, and 100-200 A fr o m s ilv e r in tim e s o f a few m illis e c o n d s . T h e s e 6He atom s w e r e r e c o il produced, m ade by c o s m o tro n bom bardm en t at B rook h a ven - m ade by a n u clea r re a c tio n at high e n e rg y , and im plan ted h om ogen eou sly in the bulk o f the s ilv e r p late, fo llo w e d by d iffu sio n out of the p late into the su rrou nding gas volu m e in a v e r y sh ort tim e . M y question is th is: What a re you r lo s s e s o f h eliu m goin g into the s u rfa ce and then com in g rig h t back out? H. M IG G E : You w on d er why the h eliu m does not com e b a ck out again during ir r a d ia tio n ? F .S . R O W L A N D : Y e s . H. M IG G E : What was the e n e rg y o f the h eliu m atom s you im planted? F .S . R O W L A N D : T h e y w e re b ein g m ade by the bom bardm en t o f the p la tes w ith G e V p roton s and w e r e com in g o ff w ith M e V e n e r g ie s . H. M IG G E : And what was the im p lan tation ra te? F .S . R O W L A N D : The im p lan tation w as done at t r a c e r le v e l — n e g lig ib le co n cen tra tion s. H. M IG G E : O ur im p lan tation takes p la ce in a n u clea r r e a c to r , with h eavy ra d ia tio n dam age in th is s u rfa ce la y e r , w hich w ill in flu en ce the b eh a vio u r o f h eliu m . T h e s e phenom ena a re w e ll known by the p eop le who w o rk on v o id s , f o r in stan ce. S. A M IE L : L e t m e be c le a r about the qu estion. A r e you asking about bouncing back? F .S . R O W L A N D : N o — about atom s com in g out. G. H A R B O T T L E : The qu estion is p e r fe c t ly c le a r — i f the 6He d eso rb s ra p id ly fr o m g r e a t depths .... F .S . R O W L A N D : It is not d eso rp tio n but the atom s a re gettin g out ... . G. H A R B O T T L E : W e ll, g ettin g out, d iffu sin g out ra p id ly . What you a re asking is - why do th ese atom s stic k in M ig g e 's niobium and in his s ilv e r ? D .M . R IC H M A N : P e rh a p s th is is an e q u ilib riu m situ ation and what he is o b s e rv in g is a net re s u lt. G. H A R B O T T L E : I think the on ly a n sw er is that atom s com e out o f M ig g e 's s ilv e r , to o . What he o b s e r v e s is only that w hich is fo r som e re a s o n fir m ly im b ed d ed . T h e r e can be m any tra p p in g s ite s in s ilv e r , and som e o f them w ill hold heliu m , and som e o f them would not. F .S . R O W L A N D : But, is the usual e ffe c t o f ra d ia tio n dam age to m ake the m a t e r ia l hold g a s e s w ith in them b e tte r? I w ould have thought it would m ake them hold g a s e s le s s w e ll.
1А Е А -Р Ь 6 15 /4
57
G. H A R B O T T L E : N ot n e c e s s a r ily . Y ou m igh t f i l l up the channels through w hich th ey would n o rm a lly com e out. G. S T O C K L IN : I think that the p e rm e a tio n ra te o f the o v e r a ll p ro c e s s is d e te rm in e d by d iffu sio n and s o lu b ility . So, it f ir s t o f a ll depends upon the m a te r ia l, and then it w ill a ls o depend upon the extent o f ra d ia tio n dam age — upon how m any tra p s you have. T h en it b e c o m e s a qu estion of s o lu b ility as w e ll. P ro b a b ly the net e ffe c t is not v e r y im p ortan t in this ca se. I have another qu estion .... H. M IG G E : I have an im p orta n t com m en t to in s e r t h e re . T h ese e x p e rim e n ts have a ll b een c a r r ie d out at ro o m te m p e ra tu re . W e w ill have to c a r r y th ese e x p e rim e n ts out again at much h ig h e r te m p e ra tu re s to d e te rm in e th e ir re le v a n c e fo r r e a c to r tech n ology. G. H A R B O T T L E : C e rta in ly annealing and d iffu sio n w ill be m uch d iffe r e n t at h ig h e r te m p e ra tu re s . G. S T O C K L IN : C e rta in ly it w as a good id ea to in v ite D r. M ig g e to p re s e n t th is g e n e r a l p ap er, even though he w as v e r y relu ctan t to com e b ecau se he does not c o n s id e r h im s e lf a hot atom ch em ist. I think th is is a fie ld which hot atom c h em ists should know about, and m o st o f the hot atom c h em ists liv e a lit t le too much in th e ir iv o r y to w e r, not c o n sid erin g much ou tside th e ir own in te r e s ts . On the oth er hand, w e should not go o v e rb o a rd . T h is is m a te r ia l s c ie n c e . What we should ask is this — a re the hot atom c h em ists able to con tribu te to the fundam ental understanding o f the fie ld o f m a te r ia l sc ie n c e ? I think they a re , and that w e should be m o re in touch w ith the p eop le w o rk in g in th is fie ld . N ow fo r m y question, a s p e c ific qu estion fo r the ca ses o f h ydrogen, d eu teriu m and lith iu m . How fa r a re you able to avoid d iffu sio n and c h e m ic a l r e a c tio n o v e rla p p in g with you r im plan tation ? F o r ex a m p le, in the h yd rogen ca se it is c le a r that an im p u re m e ta l containing oxygen m igh t e a s ily re a c t. A ls o , p r a c tic a lly nothing is known about the d iffu sion o f lith iu m through m e ta ls . A p r e lim in a r y e x p e rim e n t in our la b o ra to ry has shown with m a s s s p e c tr o m e te r techniqu es that lith iu m is able to d iffu se through niobium fo il. W e need to know m o r e about the d iffu sion o f lith iu m through th ese v a rio u s m e ta ls . But the b a s ic question is re la te d to the one R ow land asked e a r lie r — how can you be su re that you r e ffe c ts a re a ll im p la n ta tion and not diffu sion ? H. M IG G E : F o r helium , it is c le a r that when you anneal h eliu m and s ilv e r that you do not get any h eliu m into the s ilv e r . It is im p o s s ib le . W e a ls o annealed the s ilv e r in d eu teriu m gas, and did not find any deu teriu m in the s ilv e r a fte rw a rd . H o w e v e r, w e ’annealed them only at about 80°C, the te m p e ra tu re o f the ir ra d ia tio n . T h e r e is a n oth er p o s s ib le exp lan ation fo r the la r g e e x c e s s o f d eu teriu m found above that w hich you can ca lcu la te u sin g th is fo rm u la . T h e r e m a y be in c re a s e d s o lu b ility in the n e a r -s u r fa c e r e g io n becau se the im p la n ta tio n — and the ra d ia tio n dam age - is high in the su rfa ce, and it is w e ll known that h yd ro gen can accum ulate n ea r d is lo c a tio n lin e s . H ow e v e r , the s o lu b ility o f h yd ro gen in s ilv e r at ro o m te m p e ra tu re is v e r y , v e r y low . If you assu m e in c re a s e d so lu b ility , you have to in c r e a s e it by s e v e r a l o r d e r s o f m agnitude. W ith cop p er it is known that the in c re a s e in s o lu b ility can be that la r g e . I f you assu m e such an e ffe c t in s ilv e r , then you have to assu m e a v e r y , v e r y la r g e in c r e a s e , and we think this in c r e a s e m ay re s u lt m o re fr o m the c h e m ic a l d iffe r e n c e s in the nature o f deu teriu m .
58
A N D R E S E N et al.
G. H A R B O T T L E : One can c e rta in ly argu e that w e ought not to go o v e rb o a rd in c o n s id e rin g h eliu m , w hich can be c o n s id e re d a qu estion o f d iffu sion , trap p in g, and so fo rth . But, as you have pointed out, when you sta rt c o n s id e rin g r e a c tiv e g a s e s , m any o f w hich w ill be used in n u clear r e a c to r s - h ydrogen , deu teriu m , carb on d ioxid e â&#x20AC;&#x201D; w hich a re a lre a d y b ein g used in p r e s s u r iz e d CO 2 r e a c to r s . It s e e m s to m e that when you sta rt im p la n tin g th ese g a s e s into s te e l o r niobium , o r som e o th er su rfa ce, then you a re s q u a re ly into the fie ld o f hot atom c h e m is tr y in te r m s o f what happens under the s u rfa c e and how the g a s e s a re tra p p ed th e re . J. D A N O N : Im plan tation w o rk has a ls o been done w ith h yd rogen into p a lla d iu m . It is w e ll known that by e le c t r o ly s is you can m ake a lot o f h yd ro gen go into p allad iu m . But fr o m im p lan tation you get much h igh er co n cen tra tio n s o f produ cts than by e le c t r o ly s is o r by d ir e c t ab sorp tion . T h e r e is an in c r e a s e in so lu b ility , p rob a b ly fr o m ra d ia tio n dam age accom p an yin g the im plan tation . H. M IG G E : Y ou m ean when p roton s a re im p lan ted into palladiu m ? A t h igh er te m p e ra tu re s ? J. D A N O N : I do not r e m e m b e r the d e ta ils ex a c tly , but it is not at h ig h e r te m p e ra tu re s . P ro b a b ly it was at ro o m te m p e ra tu re , perhaps even lo w e r . You can re a c h h ig h e r con cen tration s o f h yd rogen in palladium by im p lan tation tech n iqu es than you can by e le c t r o ly s is . H. M IG G E : One w ay in w hich you can accum ulate v e r y la r g e amounts is through bubble fo rm a tio n . When bubbles appear, la r g e amounts can be held, but bubbles a re n o rm a lly not fo rm e d at n o rm a l te m p e ra tu re s . S. A M IE L : D on 't you think that the type o f e x p e rim e n t d e s c rib e d y e s te r d a y by M addock is m o re su itable in the sen se that they can be run under m o r e c o n tro lle d con d ition s. Y ou can study the b eh a viou r with and without p re -c o n d itio n in g o f the s u rfa c e , and so on. In y o u r p a rtic u la r ca se m any m ech a n ism s m ay be o c c u r r in g sim u ltan eou sly, w h ile in the c o n tro lle d b eam im p la n ta tion you can t r y to is o la te the v a rio u s e ffe c ts . H, M IG G E : I think that when you a re tr y in g to a n alyse what happens d ir e c t ly at the s u rfa ce that it is b e tte r to use an a c c e le r a to r . But when you a re tr y in g to a n sw er a p ro b le m o f b lis te r in g o r som eth in g s im ila r to it, then you have to take a r e a l p r o file w hich bu ilds up in the sam p le, and it is p o s s ib le , but som ew hat d iffic u lt, to r e a liz e th is p r o file in an a c c e le r a to r e x p e rim e n t. G. H A R B O T T L E : It is a qu estion o f sim u lation . S. A M IE L : T h is b e a rs on an oth er point which R ow land m entioned e a r l i e r about the r e m o v a l o f h eliu m fr o m the s u rfa c e . In an isotop e s e p a ra to r w e t r ie d to c o lle c t k rypton is o to p e s on an alum inium s trip . W e found that i f we used the shiny sid e, we lo s t the k rypton im m e d ia te ly , but i f we used the m atte sid e th ey re m a in e d in the s trip . G . H A R B O T T L E : The o x id iz e d side? S. A M IE L : Y e s . In oth er w o rd s, it is the su rfa c e conditioning â&#x20AC;&#x201D; u n derstanding the s u rfa c e stru ctu re â&#x20AC;&#x201D; w hich counts fo r a lo t in th ese e x p e rim e n ts . B e fo r e you sta rt it is u su ally a kind o f b la ck m a g ic un less one g o es to the pains o f studying the re le v a n c e o f the p a r tic u la r su rfa ce p r o p e r tie s to the containm ent o r re te n tio n o f d iffe re n t s p e c ie s under the s u rfa ce. A .G . M A D D O C K : That is connected with a point which I have wanted to r a is e . I think that you have to think v e r y c a r e fu lly about the a p p ro p ria te s u rfa c e fo r d ea lin g w ith c h e m ic a lly r e a c tiv e im p lan ts. F o r in stan ce, it
I A E A -P L -6 1 5/4
59
would be b e tte r to w o rk w ith som eth in g lik e gold w hich has a w e ll-d e fin e d and ra th e r e a s ily obtainable clea n su rfa c e . A f t e r a ll, s ilv e r is v e r y much in fe r io r to g o ld fr o m th is point o f v ie w , sin ce the s u rfa c e o f s ilv e r is ra th e r p o o r ly d efin ed u n less you go to g r e a t trou b le. F .S . R O W L A N D : G old is a ra th e r u n lik ely r e a c t o r m a te r ia l. G. H A R B O T T L E : G old - o r s ilv e r e ith e r. H. M IG G E : The f ir s t e x p e rim e n ts in th is fie ld w e r e done w ith s ilv e r and d eu teriu m . A .G . M A D D O C K : Y ou have m entioned the d e s ir a b ility o f doing e x p e r i m en ts at h ig h e r te m p e ra tu re s , but I w ould a lso su ggest that one should not o v e r lo o k the advan tages o f doing e x p e rim e n ts at lo w e r te m p e ra tu re s as w e ll. H o w e v e r, I am not c le a r about th is d eu teriu m an om aly. A r e the ca lcu la ted d eu teriu m fig u r e s dependent upon the h eliu m data at a ll? It could w e ll be that the o b s e rv e d h eliu m im p lan t co n cen tra tion is too lo w b ecau se — I think w e would a g r e e even at ro o m te m p e ra tu re — som e o f it m igh t e s c a p e . W h e re a s th is m igh t not happen to the sam e extent w ith im p lan ted deu teriu m . H. M IG G E : The e la s tic neutron s c a tte rin g o f h eliu m and d eu teriu m a re both w e ll known. A ls o w e ll known a re the ra n g es o f d eu teriu m and h eliu m in the gas phase, and a lso the r a n g e s 'in the ta r g e ts . T h e m ea su red d iffe r e n c e is th re e o r d e r s o f m agnitude, and we do not think that th is can be m ista k en in the calcu lation . G. H A R B O T T L E : I would lik e to change the su bject fo r a fe w m inutes, and re tu rn to a point that has com e up p r e v io u s ly . T h e s c ie n tific s e c r e t a r ie s have in v ite d a r e p r e s e n ta tiv e o f the n u clea r data section . D r. A le x L o r e n z , to jo in us and to d iscu ss with us the type o f n u clear data tabu lation s which would be u sefu l to us. D r. W o lf has a lre a d y m en tion ed the v e r y g r e a t d iffic u lty w hich they have in g e n e ra tin g the kind o f data th ey re q u ir e on ch a rged p a r tic le c r o s s - s e c tio n s w h en ever th ey c o n s id e r the prod u ction o f a new is o to p e fo r ra d io p h a rm a ceu tica l w o rk . I have m en tion ed that in a ll hot atom w o rk we would lik e to know m o r e about neutron captu re gam m a ra y s , and p a r tic u la r ly need the p r o b a b ilitie s fo r in te rn a l c o n v e rs io n . In A d lo ff's talk, th e re w as a d is c u s s io n o f g a m m a -r a y d ecay sch em es fo r p e rtu rb ed an gu lar c o r r e la tion, fo r w hich one would lik e m o r e in fo rm a tio n about m u ltip o la ritie s , e tc . W e have asked D r. L o r e n z to jo in us to d iscu ss the typ es o f tabu la tio n s w hich w ould be m ost u sefu l f o r bot atom ch e m is ts . What would hot atom ch e m is ts lik e to have in a b o o k ; o r in a r e t r ie v a b le fo r m fr o m a n u c le a r data bank? A . P . W O L F : W e can s ta rt w ith L e d e r e r , which is so rt o f the B ib le fo r fin din g things. L e d e r e r could be im p ro v e d upon by lis tin g a com p lete lite r a tu r e s e a rc h on m eans o f p rod u ction . T h is book is v e r y good u n til you g e t to the la s t colum n — p rin c ip le m ean s o f prod u ction a re w o e fu lly in ad equ ately s e a rch ed in the lite r a tu r e . M o st hot atom ch em ists can ju st gla n ce at a p a rtic u la r iso to p e and say, "O h, th ey m is s e d th is, and th is, and t h is " . And, it should inclu de adequate r e fe r e n c e s . Th e secon d thing w hich would be u sefu l fo r n u clea r m ed icin e p u rp oses w ould be e x c ita tio n functions fo r ch a rged p a r tic le re a c tio n s o v e r the w h ole ran ge o f the p a r tic le . T h is is p a r tic u la r ly im p ortan t when you a r e d ea lin g w ith m u ltip le is o to p e prod u ction and you need to ch oose the ch a rged p a r tic le ran ge such that you obtain m axim u m p u rity fo r the is o to p e w hich you a re tr y in g to prod u ce. T h is r e q u ir e s good
60
A N D R E S E N et al.
e x c ita tio n functions in the ran ge fr o m 2 M e V up to about 80 M e V . A b o v e 80 M e V , the kinds o f a p p roxim a tion s one can m ake on s p a lla tio n a re u su a lly adequate fo r the pu rp ose. G. H A R B O T T L E : A s I r e c a ll th e re is a book on ch a rged p a rtic le c r o s s - s e c tio n s . What is its d efect? A . P . W O L F : Y e s , th e re is such a book. F i r s t o f a ll, it is lim ite d in the num ber o f is o to p e s it c o v e r s ; it is in ad equ ately r e fe r e n c e d ; and it is r e la t iv e ly hard to u se. The la s t could be fix e d up with ed itin g. It is a v e r y th ick book, but it has v e r y lit t le in it. A th ird thing which w e could use is v e r y a ccu ra te d eterm in a tio n s o f the d ecay sch em es. A good exa m p le is the iso to p e 123I. I f you calcu late sim p ly fo r the p r in c ip a l photon o f 1231 - about 90% - then you m ake about a fa c to r o f ten e r r o r in the ca lcu la ted ra d ia tio n dose in any system . The d oses w hich a r e r e c e iv e d fr o m the m in o r photons a re much g r e a t e r than th ose fr o m the m a jo r e m is s io n . Such ca lcu la tion s have been c o lle c te d in a set o f ta b le s c a lle d the M IR T a b le s - M e d ic a l In tern a l R ad iation D ose C o m m ittee, and th ose a re v e r y good, but th ey on ly c o v e r a v e r y , v e r y s m a ll num ber o f is o to p e s . G. H A R B O T T L E : F o r those ca lcu la tion s you need v e r y , v e r y d eta iled in fo rm a tio n about d ecay sch em es. A . P . W O L F : Y ou can get a lo t o f the in fo rm a tio n fr o m L e d e r e r . But L e d e r e r tends not to be p a r tic u la r ly a ccu ra te when they lis t p e rc e n ta g e s o f decay. G. H A R B O T T L E : Is th e re not a group in W ashington that is supposed to be tabu latin g d eca y sch em es? A . P . W O L F : I am a p e s s im is t, o r m ayb e a cyn ic. T h is point was r a is e d not on ly fo r hot atom c h e m is try but fo r n u clear m ed icin e fou r y e a r s ago, and nothing happened. The sam e point was r a is e d in the IA E A two y e a r s ago, and som e gen tlem an in the F e d e r a l R ep u b lic o f G erm an y is supposed to s ta rt tabu lating th ese things too, and now th e re is a c o m m itte e in the U SA that is supposed to be tabu lating them , sta rtin g a y e a r ago. A s fa r as I know, nothing has been done an yw h ere.
IAEA-PL-615/5
PHYSICAL METHODS IN HOT ATOM CHEMISTRY J.P. A D L O F F Laboratoire de chim ie nucléaire, Centre de recherches nucléaires de Strasbourg, Strasbourg, France
Abstract PHYSICAL METHODS IN HOT ATOM CHEMISTRY. Physical methods have found considerable use in the field of hot atom chemistry. Methods that have found the greatest use in the past or are used to generate hot species are briefly mentioned, while those which are either relatively unexplored or are used for the in-situ analysis of the chemical state of atoms in solids, are discussed in some detail. These include hyperfine interactions, Mossbauer spectroscopy, per turbed angular correlations, alteration of nuclear decay rates by chemical environmental influence, nuclear resonance fluorescence, and optical excitation in beta decay.
1.
IN T R O D U C T IO N
T h e use o f p h ysica l m ethods in hot atom c h e m is tr y sta rts p r a c tic a lly w ith the advent o f ch arge s p e c tr o m e tr y in the la te 1950s. T h e m ea su rem en t o f the ch a rge d istrib u tio n s o f ion s fo rm e d in n u clea r d e c a y s , although lim ite d to the lo w -p r e s s u r e gas phase, has been an u n preceden ted step to w a rd s captu rin g the p r im a r y e ffe c ts o f n u c le a r tra n s fo rm a tio n s [ 1 ] . In a r e la t iv e ly sh ort but h ig h ly p r o lific tim e , m o s t o f the con ven ien t and a v a ila b le |3" and ¡3+ e m itte r s and ra d io n u clid es d eca y in g by e le c tr o n capture and is o m e r ic tra n s itio n s w e r e s y s te m a tic a lly in v e s tig a te d , in clu din g, in s o p h istica ted e x p e rim e n ts , n u clides as s h o r t-liv e d as 0 .8 s 8H e. In a fu rth e r step the con sequ en ces o f in n e r - s h e ll io n iza tio n s w e r e m o r e su itably in v e s tig a te d a fte r X - r a y p h otoion ization in s e le c te d a to m ic sh e lls ; fr e e d fr o m the r e s t r ic t io n o f ra d io a c tiv e p aren t n u clid es, the study could be extended to gaseou s com pounds o f a set o f oth er e lem en ts in clu d in g lea d . T h a t the c h a rg e s p e c tr o m e tr y m e a su rem en ts have been n e a rly exh au stive, is r e fle c t e d in the sharp drop o f new data sin ce 1968. T w o grou ps o f . au th ors, h o w e v e r, have d eterm in ed the c h a rg e d istrib u tio n o f a - r e c o il atom s e m itte d fr o m so lid s u rfa c e s using t im e - o f- flig h t and coin cid en ce m eth o d s [ 2- 3] . T h e m ea su rem en t o f the ch a rge s p e c tra o f r e c o i l a tom s fo r m e d in n u clea r re a c tio n s on gaseou s com pounds is s t ill an open but ch allen gin g e x p e rim e n t. P h y s ic a l m eth ods a r e now used in hot atom c h e m is try m o s tly fo r the n o n -n u clea r g en era tio n o f hot s p e c ie s (th e rm a l and p h o to lytic m o le c u la r d is s o c ia tio n , beam tech n iq u es) and fo r the in -s itu a n a ly s is o f the c h e m ic a l state o f atom s in so lid s (M o s s b a u e r s p e c tro s c o p y , p ertu rb ed an gu lar c o r r e la t io n s ). O nly the la tte r top ic w ill be d iscu ssed , to g e th e r w ith a few oth er p ro m is in g tech n iqu es, w hich a re s t ill s c a r c e ly e x p lo re d .
61
62
A D LO FF
T A B L E I. E X P E R IM E N T A L IN V E S T IG A T IO N O F H Y P E R F IN E IN T E R A C T IO N S A N D D E R IV E D P R O P E R T IE S Multipolarity of the interaction EO Ml
2.
E2
Experimental method
Measured properties
Môssbauer emission spectroscopy
Electron density at the nucleus Internal magnetic field Electric field gradient
Ml E2
Perturbed angular correlation
Internal magnetic field Electric field gradient
EO
Half-life measurement
Relative change ДХ/Х of the decay constant
EO
Conversion electron spectroscopy
Intensities of conversion lines
T H E H Y P E R F IN E IN T E R A C T IO N S
T h e c h e m ic a l e ffe c ts a s s o c ia te d w ith n u clea r tra n s fo rm a tio n s a r e a o n e -s id e d v ie w o f the in te rre la tio n s h ip b etw een the nucleus and its e n v ir o n m en t. C o n v e r s e ly , the n u c le a r tra n s fo rm a tio n s m a y be dependent on the en viro n m en t, and a c tu a lly a fe w n u clea r p a ra m e te rs a re s e n s itiv e to the e le c tr o n ic con figu ration : (i ) T h e p r e c is e valu e o f the e n e rg y o f the photons e m itted in the d eca y o f an e x c ite d n u clea r state as re v e a le d by the is o m e r sh ift in M ôssb au er s p ectra ; ( i i ) T h e an gle o f the d ire c tio n s o f tw o s u c c e s s iv e ly em itted ra d ia tion s fr o m an e x c ite d state o r in a ra d io a c tiv e decay; ( i i i ) T h e d e c a y p r o b a b ility o f a nucleus as fa r as the d is in te g ra tio n p r o c e s s in v o lv e s d ir e c t ly bound e le c tr o n s , as in the e le c tr o n captu re and the in te rn a l c o n v e rs io n . F u r th e r m o r e , the a to m ic e le c tro n s and the b ro a d e r en viron m en t o f atom s and ion s in a la ttic e produ ce e le c t r ic and m a gn etic fie ld s at the s ite o f the nucleus, which again m akes the o rie n ta tio n o f the nucleus s tro n g ly dependent on the su rrou n din gs through the coupling o f the n u clea r m om ents w ith th ese h y p e rfin e fie ld s . T h e s e in te ra c tio n s a re b est d e s c rib e d in te rm s o f th e ir m u ltip o la ritie s (T a b le I) [ 4 ] : EO is the coulom b in te ra c tio n o f the e le c tr o n ic c h a rg e w ith the n u clea r c h a rg e . T h e e le c tr o n s o f in te r e s t a r e the s type e le c tr o n s , but the mutual s c re e n in g o f the e le c tr o n s in the v a rio u s o rb ita ls m akes the d en sity at the nucleus v e r y s e n s itiv e to subtle changes in the e le c tro n a rra n g e m e n t.
1АЕА -РЬ615/5
63
M l is the m a gn etic d ip o la r in te ra c tio n a s s o c ia te d w ith the e n e rg y o f o r ie n tation o f the n u c le a r m a g n etic m om en t in the m a gn etic fie ld at the nucleus. E2 is the e le c t r ic qu ad ru p olar in te ra c tio n o f the n u clea r qu adru pole m om en t w ith the e le c t r ic fie ld g ra d ie n t. V a rio u s e x p e rim e n ta l m ethods a re b ased on the th re e types o f i n t e r a c tio n s . A d va n ta ge is taken som eh ow o r oth er o f the ra d ia tio n s em itted by the ra d io a c tiv e r e c o il atom s o r daughter n u c le i. T h e m ea su red p r o p e r tie s p ro v id e d ir e c t in fo rm a tio n on the v a le n c y , bonding and su rrou n din gs o f n u cleo gen ic a tom s. In M o s s b a u e r s p e c tro s c o p y and in p ertu rb ed angular c o r r e la tio n s m e a su rem en ts the data a r e re le v a n t to the state o f the atom s s h o rtly a ft e r th e ir fo rm a tio n , i . e . a fte r a p e rio d w hich is often o f the o r d e r o f the life t im e o f e x c ite d n u c le a r le v e ls . In a fe w fortu n ate c a s e s , the re la x a tio n o f tra n s ie n t s p e c ie s can be v ie w e d .
3.
M OSSBAUER S PE C TR O S C O PY
M o ssb a u er s p e c tro s c o p y re m a in s by fa r the m o s t w id e ly used p h ysica l m eth od in hot atom c h e m is tr y . S e v e r a l exh a u stive r e v ie w s on the a p p li cation s o f M o ssb a u er s p e c tro s c o p y in the study o f the c h e m ic a l e ffe c ts o f n u clea r tra n s fo rm a tio n s in s o lid s h ave ap p eared r e c e n tly [ 5 - 8 ] . M ossb a u er e m is s io n s p e c tro s c o p y has now been extended to a b ro a d e r set o f nu clei, in clu d in g, b e s id e s 51Co and u9Snm, a ls o n9Sb, 129T e , 125I, 99Rh, 193O s, 191P t, e tc . O f p a rtic u la r in te r e s t a re the couple o f n u clides w hich fe e d the sam e M o ssb a u er le v e l in tw o d iffe r e n t w avs [ 119Sb E C and 119Snm, 197H g EC and l97P t |3\ 153Gd E C and 153Sm (3‘ , l51Gd E C and 151Sm |3\ e tc . ] . Thu s, the M o ssb a u er le v e l o f 119Sn can be re a c h e d fro m the neutron d e fic ie n t sid e by E C d eca y o f 119Sb. In an tim on y (II I ) m a t r ic e s , the v a le n c e state o f the paren t atom lie s b etw een the tw o p o s s ib le ch a rge sta tes o f tin.(Sn2+ and Sn4+). O w in g to the A Z change, the daughter atom s m ust adapt th e m s e lv e s to a fo r e ig n host as c o m p a red with the 1 9Snm IT in tin com pounds. C o m p a r a t iv e e x p e rim e n ts on w e ll-c h o s e n an tim on y and tin com pounds could help to elu cid a te the r o le o f the h ost m ed iu m on the c h e m ic a l sta te o f daughter a to m s . F u r th e r m o r e , the gra n d -p a re n t o f 119Snm is the 4. 7 d u9T e m which d eca y s by E C to the 38 h 119Sb. T h is re n d e r s p o s s ib le the study in independent e x p e rim e n ts o f the e ffe c ts o f one o r tw o s u c c e s s iv e e le c tr o n cap tu res; the M o ssb a u er m e a su rem en ts can be c o m p leted by ra d io c h e m ic a l in v e s tig a tio n on the 119T e m- 119Sb filia tio n . R e s e a r c h has s ta rte d la te ly on this v e r y p r o m is in g s y s te m [ 9 - 1 1 ] . T h e c u rre n t status o f M o ssb a u er s p e c tro s c o p y in hot atom c h e m is try w ill be an alysed fr o m the s a lie n t fe a tu re s o f re c e n t w o rk in the fie ld , d istin gu ish in g la b e lle d o r doped s o u rc e s fr o m those in w hich the ra d io a c tiv e p aren t is produced by a n u clea r re a c tio n .
3 .1 . L a b e lle d and doped M o ssb a u er s o u rc e s In m o st o f the p r a c tic a l c a s e s , the M ossb a u er le v e l is populated by a r a d io a c tiv e d eca y o r fr o m a lo n g -liv e d is o m e r ic sta te. T h e a ft e r - e ffe c t s o f the n u c le a r p r o c e s s a r e in v e s tig a te d in s to ic h io m e tr ic com pounds o f the s o u rc e a tom s o r in any m a t e r ia l doped w ith the ra d io a c tiv e paren t atom s.
ADLOFF
64
(a) FIG. 1(a).
У1-/2 cascade populated by the decay of a parent nuclide.
D e te c to r
1
FIG. 1(b). Angular correlation of yj and у г . kj is fixed by the source-detector-1 direction. I is the spin direction of the intermediate nuclear state B.
In la b e lle d s o u rc e s , the c h e m ic a l state o f ra d io n u clid e is id e n tic a l with that o f the is o to p ic stab le c a r r ie r a tom s. In doped m a te r ia ls c o n s id e ra b le doubt m ay su bsist on the c h e m ic a l state o f the paren t sin ce w h a te v e r ch arge com p en sation is re q u ir e d i f the la ttic e has to a ccom m od ate an a lio v a le n t im p u rity atom . T h e im p lan tation o f a c c e le r a te d M o ssb a u er n u clei in su itable ta r g e ts ap p ear v e r y p r o m is in g . P o s t-p r e p a r a tio n tre a tm e n ts o f the so u rce such as an n ealin g, fir in g , ir r a d ia tio n can a lte r the type and d istrib u tion o f the d e fe c ts .
IA E A -P L-61 5/5
65
M ô ssb a u er so u rce e x p e rim e n ts can be a ch ieved in the m o s t d iv e r s ifie d m e d ia , ra n g in g fr o m sin g le c r y s ta ls o f a lk a li h a lid es and o x id es to o rg a n o m e ta llic s and v e r y e la b o ra te d b io m o le c u le s such as v ita m in В 12. T h e en viron m en t o f the p a ren t atom in the s o u rce m a te r ia l and the c h e m ic a l s ta b ility o f the daughter products have such a param ount in flu en ce that m any e x p e rim e n ts w ill be r e q u ir e d b e fo r e a ra tio n a le o f the re s u lts can be a ttem p ted . M o re attention should be g iv e n to the s im p le s t s y s te m s . One o f the e a r ly hopes o f M ô ssb a u er s p e c tro s c o p y was the d etection of unusual ch arge states o f n u cleogen ic atom s fo rm e d in an E C o r in a co n v e rte d I T . F o r in stan ce the p re d icted ch arge state o f ir o n in the d e c a y o f 57Co is about 5 to 6 units. In a ll in sta n ces, the n e u tra liza tio n p r o c e s s e s have been found to be c o m p lete w ithin 1 ns o f the d e c a y so that by the tim e the M ôssb a u er photons a re em itted the ch arge state and the en viron m en t o f the ion atom s a re w e ll d efin ed . One could ex p e c t that in a v e r y in e r t m a tr ix without fr e e e le c tro n s the chances o f d e te c tin g h igh er than n o rm a l ch a rge sta tes o f ir o n m ig h t be in c r e a s e d . A c tu a lly , M ôssb au er e m is s io n s p e c tro s c o p y o f 51 Co atom s is o la te d in a so lid xenon m a tr ix has s u rp ris in g ly r e v e a le d on ly n eu tral iro n in the 3d64s 2 co n figu ra tio n and F e + in the 3d 7 state 1 1 2 ]. Thus even in a r a r e gas m a tr ix , the h ig h e r ch a rge states re c o m b in e v e r y r a p id ly , p re s u m a b ly by a c h a rg e tr a n s fe r p r o c e s s with the r a r e g a s . E xten sion o f m a tr ix is o la tio n M ô ssb a u er e x p e rim e n ts to oth er n u clid es and in d iffe r e n t m ed ia (r a r e g a s e s , n itro g en , h yd ro ca rb o n ) is s tr o n g ly su ggested . Althou gh M ô ssb a u er s p e c tro s c o p y has p rom p ted the p o s s ib ility o f d e tectin g m eta sta b le states fo rm e d in a r a d io a c tiv e d eca y and studying re la x a tio n p r o c e s s e s , no con vin cin g e x p e rim e n ta l e vid en ce has so fa r been g a in ed . In the fe w in sta n ces w h ere the M ôssb a u er e m is s io n s p e c tra have been tra c e d as a function o f the tim e ela p sed a fte r 57Co d eca y, th ere was no e v id e n c e o f any change in the c h e m ic a l fo r m o f the iro n betw een 5 ns and 300 ns a fte r the d eca y [ 13-14] . H o w e v e r, m o st in te r e s tin g is the re c e n t fin d in g o f a tim e-d ep en d en t r e c o ille s s fr a c tio n [ 15] . T h e ra d io a c tiv e d ecay w hich feed s the M ôssb a u er tra n s itio n sets the nucleus in v ib ra tio n with r e s p e c t to its e q u ilib riu m p ositio n in the la ttic e . I f the duration o f this v ib ra tio n m ode is o f the o r d e r of the life t im e o f the n u c le a r le v e l, its re la x a tio n w ill be apparent in the M ôssb a u er sp ectru m by a d e c r e a s e o f the M ô ssb a u er e ffe c t and a n a rro w in g o f the peaks. T h is re la x a tio n has been o b s e rv e d , using d e la y e d -c o in c id e n c e M ôssb a u er s p e c tro s c o p y , at the F e 3+ but not at the F e 2+ s ite in C o S 0 4 - 7HaO s o u rc e s , w ith a c h a r a c te r is tic tim e o f 10 to 100 ns. S u rp ris in g ly no e ffe c t is apparent in oth er so u rc e s such as C o C l 2 • 6 H 20 .
3 .2 .
Ir r a d ia te d s o u rc e s and o n -lin e e x p e rim e n ts
M ô ssb a u er paren t atom s with lon g enough h a lf- liv e s can be d ir e c t ly g e n era ted in a s o u rce m a te r ia l containing the ta r g e t a to m s. T h is p ro ced u re has been ap p lied to lo n g - liv e d is o m e r s produced by neutron captu re, a ctu a lly so fa r s o le ly to 118 Sn(n, 7 ) 119 Snm. T h e ir r a d ia te d m a te r ia l s e r v e s as a M ô ssb a u er s o u rce and can a ls o be subm itted to a ra d io c h e m ic a l a n a ly s is . T h e s p e c tra show the e ffe c t o f both the ra d io a c tiv a tio n p ro c e s s and the d ecay to the M ô ssb a u er tra n s itio n . Such e x p e rim e n ts a r e w id e ly needed; they re q u e s t h ig h -a c tiv a tio n c r o s s - s e c tio n s , la r g e r e c o i l - f r e e fra c tio n s and a
66
ADLOFF
s u ffic ie n tly n a rro w lin e fo r the re s o lu tio n o f the h y p e rfin e p a ra m e te rs . F o r e x a m p le, a thorough in v e s tig a tio n o f tin sulphate SnSC>4 has shown that the s p e c ie s o b s e rv e d in the e m is s io n sp ectra o f 119 Snm(Sn4+and a new Sn2+ s ite ) a re s im ila r in nature w h eth er the m e ta sta b le 119 Snm n u clei a r e in c o rp o ra te d in the host by c h e m ic a l la b e llin g o r a r e produ ced b y neutron a c tiv a tio n o f usSn in the m a tr ix [ 1 6 ]. F u r th e r m o r e , the sam e s p e c ie s a r e produced by the m a c r o s c o p ic ra d ia tio n dam age o f the m a te r ia l. In te r e s tin g ly , the prod u ction o f Sn4+ s p e c ie s in c r e a s e s w ith the fa s t neutron d ose. A lte r n a tiv e ly , the M ossb a u er le v e l can be populated fro m a n u clea r re a c tio n o r by cou lom bic e x c ita tio n in o n -lin e e x p e rim e n ts ; this is e s s e n tia l i f th ere is a la ck o f a su itab le ra d io a c tiv e parent atom . T h e e x p e rim e n t is m o r e d iffic u lt to a ch ieve and at p resen t few re s u lts o f in te r e s t in hot atom c h e m is tr y have been c la im e d . T h e m o s t n otable is the p a rtia l oxid ation o f 57F e a fte r the s6F e (n , y) re a c tio n in h ydrated fe r r o u s sulphate [ 17] . O ther attem pts have been ra th e r u n su ccessfu l, g iv in g s p ectra with v e r y p oor reso n a n ce e ffe c ts o r no reso n a n ce at a ll [ 18] . On the oth er hand, o n -lin e fe e d in g o f the iro n M o ssb a u er tra n sitio n s b y (n , 7 ) [ 1 7 ] , o r (d, p) [1 9 ] in РегО з, g iv e s p e c tra w ith h y p e rfin e p a r a m e t e r s and D ebye W a lle r fa c to rs which m atch c o m p le te ly w ith those o f the ab so rp tio n sp ectru m . Th u s, 10' 7 s a fte r the a ctiva tio n p r o c e s s , e s s e n tia lly a ll e x c ite d r e c o il atom s a r e at n o rm a l la ttic e s ite s , the n e a re s t neighbours b ein g u ndisturbed. T h e s im ila r it y o f the beh aviou r o f r e c o il atom s with v e r y d iffe r e n t e n e r g ie s (500 eV in the (n, 7 ) and 105 eV in the (d, p) re a c tio n ) s u ggest that the hot atom s com e to r e s t by re p la c e m e n t c o llis io n s at low en ergy. Q u ite g e n e r a lly it ap p ea rs that a ll the rap id annealing p ro c e s s e s that d e te rm in e the in itia l reten tio n and the y ie ld s o f oth er products a re com p lete w ithin 1 ns. Thu s, M o ssb a u er s p e c tro s c o p y is unable to g iv e a p ictu re o f the p r im a r y e ffe c ts o f n u clea r tra n s fo rm a tio n s . H o w e v e r, it c o n s e rv e s a ll its unique advan tages fo r the d eterm in a tio n o f the fin a l ch e m ic a l state o f n u cleo gen ic s p e c ie s and as such is an in d isp en sa b le technique in hot atom c h e m is tr y (in clu d in g an n ealin g phenomena and s o lid -s ta te exchange r e a c tio n s ), and at p resen t re m a in s u nsurpassed by any o th e r m ethod.
4.
P E R T U R B E D A N G U L A R C O R R E L A T IO N S (P A C )
T h e p rin c ip le and the ch e m ic a l a p p lica tio n s o f P A C have been r e v ie w e d b y De B en ed etti [2 0 ] and V a rg a s [2 1 ], the la tte r with s p e c ia l m ention o f hot atom c h e m is try . T h e m ea su rem en t with P A C techniques and the s ig n ific a n c e o f h y p erfin e in te ra c tio n s o f r e c o il n u clei have been discu ssed b y G e lb e r g [ 22] a ls o . P A C m e a su rem en ts a r e m a in ly used in n u clea r ph ysics and so lid state p h y s ic s . A p p lic a tio n s in c h e m is try a r e s t ill s c a r c e and no m o re than h alf a d ozen p ap ers a re r e le v a n t to hot atom c h e m is try . T h e r e fo r e , it a p p ears a p p ro p ria te to outline b r ie f ly the p rin c ip le s o f P A C e x p e rim e n ts . In the m o st com m on c a s e a P A C e x p e rim e n t re q u ir e s a ra d io a c tiv e s o u rc e that decays through 7 - 7 cascad e (F ig . l a ) . T h e upper le v e l A is populated by any type o f r a d io a c tiv e d eca y. A s in M ossb a u er s p e c tro s c o p y , the r a d io a c tiv e paren t m ay be in trod u ced in the sam p le by ch em ica l la b e llin g , doping o r , a lte r n a tiv e ly , by a n u clea r re a c tio n .
1АЕА-РЬ615/5
67
FIG.2. Attenuation function G2(t) for an axially symmetrical electric field gradient acting on a I = 5/2 spin state.
T h e e m is s io n p ro b a b ility o f a 7 - r a y in a g iven d ire c tio n depends on the o rie n ta tio n o f the n u c le a r spin a x is r e la t iv e to that d ire c tio n . In an o rd in a ry r a d io a c tiv e so u rce the n u clei have th e ir spin o rie n te d at random in space and the ra d ia tio n e m itte d is is o tr o p ic . T o produ ce an a n is o tro p ic in ten sity p a ttern , the n u clea r spins m u st be o rie n te d . In angular c o r r e la tio n w ork , the o rien ta tio n is obtained sim p ly b j s e le c tin g the n u clei w hich have em itted the f ir s t 7 -r a y in a fix e d d ir e c tio n k1; which is in p r a c tic e the s o u r c e d e te c to r 1 a x is (F ig . lb ) . T h e in te rm e d ia te n u cle a r states В w ill have th e ir spins a lign ed r e la t iv e to k j and th e r e fo r e , the second photon 72 d etected in coin cid en ce with 7 1 is no m o r e is o tr o p ic . T h e an gu lar dependence o f the e m is s io n o f 7 2 is g iv e n by the c o r r e la tio n function W (0 ) w hich m e a s u re s the r e la t iv e p r o b a b ility that the tw o 7 -ra y s be em itted at an an gle 0 o f each oth er: W(0)
=
I + A 2P2 ( c o s 0) + A^P^(cos 0)
w h e re the A c o e ffic ie n ts depend on v a rio u s n u clea r p a ra m e te rs c h a r a c te r is tic o f the casca d e, and the P (c o s 0) a r e L e g e n d re p o ly n o m ia ls . T h e angular c o r r e la tio n m ay be p ertu rb ed when a cou plin g is esta b lish ed betw een the m om en ts o f the in te rm e d ia te n u clea r state and e x tra n u c le a r m a gn etic fie ld s (M l and E2 in te r a c tio n s ). T h e n u clea r spin w ill p r e c e s s around the m a gn etic fie ld o r around the e le c t r ic fie ld g ra d ien t a x is . C onsequ ently, the c o r r e la tio n w ill change in the tim e in te r v a l ela p sed b etw een the e m is s io n o f the f ir s t and second 7 - r a y s , and b eco m es W (0 ,t)
=
1 + A 2G2 ( t ) P 2 ( c o s 0 ) + A 4G4 ( t ) P A ( c o s 6 )
T h e e n tir e in fo rm a tio n on the h y p e rfin e in te ra c tio n is con cen trated in the tim e-d ep en d en t attenuation c o e ffic ie n ts Gk(t ). T h e o r e t ic a l e x p re s s io n s o f G]((t) h ave been e la b o ra te d fo r each type o f in te ra c tio n : e le c t r ic and m a g n e tic , s ta tic o r d yn a m ic. A s a ty p ic a l e x a m p le , the G 2(t) c o e ffic ie n t fo r
FIG.3. DPAC measurement on y-y cascades in luCd. Perturbations observed in Ag2(mAg, Cd(IUCdm,IT)Cl2and In(ulIn,EC)P04. Reproduced from Ref. [23].
A 9j S 0 4
6 )S04,
68 ADLOFF
IA E A -P L-61 5/5
69
an a x ia lly s y m m e tr ic a l fie ld g ra d ie n t a c tin g on a nucleus w ith spin I = 5/2, takes the p e rio d ic fo rm (F ig . 2) G ,(t) 2
=
s
2o
+ s
2J
c o s io t + s - . c o s 2ш t + s , , c o s З и t о 22 о 23 о
w h ere the s2n a r e tabulated g e o m e tr ic a l c o e ffic ie n ts and uq is the fundam ental fre q u e n c y r e la te d to the qu adru polar fre q u e n c y u q = e2 qQ /h 41(21 - 1) by the s im p le r e la tio n uo = 6 ljq. H e re eq stands fo r the e le c t r ic .field gra d ien t, and Q is the n u clea r quadrupole m om en t. A s can be seen , the attenuation c o e ffic ie n t lea d s s tra ig h t to the quadrupole cou plin g constant e 2q Q . In a n o n -a x ia l e le c t r ic fie ld gra d ien t, the G2 (t) function is s till p e r io d ic but depends on the a s y m m e try p a ra m e te r r¡. In p r a c tic e one r e g is t e r s the num ber o f y 2 e m itted in co in cid en ce with Y i as a function o f tim e , the s ta rt sign a l b ein g g iv e n by the d etection o f Y i D epending on the r e la t iv e va lu es o f the m ean life т o f the in te rm e d ia te le v e l, and the re s o lu tio n tim e t r o f the coin cid en ce c ir c u it, two d iffe r e n t types o f e x p e rim e n ts a re p e r fo r m e d . T h e m o s t valu ab le in fo rm a tio n is obtained fr o m the tim e d iffe r e n tia l m ethod (D P A C ), w hich is fe a s ib le i f t, « т and w hich d e liv e r s the G ^ t) c u rv e s . On the oth er hand, the tim e in te g r a l m ethod (IP A C ) has to be used w h en ever т is so sh ort that on ly the t im e in te g ra te d p ertu rb a tio n can be m ea su red , which takes the fo rm (I = 5/2, П = 0)
G,(“)
=
s
+
3 . 2 s n=l n 1 + (nco t ) 2 о
C le a r ly , Р А С and M ossb a u er s p e c tro s c o p y sh are com m on fe a tu re s . Both use y - r a y s e m itted a ft e r a n u clea r even t as a p ro b e, p ro v id e in -s itu an alysis on a 1 -ns to a 100-ns tim e in te r v a l a fte r the d eca y. Both s u ffe r a ls o fro m a lim ite d re s o lu tio n w hich m a y re n d e r the in te rp re ta tio n o f data d iffic u lt o r even im p o s s ib le , in p a rtic u la r when s e v e r a l d iffe r e n t n u cleogen ic sp e c ie s a r e fo r m e d . M o ssb a u er s p e c tro s c o p y shows a d e fin ite advantage o v e r Р А С in p ro v id in g the is o m e r sh ift fro m the EO in te ra c tio n . H o w e v e r, it is r e s t r ic t e d to s o lid phases and often to low te m p e ra tu re s , d e liv e r s only a tim e -in te g r a te d in fo rm a tio n , and has m o r e s e v e r e n u clea r lim ita tio n s . 4 .1 .
Р А С m e a s u re m e n ts on la b e lle d so u rces
In la b e lle d s o u rc e s , Р А С te lls o f the ch a rge state and en viron m en t of the atom s in the В le v e l as a consequ ence o f the ra d io a c tiv e d ecay which p re c e e d s the c a s c a d e . An illu s tr a tiv e ex a m p le is p ro vid ed by y - y cascad es in 11:1Cd w hich can be fed by IT fro m u l C dm, j3~ fro m m A g and E C fro m m In ( F i g . 3 ) [ 2 3 ] . T h e 84 ns - I = 5/2 -247 keV le v e l o f m Cd is quite convenient fo r D P A C m e a s u re m e n ts . A s exp ected , no change o f the a to m ic en viron m en t is o b s e rv e d a fte r the p o o r ly c o n verted IT ; the o b s e rv e d G 2(t) is c h a r a c te r is tic o f the s ite o f the nucleus b e fo r e the d e c a y . In the c a se o f 0 -d eca y, in a la r g e fr a c tio n o f the e v e n ts , a w e ll-d e fin e d fie ld g ra d ie n t acts on the daughter n u clei, w hich o b v io u s ly occu py the site o f the p aren t. In te r e s tin g ly enough, the daughter has to adopt the stru ctu re o f the p aren t com pound, which can lea d to unusual c o n fig u ra tio n s . F o r in stan ce, Р А С m ea su rem en t on ls lH f F »" 181 2" have shown that the daughter ion Т а F 7 is fo r c e d to take the pentagonal
70
ADLOFF
0
10
20
30 ns
FIG. 4. DPAC measurements on (a) 181Hf-labelled and (b) neutron-irradiated Hf CDTA NH4. The full line is drawn from the fitted data. A new hafnium site is apparent in the irradiated material. The radiochemically measured retention is ЗОУо.
b i-p y r a m id a l s tru c tu re o f the hafnium c o m p le x , ra th e r than the trig o n a l p r is m a tic s tru c tu re ty p ic a l fo r n o rm a l T a F 2" [ 21] . When the ca sca d e is fed by an E C d ecay, the an gu lar c o r r e la tio n is s tr o n g ly attenuated and no p e r io d ic m odu lation is o b s e rv e d . T h is re s u lt is co h eren t w ith the A u g e r ch a rgin g p ro c e s s and the a u to ra d io ly s is w hich a re o p e r a tiv e b e fo r e the e m is s io n o f the photons, and p resu m a b ly g iv e r is e to a b road sp ectru m o f in te ra c tio n fre q u e n c ie s . In liqu id s y s te m s it is exp ected that the m o le c u la r m otion a v e r a g e s the lo c a l fie ld g ra d ien t at the nucleus and the a ft e r - e ffe c t s a r e annealed v e r y ra p id ly .
IA E A -P L-61 5/5
4 .2 .
71
P A C m ea su rem en ts on {n,y) r e c o ils
T h e f ir s t P A C e x p e rim e n t in hot atom c h e m is try has been p e r fo r m e d on neutron ir r a d ia te d potassiu m p errh en a te [ 27] . A m ea su rem en t o f the a n gu lar c o r r e la tio n o f 18sOs showed that the 188R e r e c o il atom s w e r e not in n o rm a l la ttic e s ite s and p rob a b ly a ls o not in the sam e c h e m ic a l fo rm as the ta r g e t m o le c u le . I P A C m ea su rem en ts on s e v e r a l p e rrh en a tes and K^ReCle a re con sisten t w ith the action o f a com bin ed m a gn etic and e le c t r ic p e r t u r bation at the s ite o f the r e c o il n u clei [ 25] . On th e rm a l an n ealin g, the c h a r a c te r o f the p ertu rb a tio n changes to pure qu adru polar e le c t r ic in the c h lo ro c o m p le x , w h ile the m a g n e tic c h a ra c te r in c r e a s e s in the p e rrh e n a te s , show ing the fo rm a tio n o f p a ra m a g n etic com pounds. O f p a rtic u la r in te r e s t a re D P A C m e a s u re m e n ts on hafnium compounds and the re la tio n o f P A C data to re te n tio n valu es [ 22] . G 2 (t) cu rv e s o f ir r a d ia te d hafnium com pounds m ay be com p a red w ith the attenuation fa c to r in the la b e lle d m a te r ia ls . In the fo r m e r c a se, the p e r io d ic m odu lations a re s m e a re d so much m o re that the re te n tio n has a lo w e r v a lu e . H o w e v e r, the s tru c tu re o f G 2(t) re m a in s apparent in m o s t c a s e s , su pporting the id ea that the r e c o il atom s take a few d istin ct c h e m ic a l fo r m s . Illu s tr a tiv e ex a m p les a re shown on F ig . 4 [ 26 ] . On the oth er hand, no p ertu rb a tio n o f the an gu lar c o r r e la tio n o f the 140C e y - y cascad e has been o b s e rv e d when the 140L a parent atom s a re p rodu ced in io n ic o r co m p le x lanthanum com pounds [ 2 7 ]. 181 H f and 188R e a r e v e r y fortu nate ca s e s f o r P A C m e a su rem en ts in hot atom c h e m is tr y and fu rth e r w o rk on th ese and on o th e r n u clei is h igh ly d e s ir a b le , m a in ly in connection w ith r a d io c h e m ic a l a n a ly s e s .
5.
A L T E R A T IO N O F N U C L E A R D E C A Y R A T E S
T h e in flu en ce o f the ch e m ic a l en viro n m en t on the d eca y p ro b a b ility o f a r a d io a c tiv e nucleus is a w ell-k n o w n phenom enon w hich has been r e v ie w e d r e c e n t ly b y V a rg a s [2 1 ] and E m e r y [ 2 8 ] . T h e d eca y p ro c e s s m o st s e n s itiv e to c h e m ic a l bondings is the e le c tro n cap tu re. T h is is a ty p ic a l EO i n t e r a ctio n and has a p ro b a b ility d ir e c t ly re la te d to the К o r L e le c tr o n d en sity at the nucleus and thus to the o v e r a ll o rg a n iza tio n o f the a to m ic c o r e , i . e . to c h e m ic a l bonding and s c re e n in g p a r a m e te r s . H o w e v e r, the ou term o st e le c tr o n s w hich a r e in v o lv e d in the bondings con tribu te v e r y lit t le to the e le c tr o n d en sity at the nucleus w ith the ex cep tio n o f the lig h te r elem en ts am ong w hich 7B e is the b est can didate. R e la tiv e h a lf- life changes o r ДХ/Х am ounts to 0 . 1 % in b e r y lliu m com pounds, w hich s e e m s to be an u pper lim it o f the c h e m ic a l p ertu rb a tion o f a d ecay ra te , with the v e r y notable excep tion o f 90Nb fo r w hich a change as la r g e as 3.5% has been r e p o rte d . W h e re a s the a lte r a tio n o f E C d eca y r a te s w ith the c h e m ic a l stru ctu re has been v e r ifie d in d e fin itiv e though s o p h istica ted and pain stakin g m e a s u r e m e n ts , it re m a in s doubtful if, c o n v e r s e ly , the c h e m ic a l state o f a n a to m can be in fe r r e d fr o m ДХ/Х v a lu e s . It is e a s ily shown that the f i r s t re q u is ite fo r a s u c c e s s fu l m ea su rem en t is the r a d io c h e m ic a l p u rity o f the sa m p le and the c h e m ic a l id e n tity o f a ll the r a d io a c tiv e a to m s . T h is re q u ire m e n t is h a rd ly fu lfille d in hot atom c h e m is tr y . H o w e v e r, in one in stan ce at le a s t it has b een r e p o r te d that the d e c a y ra te o f r e c o il a tom s d iffe r s fr o m th ose o f the atom s c h e m ic a lly reta in ed in the paren t compound: in n e u tro n -irra d ia te d a -c o p p e r phthalocyanine the X va lu e o f r e c o il 64Cu (w h ich has a h a lf- life o f
72
ADLOFF
12.8 h and d ecays by negaton e m is s io n (40% ), p o s itro n e m is s io n (19%) and e le c tr o n captu re (41% )) is s m a lle r than that o f 64Cu in the la b e lle d compound by an amount ДЛ/Х = (10 ± 1. 6 ) X 10-4 [ 2 9 ] . H o w e v e r, another grou p o f au th ors, as a m a tte r o f fa ct with a d iffe r e n t counting d e v ic e , did not find w ithin e x p e rim e n ta l e r r o r s any d iffe r e n c e in the h a lf- life o f 64Cu in both c h e m ic a l fo r m s I 30J . A m on g oth er n u clides w hich could be a p r io r i m o re su itable fo r ДЛ/Х m e a su rem en ts, one finds 51C r (T = 27.8 d, E C 100%), this atom h avin g a lo w e r Z than 64Cu and a g r e a t e r ra n ge o f oxid ation states [3 0 ] . A lte r a tio n o f IT d ecay ra te s could in p rin c ip le be used fo r the sam e p u rp ose. H ig h ly c o n verted and lo w - e n e r g y tra n s itio n s a r e m o s t a p p ro p ria te . Just as in the p re v io u s c a s e , thorough d iffe r e n tia l m ea su rem en ts must be p e r fo r m e d sin ce the exp ected e ffe c t is o f the o r d e r o f 0 .1 to 0 . 0 1 % [2 8 ]. M o re subtle d e ta ils on the ch e m ic a l bonding o f r a d io a c tiv e atom s m ay be gained fr o m the m ea su rem en t o f the in te n s itie s o f e le c tro n c o n v e rs io n lin es w hich a re p ro p o rtio n a l to the d e n s itie s in the sh ells and su b sh ells. No a p p lica tio n o f such m ea su rem en ts to hot atom c h e m is try have been re p o rte d up to now.
6.
N U C LE A R RESONANCE FLU O RESCENCE
M o st n u clea r re a c tio n s and ra d io a c tiv e decays lead to e x c ite d states o f the produced n u clei. T h e photons em itted in the subsequent d e -e x c ita tio n to the ground sta te can act as a v e r y u sefu l p rob e in hot atom c h e m is try , as in M ô ssb a u er s p e c tro s c o p y and an gu lar c o r r e la tio n m e a s u re m e n ts . A n oth er w a y to take advantage o f the photons is to esta b lish con dition s fo r a reson an t ab sorp tion o r s c a tte rin g o f the y -r a y s . T h e o b s e rv a tio n o f the reson an t flu o re s c e n c e o f y -r a y s is in h eren tly d iffic u lt b ecau se o f the e n e rg y lo s s o f the photons in a b so rp tio n , em is s io n and s c a tte r in g p r o c e s s e s . T h e natural width Г o f 7 -lin e s g iv e n by the u n certain ty re la tio n Г T =
h
=
6.56 x ]0~J 6 eV • s
is o f the o r d e r o f 10" 8 eV f o r n u clear le v e ls with 10-ns life t im e s . On the o th e r hand, the r e c o il e n e rg y E R o f an atom o f m a ss M = 100 amu, having ab sorb ed o r em itted a 100-keV photon, is 5 X 10" 2 e V . Thus, the e n e rg y lo s s o f the photon e x c e e d s by a la r g e amount the w idth o f the 7 -lin e s . The e m is s io n and a b sorp tion lin e s a r e sep a ra ted by an amount E 2/M c 2 (F ig . 5). T h e conditions in w hich the n u clea r reso n a n ce flu o r e s c e n c e (N R F ) can be esta b lis h e d and the fo rm u la tio n o f the p ro b le m have been th orou gh ly r e v ie w e d by M e t z g e r [ 31 ] . T h e o v e r la p o f the lin e s can re s u lt fr o m the D o p p ler b road en in g due to the th e rm a l m otion s o f the e m ittin g (o r ab so rb in g) atom s o r m o le c u le s . Th u s, h eatin g the e m itte r (o r s o u rc e ) m a y be s u fficien t to o b s e r v e the N R F , as shown by the dotted c u rv e on F ig . 5. T h e calcu lation shows that the c r o s s - s e c t io n o f the reson an t ab sorp tion o r s c a tte rin g re m a in s v e r y s m a ll. H o w e v e r, the h eatin g o r co o lin g o f the s o u rce (o r the a b s o rb e r, o r the s c a t t e r e r ) m a y be a con ven ien t w ay to ch eck the v e r y o c c u rre n c e o f the N R F . A n o th er w ay to set up the re so n a n ce conditions r e lie s on the D o p p ler sh ift o f the e n e rg y o f the photon em itted by a m o v in g nucleus. C o n sid er the s o u rce and a b s o r b e r n u clei in itia lly at r e s t . T h e r a d io a c tiv e d eca y o r the
IA E A -P L-61 5/5
Emission
73
Absorption
FIG. 5. Profiles of у-emission and absorption lines. The lines are shifted by an amount ± ER (ER = recoil energy). The dotted curve represents the Doppler-broadened source line which partially overlaps the absorption line.
n u c le a r re a c tio n in the so u rce im p a rts an in itia l m om entum p¡ to the e x c ite d n u cleu s. T h e photon m om entum is p^, = E^/c. F o r fr e e atom s, the reson a n ce con dition is such that Pi
E
~ Б 0
R
PY
+ — --M
=
E O
+ E R
w h ich is s a tis fie d i f the in itia l v e lo c it y o f the r e c o ilin g atom in the d ir e c tio n o f the photon v e r if ie s v . cos о 6 = v 1
E
о
=
—°
Me
T h e in itia l m om entum and v e lo c it y d istrib u tio n s depend on the type o f d eca y w hich populates the e x c ite d le v e l. In an E C d ecay, the so u rce lin e has a re c ta n g u la r shape, assu m in g the e m is s io n o f a m o n o e n e rg e tic n eu trino. H o w e v e r , b ecau se o f the subsequent e m is s io n o f X - r a y s and A u g e r e le c tr o n s , the actu al shape is v e r y c o m p lic a te d . I f one in clu des a M a x w e llia n d i s t r i bution fo r the th e rm a l v e lo c it y o f the a tom s, the shape o f the s o u rce lin e is sm oothed out and b e c o m e s a p p ro x im a te ly G aussian. A broad e m is s io n lin e is a ls o exp ected a ft e r /3-d eca y. A c tu a lly , the lin e p r o file depends on the slo w in g-d o w n p ro c e s s o f the r e c o il a tom s, w hich lo s e e n e r g y in c o llis io n s with the su rrou n din g. The photons, o f c o u rs e , m u st be e m itted b e fo r e the nucleus co m es to r e s t . T h e c o llis io n tim e is about 1 0 " 13 s in the condensed phase and 1 0 " 9 s in g a ses at a tm o s p h e ric p r e s s u r e . T h e s e va lu e s set the lim it on the life tim e s o f the n u clea r le v e ls fo r w hich the reso n a n ce condition can be r e s to r e d by the e ffe c t o f the n u clea r tra n s fo rm a tio n . It is obviou s that N R F e x p e rim e n ts a r e e a s ie r to o b s e r v e with gaseou s s o u rc e s .
ADLOFF
74
T h e a p p lica tio n o f N R F e x p e rim e n ts to hot atom c h e m is tr y is s tr a ig h t fo r w a r d and has been d iscu ssed e ls e w h e r e [ 3 2 ]. In the gas phase, m o le c u la r d isru p tio n s fo llo w in g the r a d io a c tiv e d eca y a r e r e a d ily d em on stra ted . O f p a r tic u la r in te r e s t is the d etectio n o f co u lom b ic fra g m en ta tio n a fte r a pure 0 -d e c a y . R esonant s c a tte rin g e x p e rim e n ts o f the photons em itted by 131X e fo llo w in g /3-d e c a y o f 131I in gaseou s I 2, C H 3I, a lk a li io d id e s have shown that the m o le c u le s a r e ru ptu red a fte r the d eca y [ 3 3 ]. T h e e x p e r i m en ta l re so n a n ce c r o s s - s e c t io n is in good a g re e m e n t w ith p re d ic tio n s b ased on the s h a k e -o ff o f an in n e r e le c tr o n w hich t r ig g e r s A u g e r ca sca d es. T h e fra g m en ta tio n o f 131I - la b e lle d o rg a n ic io d id e s was in v e s tig a te d a lo n g tim e b e fo r e b y c h a rg e s p e c tr o m e tr y . T h e d iffe r e n c e o f both techniques lie s in the t im e - s c a le . T o c o lle c t and d e tect ions in the m a ss s p e c tr o m e te r r e q u ir e s 1 0 " 4 to 1 0 ‘ 5 s, w h ile the m ea su rem en t o f the reson an t photons n e c e s s a r ily p ro c e e d s in a tim e s h o r te r than the life tim e o f the e x c ite d le v e ls , fr o m 0. 7 to 70 ps in the p re s e n t c a s e . In addition, ch a rge s p e c t r o m e tr y g iv e s the net consequ ence o f the to ta l d eca y (tran sm u tation and in te rn a l c o n v e rs io n o f e x c ite d le v e ls ), w h ile N R F e x p e rim e n ts an alyse the e ffe c ts o f the 0 -d e c a y s o le ly o r the e ffe c ts o f the d eca y and o f a s e le c te d d e - e x c i tation c a sca d e. A fu rth e r in te r e s t lie s in the dependence o f the so u rce lin e p r o file on the slo w in g -d o w n m ech a n ism . Ion m o le c u le re a c tio n s have thus been in v e s tig a te d in N R F e x p e rim e n ts [ 34 ]. In the liq u id and s o lid ph ases, N R F m e a su rem en ts g iv e unique i n f o r m a tio n on the slo w in g-d o w n p r o c e s s o f hot atom s in the e n e rg y ran ge b e lo w 300 e V , which can h a rd ly be in v e s tig a te d by any o th e r m ethod. T h e re le v a n t e x p e rim e n ta l data a r e the shape o f the e m is s io n lin e and the in te n s ity o f the re s o n a n tly s c a tte re d photons. B e s id e s the slow in g-d ow n p r o c e s s o f the r e c o il atom s the r o le s o f r a d ia tio n - and n eu tron-indu ced d e fe c ts o f the la ttic e p o ten tia l, o f the a n is o tro p y o f the r e c o il d ire c tio n in m o n o c r y s ta l, e tc . can be a s s e s s e d . T h e v e r y p e c u lia r advantage o f N R F lie s in the d y n a m ica l aspect: the in v e s tig a tio n co n c e rn s " liv in g " hot s p e c ie s sin ce, as soon as the r e c o il atom s co m e to r e s t, no m o r e reso n a n ce is o b s e rv e d .
7.
O P T I C A L E X C IT A T IO N IN 0 -D E C A Y
T h e s h a k e -o ff e x c ita tio n o f a tom s and m o le c u le s a fte r )3-decay has been the su b ject o f num erous th e o r e tic a l in v e s tig a tio n s [ 1 ]. B r ie fly , b e fo r e the e m is s io n the e le c tr o n s a re in a cou lom b fie ld o f n u clea r c h a rg e Z with e ig e n s ta te s d e s c rib e d b y the eigen fu n ction s (//„(Z). T h e 0’ decay p ro m o tes a sudden change o f the n u c le a r c h a rg e fr o m Z to Z + 1 and the new e ig e n functions o f the s y stem b eco m e <//„■ (Z + l ) . E xpanding the in itia l function in a lin e a r com bin ation o f the new ones,
* ° < Z )
=
2
j
с -
J
Ф -
i (Z
+
1)
J
the squ ared c^'s g iv e the tra n s itio n p ro b a b ility fro m the in itia l state to a fin a l state Pnn'
=
< ' V
Z + 1)1 * n (Z ) >2
1АЕА-РЬ615/5
75
T h e old and new eigen fu n ction s a r e not orth ogon al and the new states b elon g not o n ly to the ground state o f the daughter io n s, but a ls o to e x c ite d sta tes. V a rio u s authors h a ve com puted the P nn' p r o b a b ilitie s fr o m m o r e o r le s s r e a lis t ic w a ve fu n ction s. T h e m a tr ix ele m e n ts o f the P nn' e x p re s s io n a r e n o n -z e r o on ly fo r n 1 f n. N e g le c tin g the d ir e c t in te ra c tio n o f the em itted 0 - p a r tic le w ith the a to m ic e le c tr o n s , the to ta l a n gu lar m om entum o f the e le c tr o n s h e lls m ust re m a in unchanged, w hich lea d s to the A J = 0 and A M = 0 s e le c tio n r u le s fo r a m on op ole e x c ita tio n . F u r th e r m o r e , in the c e n tra l fie ld a p p ro x im a tio n it is found that the p ertu rb a tion o p e ra to r acts o n ly on the ra d ia l p a rt o f the w a ve functions so that each e le c tr o n k eep s its i, m { , m and j quantum n u m b ers. Thu s, the 0 -d e c a y -in d u c e d ex cita tio n ex h ib its stro n g s e le c tio n r u le s a llo w in g on ly a lim ite d num ber o f e x c ite d s ta te s . T h e r e a r r a n g e m e n t o f the a to m ic c o r e in gaseou s r a d io a c tiv e sou rces m a n ife s ts it s e l f by the lu m in escen ce [ 3 6 ], w h ile in s o lid s o u rc e s the e m is s io n sp ectru m o f the o p tic a l photons shows the in flu en ce o f the su rrou n din g on the d e c a y p ro c e s s o f the dau ghter io n s. F e w e x p e rim e n ta l data a r e s t ill a v a ila b le . W e x le r and P o r t e r [3 5 ] have been s u c cessfu l in m e a s u rin g w ith a R am an s p e c tro g ra p h the 4686 Â lin e o f the 4s -*■ 3p o p tica l tra n s itio n in (3 H e)+ fo llo w in g |3-d e c a y o f tritiu m . M ic k litz , L u ch n er et al. 138-43] h ave m ad e an e x te n s iv e in v e s tig a tio n o f o p tica l e x c ita tio n p r o c e s s e s a fte r /3-d e c a y o f ra d io a c tiv e atom s em bedded in s o lid r a r e - g a s m a tr ic e s . In th e s e e x p e rim e n ts the photons e m itted in the d ip ole tra n s itio n fr o m the e x c ite d dau ghter ions a r e m e a s u re d in d e la y e d coin cid en ce w ith the j3p a r tic le . W ith this p ro c e d u re the tim e o f the e x c ita tio n is w e ll defin ed, m a k in g it p o s s ib le to m e a s u re the life t im e o f the e x c ité d sta te s . B e s id e s the s h a k e -o ff o f the e le c tr o n cloud o f the dau ghter ion s, c h a rg e and e n e rg y t r a n s fe r p r o c e s s e s betw een the n u cleo g en ic s p e c ie s and the surroundings a re o f im p o rta n c e in d e te rm in in g the shape and in te n s ity o f the photon sp e c tru m . A to m ic d is p la c e m e n ts fo llo w in g 0 -d e c a y in r a r e - g a s m a tr ic e s and d is p la c e m e n t e n e rg ie s a r e a ls o in fe r r e d fr o m the e x p e rim e n ta l r e s u lts . C le a r ly o p tic a l m e a s u re m e n ts open new paths fo r the studies o f the p r im a r y e ffe c ts o f n u clea r tra n s fo rm a tio n s .
8.
C O N C L U S IO N
T h e p h ysica l m eth ods which have been d e s c rib e d a re not exh au stive, but th ey r e p r e s e n t the m o s t used o r m o s t p ro m is in g techniqu es a v a ila b le at p re s e n t. No one o f th ese m eth ods is o f g e n e ra l use and each p ro b le m m ust be c o n s id e re d s p e c ific a lly . A s fa r as p o s s ib le it is h ig h ly reco m m en d ed to use p h y sica l m eth od s in conjunction w ith ra d io c h e m ic a l a n a ly s is .
REFERENCES [1 ]
WEXLER,
S..
"Prim ary physical and c h em ic a l effects associated w ith emission o f radiation in nuclear
processes” , A ction s Chim iques et Biologiques des Radiations (H A IS SIN S K Y, M ., Ed.). H u itièm e Série, Masson, Paris (1965) 105 - A re v ie w . [2 ]
DE W IE C LAW IK , W . , PERRIN, N . , J. Physique 30 (1969) 877.
[3 ]
MEYER, J. , PAULUS, J . , ABBE, J .C h ., Radiochim . A cta 17 (1972 ) 76.
[4 ]
SHIRLEY, D . A . , S cien ce 161 (1968) 745.
76
ADLOFF
[5 ]
W ERTHEIM , G .К . , Accounts Chem . Res. 4 (1971) 373.
[6 ]
ADLOFF, J.P. , FRIEDT, J. M . , "Mossbauer studies o f ch em ic a l effects o f nuclear transform ations",
[7 ]
M A D D O C K , A .G . , "Mossbauer spectroscopy in the study o f the ch em ic a l effects o f nuclear
Mossbauer E ffect and its A pplications, IA E A , Vienna (1972) 301. transform ations", M T P Int. Rev. S c i., Radiochem istry, Inorg. Chem . Ser. 1, 8, Butterworths, London (1972) 213. [8 ]
Z A H N , U . , PO TZ E L, W. , WAGNER, F. E . , "Consequences o f nuclear transformations in ch em ical compounds studied b y th e Mossbauer e ff e c t " , Perspectives in Mossbauer Spectroscopy (COHEN, S. G. , PASTERN AK, M . , Eds), Plenum Press, New York (1973) 55.
[9 ] [1 0 ]
AMBE, F . , SHOSI, H . , AMBE, S ., TA K E D A , М . , S A IT O , N . , Chem . Phys. Lett. 14 (1972) 522. AMBE, F . , AMBE, S ., Phys. Lett. 43A (1973) 399.
[1 1 ]
LLABADOR, Y . , J. Inorg. Nu cl. C h e m ., in press.
[1 2 ]
M I C K U T Z , H ., BARRET, P. H. , Phys. Rev. Lett. 28 (1972) 1547.
[1 3 ]
TRIFTS HAUSER, W . , CRAIG, P . P . , Phys. Rev. 162 (1967) 274.
[1 4 ]
TRIFTSHÀUSER, W . , SCHROERR, D . , Phys. Rev. 187 (1969) 491.
[1 5 ]
H O Y, G . R . , WINTERSTEINER, P . P . , Phys. Rev. Lett. 28 (1972)
[1 6 ]
FRIEDT, J .M . , VO G L, W . , Phys. Stat. Sol. , in press.
[1 7 ]
BERGER, W .G . , FINK, J. , OBENSHAIN, F. E. , Phys. L ett. 25A (1967) 466. J . , Riso Rep.
877.
[1 8 ]
FENGER,
[1 9 ]
CHR ISTIANSEN ; J. , M A H NK E, H .E . , MORFELD, U. , RECKNAGEL, E. , RIEGEL, D. , W IT T H U H N , W. ,
Danish A to m ic Energy Comm ission (1971) 255.
[2 0 ]
D EB ENEDETTI, J . , DE BARROS, F ., H O Y, G. R . , Ann. Rev. N u cl. S cien ce ¿6 (1971) 31.
Z . Phys. 261 (1973) 13. [2 1 ]
VA RG A S, J .I. , "T h e ch em ic a l applications o f angular correlations and h a lf - life measurements", M T P Int. Rev. S c i., Radiochem istry, Inor. Chem . Ser. 1, 3 , Butterworths, London (1972) 45.
[2 2 ]
GELBERG, A . , Rev. Roum. Phys. 17 (1972) 459.
[2 3 ]
H A A S , A . , SHIRLEY, D . A . , J. Chem . Phys. 58 (1973 ) 3 339.
[2 4 ]
S A T O , J . , Y O K O Y A M A , Y . , Y A M A Z A K I, T . , Radiochim . A c ta 5 (1966) 115.
[2 5 ]
B A D IC A , T . , GELBERG, A . , SALAG EAN U, S ., IO N -M IH A IR , R. , IA N O V IC I, E. , Z A IT S E V A , N . G. , Radiochim . A cta 16 (1971) 36.
[2 6 ]
M ARQ U ES-NETTO , A . , ABBE, J .C h ., to b e published.
[2 7 ]
VASU D EV, P. , JONES, C . H .W . , Radiochim . A cta 17 (1972) 121.
[2 8 ]
EMERY, G .T . , Ann. Rev. N u cl. S cien ce 22 (1972) 165.
[2 9 ]
A U R IC , P ., V ARG AS, J .I. , Chem . Fhys. Lett. 15 (1972) 366.
[3 0 ]
DEMA, J . , HARBOTTLE, G. , V II Int. Symp. Hot A tom Chem istry, Jiilich, F.R. Germany (1973).
[3 1 ]
METZGER, F.R. , Progr. N u cl. Phys. 2 (1 9 5 9 ) 53.
[3 2 ]
ADLOFF, J.P. , Radiochim . A cta 15 (1971) 135.
[3 3 ]
LANGHOFF, H . , Phys. Rev. A 3 (1 9 7 1 )1 .
[3 4 ]
SCHUMACHER, M . , BORCHERT, I. , SMEND, F . , Z. Phys. 263 (1973) 49.
[3 5 ]
WEXLER, S . , PORTER, F .T . , J. Chem . Phys. 50 (1969) 5428.
[3 6 ]
PARSCHE, H. , LUCHNER, K . , Phys. Lett. 26A (1967) 9.
[3 7 ]
A N D R A , H.J. , Z . Phys. 215 (1968) 279.
[3 8 ]
M IC K U T Z ,
H .,
Z. Phys. 215 (1968) 302.
[3 9 ]
M IC K L IT Z ,
H .,
LUCHNER, K .
,Z . Naturforsch. 22a (1967)
[4 0 ]
M IC K L IT Z ,
H. ,
LUCHNER, K .
,Z . Phys. 227 (1969) 301.
[4 1 ]
LUCHNER, K. , M I C K U T Z , H . ,
1650.
J. Luminescence 12 (1970) 368.
[4 2 ]
LUCHNER, K . , PARSCHE, H. , M IC K L IT Z , H. , Z . Naturfbrsch. 26a (1971) 423.
[4 3 ]
GERTH, G. , LUCHNER, K . , M IC K U T Z , H. , Phys. Stat. Sol. b. 53 (1972) 593.
D IS C U S S IO N J. D A N O N : O n -lin e e x p e rim e n ts can be conducted in v o lv in g M ossb au er le v e ls populated by n u c le a r re a c tio n s o r by cou lom b e x c ita tio n m ethods. T h e r e a re a num ber o f n u clides — the r a r e - e a r t h re g io n , the actin id e re g io n , p ota ssiu m , fo r in stan ce — w hich, in p rin c ip le , would r e a lly be in te re s tin g fo r hot atom c h e m is tr y . Som e o f th ese n u clid es a r e lim ite d by the fa ct that the M ossb a u er sp ectru m is not w e ll r e s o lv e d . Som e o f the r a r e earth s would
IAEA-PL-615/5
77
g iv e la r g e e ffe c t s , p e rm ittin g the d etectio n s o f s m a ll fra c tio n s o f these s p e c ie s lo c a te d in d iffe r e n t oxid ation s ta te s . I think it would be w o rth w h ile to ex a m in e the p o s s ib ility o f such e x p e rim e n ts . J . F . A D L O F F : T h e r a r e earth s have been v e r y lit t le studied in hot atom c h e m is try , and it is an in te r e s tin g s e r ie s o f e le m e n ts . Although the dom inant oxid ation sta te is +3, m o st o f the r a r e ea rth s have a ls o le s s stab le v a le n c y sta tes w hich a r e v e r y in te r e s tin g fro m the fundam ental point o f v ie w . S om e o f th ese unstable v a le n c y sta tes could be o b s e rv e d by M ossb a u er stu dies — fo r in stan ce, d y sp ro siu m has been o b s e rv e d by M o ssb a u er e m is s io n s p e c tro s c o p y . S. A M IE L : Since E S C A is b ein g used fo r the study o f m o le c u la r stru ctu re, do you know o f any e x p e rim e n ts in w hich E S C A has been ap p lied to the study o f the fa te o f hot a tom s in a c e rta in m a tr ix ? J. P . A D L O F F : I ca n 't understand how E S C A could be used f o r such a p r o b le m . I f you use a p h y sica l technique in hot atom c h e m is try , you have to get the sign a l fr o m the r e c o il s p e c ie s , and in E S C A you b rin g a sign a l to the m a te r ia l. You ir r a d ia te the m a te r ia l with you r X - r a y s , e tc . , and look at the e n e rg y o f the em itted e le c tr o n s . How w ould you get the sign a l fr o m such s c a r c e s p e c ie s as the r e c o il a tom s? S. A M IE L : I m ean E S C A -lik e . Y ou g a ve ex a m p les in w hich the p re s e n c e o f ca g es was in v e s tig a te d . I am thinking o f atom s r e c o ilin g to a s u rfa c e , w ith e m is s io n o f one gam m a r a y fo llo w e d by in te rn a l c o n v e rs io n o f a secon d . A l l you need is a d e te c to r to m ea su re the f i r s t gam m a r a y plus an oth er d e te c to r to m e a s u re the e n e rg y o f the c o n v e rs io n e le c tr o n . In o th e r w o rd s , an E S C A -lik e e x p e rim e n t, w h ich could g iv e you qu ite a lo t o f in fo rm a tio n w ith the re s o lu tio n that is a v a ila b le in e le c tr o n s p e c tro s c o p y about the c h e m ic a lly unperturbed atom im p lan ted underneath a c e rta in s u rfa c e . J . P . A D L O F F : U ntil now I have no in fo rm a tio n about such an e x p e rim e n t. J. D A N O N : It w ould be a kind o f g a m m a -e le c tr o n c o in cid en ce e x p e rim e n t, which has been done. J . P . A D L O F F : In p ertu rb ed an gu lar c o r r e la tio n , y e s . S. A M IE L : A l l I'm tr y in g to say h e re is this: by know ing the e n e rg y sh ift o f the e le c tr o n w hich is co n v e rte d by the second gam m a ra y , you can get quite p r e c is e in fo rm a tio n about the c h e m ic a l state o f the im p lan ted atom , o r the r e c o ilin g atom in a c e rta in m a tr ix . A . G . M A D D O C K : T h e r e is the p o s s ib ility h e r e o f an e x p e rim e n t which is not so much o f hot atom in te r e s t but o f g e n e r a l s tru c tu ra l, c h e m ic a l to o l in the sam e w ay that E S C A , flu o re s c e n t X - r a y e m is s io n and p h o toelectron s p e c tro s c o p y can be used. F lu o re s c e n t X - r a y e m is s io n has som e in te re s t becau se the photon e m is s io n g iv e s the a d d ition a l in fo rm a tio n in v o lv e d through the s e le c tio n ru le s . So fa r , such e x p e rim e n ts h ave been pursued b y using X - r a y e x cita tio n , but the tro u b le w ith X - r a d ia tio n is that the flu o re s c e n t y ie ld s a re such that you have to bash the s a m p le until th e re is h a rd ly any o f it le ft by the tim e anything m e a s u ra b le c o m es out. T h e r e is d is tin c t in te r e s t in using the m uch m o r e e ffic ie n t — a lm o s t 1 0 0 % — ex cita tio n in v o lv e d w ith the use o f o r b it a l- e le c t r o n ca p tu rin g is o to p e as a probe fo r the com pound in w hich you a re in te r e s te d . T h is r e q u ir e s ra th e r la r g e r s p e c ific r a d io a c tiv itie s than one handles in the o rd in a ry r e c o il e x p e rim e n t — it pushes you up into the s e v e r a l m illic u r ie re g io n , ra th e r than with m ic r o c u r ie s - but the le v e ls a r e not d is tu rb in g ly la r g e .
78
ADLOFF
T . S H IO K A W A : You have com m ented that the c h a rg e s p e c tro m e try m e a su rem en ts have n e a rly exhausted the fie ld . R e c e n tly , I have published two p a p ers in w hich our r e s u lts a re in c o n flic t w ith the C a rls o n -W h ite e x p lo s io n m o d e l. T h e e x p e rim e n ts show the e x is te n c e o f som e h y d ro g en d e fic ie n t ions a fte r a to m ic tra n s itio n s in the S0B r -m e th y l b ro m id e system . C o m p lete d isru p tion o f the m o le c u le did not o c c u r. I b e lie v e that it w ill be n e c e s s a r y to c a r r y out m any m o r e such ch a rge s p e c tr o m e tr y ex p e rim e n ts in o r d e r to understand th ese phenom ena. J . P . A D L O F F : I a p o lo g iz e fo r not h avin g m entioned you r w ork , but I on ly le a rn e d about it th is m o rn in g , and the p u b lication s have not yet a p p eared . G. S T O C K L IN : A r e you ta lk in g. P r o f e s s o r Shiokawa, about y o u r paper in R a d io c h e m ic a l and R a d io a n a ly tic a l L e t t e r s in which you d is c o v e r e d these h y d ro g e n -d e fic ie n t s p e c ie s , and put up a th esis in explan ation which c o n flic ts w ith the C a rls o n -W h ite coulom b ex p lo sio n m o d el? T . S H IO K A W A : Y e s . T h is e x p e rim e n ta l data w ill be c o v e r e d in m y p a p er la te r this w eek . G . H A R B O T T L E : W h ile A d lo ff was show ing the s lid e about the n u clear flu o re s c e n c e e x p e rim e n t w ith 60Co, I s ta rte d c o m p a rin g the r e c o il e n e rg y â&#x20AC;&#x201D; I suppose that I should have r e a liz e d that the r e c o il was as la r g e as 32 eV â&#x20AC;&#x201D; w ith the r e s o lv in g p ow er o f a G e L i d e te c to r . I didn 't get v e r y fa r, b eca u se it is s t ill ra th e r s m a ll. On the o th e r hand, i f one had a ra th e r ligh t nucleus r e c o ilin g fro m a ra th e r e n e r g e tic beta d eca y, and i f in addition you w e r e fe e d in g in to a r a th e r lo w e n e rg y lin e in the dau ghter, it seem s to m e that w ith e x is tin g d e te c to rs you a re p r a c tic a lly into the r e g io n in which you could o b s e r v e the D o p p ler sh ift in the dau gh ter nucleus. C onsequ ently, one m igh t w o rk in a d ir e c t, n o n -d is p e rs iv e w ay to o b s e r v e th is sh ift. P erh a p s one could te s t at m o d e ra te a c tiv ity le v e ls the slow in g-dow n m ech an ism fo r c o o lin g o ff the r e c o il a tom s. T o go a step fu rth e r: I f one had in the daughter an in te rn a l co n versio n p r o c e s s in flig h t, would you sh ift the X - r a y s by enough to m e a s u re it through D o p p le r sh iftin g? I w on d er i f this is a p o s s ib le approach to the p ro b le m o f studying the re ta rd a tio n o f fa st atom s fo llo w in g beta d ecay? J . P . A D L O F F : It is a question o f the tim e - s c a le o f the even ts. You have the r e c o ilin g nucleus; the r e o rg a n iz a tio n o f the a to m ic c o re ; the e m is s io n o f the X - r a y , sh ifted by the v e lo c it y o f the e m itted atom . S. A M IE L : A s a m a tte r o f fa c t, you can m ea su re th is sh ift, not by the gam m a r a y , but by the e le c tro n s p rodu ced by th ese gam m a r a y s . Y o u 'll have m uch g r e a t e r re s o lu tio n fo r the e le c tr o n s . W hy go to the trou b le o f m e a s u rin g the gam m a ra d ia tio n ? Just m e a s u re the e le c tr o n s , and then you can go down to 1 eV e a s ily , and the coin cid en ce is s t ill p resen t. G . H A R B O T T L E : Is this p rop osed as a th ick sam p le e x p e rim e n t? S. A M IE L : A n yth in g th ic k e r than 20 atom s is fin e . You don 't have to go to bulk to do this e x p e rim e n t. G. H A R B O T T L E : H e 's p e r fe c t ly rig h t. In E S C A sh ifts one can use th ick s o u rc e s . M . N E W T O N : W hat type o f e n e rg y re s o lu tio n a re you ta lk in g about in E S C A -ty p e e x p e rim e n ts ? E S C A s e e m s to have the unique p ro b le m that you m u st know one w ay o r another what the w o rk -fu n ctio n te rm is , and this has been a r e a l n ig h tm a r e . I f you a re ta lk in g about a fe w e le c tr o n v o lts â&#x20AC;&#x201D; th at 1 s why I'm w o n d erin g about the kind o f re s o lu tio n you want. I f you a re not ta lk in g about e x p e rim e n ts with a con du ctor, then o f co u rse you don't know what the e ffe c t iv e con tact poten tials w ill be.
1АЕА-РЬ615/5
79
S. A M IE L : I didn 't tr y to s o p h istica te this e x p e rim e n t to this exten t. T o know the c h e m ic a l state o f the atom , the d iffe r e n t c h e m ic a l sh ifts need to be e x p re s s e d in the o r d e r o f 1 eV o r so. M . N E W T O N : I a g r e e that this e x p e rim e n t would be u sefu l to the extent that E S C A is u sefu l. But, in p r a c tic e , E S C A is often not v e r y useful, w ith u n certa in ties o f 2 to 3 eV so that you need som e s o r t o f c a lib ra tio n o f the e ffe c t iv e w o rk function. In p r a c tic e it is often v e r y d iffic u lt. S. A M IE L : T h e b est c a lib ra tio n p ro c e d u re would be to take a few oxid ation sta tes, m ake the m e a s u re m e n ts , and use this as a c a lib ra tio n am ong the d iffe r e n t sta tes. M . N E W T O N : In the E S C A op era tio n at B rookh aven — Hudis and P e r lm a n — th ey have been scru p u lou sly lo o k in g at the ch a rg in g p ro b le m , and the n o is e le v e l is about 3 eV , unless you a r e v e r y c le v e r in g e ttin g a c a lib ra tio n . S. A M IE L : But th is is induced b eca u se you have used X - r a d ia tio n o r u .v . f o r e x c ita tio n . In this e x p e rim e n t you don 't shine a thing. M . N E W T O N : N o, this is an in tr in s ic p ro b lem in d e a lin g w ith n on con d u ctors. J . P . A D L O F F : A qu estion fo r D r. H a rb o ttle . W h a t's new in B rook h aven about the a lte ra tio n o f c h e m ic a l d eca y ra te s due to the ch e m ic a l state o f the atom ? G. H A R B O T T L E : T h o s e o f you who w e r e at the Jülich m e e tin g la s t S ep tem b er w ill r e c a ll that D r. D em a and I r e p o r te d that w e could not find any a lte ra tio n in d eca y ra te fo r 64C u -la b e lle d phthalocyanine when co m p a red w ith the d e c a y r a te o f the r e c o il 64Cu. A t that m e e tin g the young lady c o lla b o r a to r o f V a rg a s pointed out that she had been w o rk in g w ith the alpha fo r m o f the co p p e r phthalocyanine and that w e w e re w o rk in g with the beta fo r m . A lthou gh D em a had only a fe w w eek s le ft at B rook h aven , he did have the op p ortu n ity to do a fe w e x p e rim e n ts w ith the alpha fo r m . T h e re s u lts w e r e not so c le a r - c u t as with the beta fo r m . W ith the beta fo r m , we o b s e rv e d a v e r y , v e r y s m a ll p o s s ib le a lte ra tio n in h a lf- life — fa r b elo w the v a r ia tio n re p o rte d by V a r g a s . W ith the alpha fo r m , w e s t ill s e e no v a r ia tio n , but our e r r o r is la r g e enough at the one sigm a le v e l that our e r r o r o v e rla p s that o f V a r g a s . O ur r e s u lts a re fa r a p a rt, but at the one sigm a le v e l th ey just touch each o th e r. W ith the alpha fo r m , w e cannot a b s o lu te ly ru le out any h a lf - lif e v a r ia tio n . W e found that it was much m o re d iffic u lt to m ake pure a lp h a -c o p p e r phthalocyanine than the beta fo r m — and p u rity is a b so lu tely o f the e s s e n c e . You m u st have p u rity to one p a rt in 107. N . S A IT O : I want to d raw y o u r attention to ou r r e c e n tly published w ork on the ch a rge s p e c tr o m e tr y o f alpha r e c o il fr o m 210P o and 241 A m , which was published in a Japanese jou rn a l p rin ted in Japan ese. W e have a lre a d y su bm itted a p ap er on 119Sn s p e c tro s c o p y by A m b e, A m b e, Shoji and Saito, and w hich w ill com e out this m onth in P h y s ic s and C h e m is try o f S olid s. G. S T O C K L IN : D id n 't the M ô ssb a u er e x p e rim e n t on cob alt in an F e C l 3 la ttic e in d icate n e u tra liza tio n a lr e a d y du rin g the A u g e r c a sca d e, and s ta b iliz a tio n without the fo rm a tio n o f tra n sien t h ig h ly -c h a rg e d s p e c ie s ? Is that the w ay you in te r p r e t th ose e x p e rim e n ts ? J . P . A D L O F F : It is an assu m ption fo r a v e r y s p e c ific m ediu m . G. H A R B O T T L E : R etu rn in g to the qu estion o f using the M ôssb a u er technique fo r the study o f p r o c e s s e s du ring the slo w in g down: T h e ex p e rim e n ts a re m uch to o slo w by th re e to fiv e fa c to rs o f ten. E ven in the nanosecond re g io n , it is s till too slo w by a fa c to r o f about a thousand. You
80
ADLOFF
have to turn to e x p e rim e n ts w ith n u c le a r reson a n ce flu o re s c e n c e — o r perhaps t im e - r e s o lv e d p ertu rb ed an gu lar c o r r e la tio n . E ven th e re , w ith e v e ry th in g sh arpen ed up, it s t ill looks too slo w to m e fo r g ettin g any in fo rm a tio n about im m e d ia te ch e m ic a l con sequ en ces. It w ill n e v e r g iv e you in fo rm a tio n about what you h ave in the sa m p le im m e d ia te ly a fte r the atom com es to r e s t, and we know that the c h a rg e re la x a tio n p r o c e s s e s a re e x tr e m e ly fa s t in s o lid s , so you w on 't get anything out o f that e ith e r. J . F . A D L O F F : O n ly in v e r y s p e c ia l c a s e s . A . G . M A D D O C K : I a g re e w ith H a r b o ttle 's su m m a ry p ro v id in g you stic k la r g e ly to ch a rge re la x a tio n . H o w e v e r, anything oth er than ch arge re la x a tio n can be slow ed down by goin g down to v e r y low te m p e ra tu re s . One o f the d isap p oin tin g fea tu res o f so m any o f these e x p erim en ts in the M o s s b a u e r fie ld is that th ey have not been p e rfo rm e d at liq u id h eliu m te m p e ra tu re s . J . F . A D L O F F : W e 'v e done M o ssb a u er e x p e rim e n ts at liqu id helium te m p e ra tu re . N ot when we f ir s t sta rted becau se w e had no p ro p e r e q u ip m en t, but now w e m ea su re a ll o f our com pounds at liqu id h eliu m t e m p e r a tu re s . I t 's a ls o v e r y e x p e n s iv e , but it is a n e c e s s ity . J. D A N O N : P e rtu rb e d an gu lar c o r r e la tio n has been m entioned fo r Co, the is o to p e a ls o used fo r M o ssb a u er e x p e rim e n ts . I don't think w e have any e ffe c ts w hich would be in te re s tin g , show ing again that this is a v e r y fa s t re la x a tio n p r o c e s s . N . S A IT O : P r o fe s s o r A d lo ff, do you have any e x p e rim e n ta l evid en ce fo r the p re s e n c e o f Sn +3 by r e c o il m eth od s? J . P . A D L O F F : Sn+3? N o. H o w e v e r, in a ll co va len t com pounds, we h ave som e evid e n c e fo r fra g m e n ts in w hich Sn could be p re s e n t in the +3 oxidation s ta te. It is not an io n ic state, but a c o v a le n tly bonded Sn s p e c ie s . T h e e v id e n c e co m e s fr o m the is o m e r sh ift. T h e peaks a r e lo ca ted in a p ositio n w hich fa lls ju st on the p la ce one would ex p ect Sn+3 fro m an e x t r a p o la tio n o f the is o m e r ic sh ift v e rs u s the fo r m a l oxidation state o f tin . T h is is the on ly e v id e n c e in o r g a n o -m e ta llic com pounds. N . S A IT O : P e rh a p s you knew that A m b e studied the e m is s io n s p e c t r o s copy o f 119 S b -la b e lle d antim ony o x id e, and found th ree peaks. One peak cannot be a ssign ed to any o xid a tion sta te. One o f the p o s s ib le explanations f o r th is o b s e rv a tio n is Sn +3 . J . P . A D L O F F : M aybe it is a tin atom w hich sits next to a d e fe c t in the o x id e . N . S A IT O : H e 's not su re o f the explanation. A . G . M A D D O C K : I think you should be e x tr e m e ly cautious in a s c rib in g anything lik e th is to S n +3. T h e ca esiu m salt o f the anion contains an Sn+2 w ith the a p p ro p ria te is o m e r sh ift. On the oth er hand, the c h lo rin e compound o f SnClâ, that is SnCl4 fr o m C l +, the is o m e r sh ift is a p p ro p ria te to Sn . B etw een th ese two lim it s , th e re a re hundreds o f com pounds known at the m om en t w hich contain the SnCl 3 m o ie ty , and in s o fa r as ch em ica l sh ift g oes, populate the w hole o f the r e g io n in betw een th ese tw o lim it s . In oth er w o rd s, it is p r o g r e s s iv e ly p o s s ib le to go fr o m Sn+2 to Sn'*4 , but I d o n 't know that I would c a r e to say w h eth er in doin g so you go through anything which is p r o p e r ly d e s c rib e d as S n +3.
IA E A -P L-61 5/6
POSITRONIUM AND MUONIUM CHEMISTRY * H.J. ACH E Department o f Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Va., United States o f America
Abstract P O S IT R O N IU M A N D M U O N IU M C H E M IS T R Y . This report b riefly examines the present status o f the chemistry o f tw o new atoms, positronium and muonium. Th e current o r future contributions which the study o f these new atoms can make towards our understanding o f the fundamental aspects o f chemical dynamics in general, and m ore specifically towards the problems involved in h o t atom chemistry, are discussed.
IN T R O D U C T IO N T h is r e p o r t b r ie fly exam in es the p resen t status o f p o sitro n iu m and m uonium c h e m is try , w ith r e g a r d to the cu rre n t o r p oten tial fu tu re c o n t r i butions w hich the study o f the re a c tio n s o f th ese new a tom s can m ak e tow ard s our understanding o f the fundam ental asp ects o f ch e m ic a l d yn am ics in g e n e r a l, and m o r e s p e c ific a lly tow a rd s the p ro b lem s in v o lv e d in hot atom c h e m is tr y . It is not intended to be a s u rv e y o f the a p p lica tio n s o f p o sitro n an n ih ilation o r muon d e p o la riz a tio n tech n iqu es as n u clea r p ro b e s ; f o r a c o m p re h e n s iv e d e s c rip tio n o f th ese la tte r techniques the r e a d e r m a y r e f e r to re c e n t r e v ie w a r t ic le s [ 1 ]. P o s itr o n iu m (P s o r e+e") and m uonium (Mu o r iu+e~) can be co n s id e re d as the lig h te r is o to p e s o f h yd ro gen in w hich the proton has been re p la c e d by a p o s itro n o r a p o s itiv e muon, r e s p e c t iv e ly . Both p a r tic le s have a ra th e r lim ite d life t im e ( 1 0 ' 6 - 1 0 ' 10 s) b e fo r e th ey undergo n u clea r p h y sica l p r o c e s s e s , w hich lea d in the f ir s t c a se to m ass an n ih ilation o f the p o s itro n and in the c a se o f the m uonium to the d e c a y o f the p o s itiv e muon to p o sitro n , neutrino and an tin eu trin o. C onsequ ently, the c h e m ic a l re a c tio n s o f th ese s p e c ie s cannot be m o n ito re d in the usual w ay b y con ven tion al produ ct a n a ly s is . T h u s, the qu estion a r is e s , what p r o p e r tie s do th ese p a r tic le s p o ssess w hich le t us b e lie v e that the study o f th e ir re a c tio n s can advan ce our k n ow led ge o f the d e ta ile d m ech a n ism s o f c h e m ic a l in te ra c tio n s and ju s tify the som ew h at e la b o ra te e x p e rim e n ta l techniqu es n e c e s s a r y fo r th e ir d etection ? T h e f ir s t p r o p e r ty b e c o m e s im m e d ia te ly obvious i f one c o n s id e rs the m a ss o f p o sitro n iu m and m uonium w hich a r e a p p ro x im a te ly 1/1000 and 1/9 the m a s s o f a h yd ro gen atom , r e s p e c t iv e ly . Although both s p e c ie s a re exp ected to behave c h e m ic a lly lik e an alogu es o f h ydrogen , as a re s u lt o f the e x tr e m e d iffe r e n c e s in th e ir m a s s e s , h o w e v e r, the s tru c tu re o f fo rm e d
* This paper was written after the meeting fo r the present book, at the request o f the Scientific Secretary.
81
82
AC H E
com pounds, th e ir fo rm a tio n e n e r g ie s , and the k in etics o f the ch e m ic a l r e a c tio n s a r e e x p ected to be r a d ic a lly d iffe r e n t fo r h yd ro gen , m uonium and p o s itro n iu m . O f s p e c ia l in te r e s t w ill be h e re the p o s s ib ility o f in te ra c tio n b etw een p o sitro n iu m o r m uonium and su b strate m o le c u le by an e ffic ie n t "tu n n e llin g " m ech a n ism , w hich takes the re a c tin g s p e c ie s through the r e s t r ic t iv e p oten tial b a r r ie r s ra th e r than o v e r them under e x p e rim e n ta l con dition s w h e re h yd ro gen atom tu n n ellin g is p r a c tic a lly n on -ex isten t; e . g . , the c h a r a c te r is tic te m p e ra tu re at w hich the p r o b a b ility o f the s y s te m to o v e r c o m e the b a r r ie r o r to p en etra te through it by tu n n ellin g b e c o m e s equal is ~ 50K fo r h yd rogen . That is to say, the tunnel e ffe c t b eco m es s ig n ifica n t fo r h yd rogen atom re a c tio n s o n ly at te m p e ra tu re s b elo w 50K, w h erea s the c o rre s p o n d in g te m p e ra tu re fo r p o sitro n iu m is ~ 1600K, w hich m akes the la t t e r p a r tic le uniquely suited fo r in v e s tig a tio n s o f the tunnel e ffe c t even at ro o m te m p e ra tu re . T h e secon d unique p ro p e rty o f the p o sitro n iu m is that its life t im e and an n ih ilation c h a r a c te r is tic s a r e a fu nction o f the c h e m ic a l en viron m en t, w hich w ill be s u b sta n tia lly a lte r e d i f the p a r tic le u n dergoes a c h e m ic a l re a c tio n . Th u s, b y a p p ly in g n u clea r p h y s ic a l m ethods it is p o s s ib le not on ly to fo llo w the re a c tio n s o f the p o sitro n iu m , but a ls o , by c o r r e la tin g its life t im e to the tim e it takes fo r the p ositro n iu m to lo s e its in itia l e x c e s s k in e tic e n e r g y , the d ir e c t a ss e s s m e n t o f the k in e tic e n e rg y w hich the p a r tic le p o s s e s s e s when it r e a c ts is fe a s ib le . M u ltip a ra m e te r tech n iqu es, i . e . life t im e m ea su rem en ts in conjunction w ith a n gu lar c o r r e la tio n e x p e rim e n ts , although not yet fu lly ap p lied to th ese p r o b le m s , could p ro v id e an independent te s t o f the v a lid ity o f the t h e o r e  t ic a lly ca lcu la ted p o s itro n life t im e - k in e t ic e n e r g y re la tio n s h ip and co m p lem en t the s im p le life t im e m e a s u re m e n ts . Th u s, in the c a se o f p o s i tro n iu m re a c tio n s a d ir e c t eva lu a tion o f the re a c tio n c r o s s - s e c tio n - k in e tic e n e r g y re la tio n s h ip , w hich has been ra th e r e v a s iv e in m o s t oth er system s studied in hot atom c h e m is try , s eem s to be a r e a l p o s s ib ility . E x p e rim e n ts based on the d e g re e o f d e p o la riz a tio n o f the p o s itiv e muon as a fu n ction o f the r e a c t iv it y o f the m uonium to w a rd s v a rio u s su b stra tes have p ro v id e d s im ila r m ean s o f d istin gu ish in g b etw een hot and th e rm a l muonium re a c tio n s and shown the poten tial use o f the m uonium atom tow a rd s the a s s e s s m e n t o f b a s ic c h e m ic a l re a c tio n m ech a n ism s. A lth ou gh the u n d erlyin g p rin c ip le s in the re a c tio n s o f both typ es o f p a r tic le s a r e e s s e n tia lly the sam e, th e re a r e b a sic d iffe r e n c e s in the e x p e rim e n ta l tech n iqu es em p loyed , which m ake it m o r e exp ed ien t to discu ss the re a c tio n s o f th ese tw o s p e c ie s s e p a ra te ly .
P O S IT R O N IU M C H E M IS T R Y G e n e ra l P o s itr o n s , w hich a r e u su ally g e n era ted a s the re s u lt o f the ra d io a c tiv e d e c a y o f n e u tro n -d e fic ie n t n u clides w ith in itia lly s e v e r a l hundred k ilo e le c tr o n v o lts o f k in e tic e n e rg y , lo s e th e ir e n e rg y in e la s tic and in e la s tic c o llis io n w ith the su rrou n din g m a tte r until th ey b e c o m e th e r m a liz e d o r n e a r ly th e r m a liz e d , in w hich c a se the p r o b a b ility fo r the an n ih ilation as " f r e e " p o s itro n s by tw o photon d e c a y a ssu m es a m axim u m v a lu e . D uring the slo w in g -d o w n p r o c e s s the p o sitro n s pass through an e n e r g y r e g io n (O re
IA E A -P L-61 5/6
83
O.I
0.25
0.5
0.7 1.5 FIG. 1.
T im e scale for the progress o f the various types o f interactions between e + or Ps and solute or solvent
species in aqueous solutions.
gap) w h e re the tra n s la tio n a lly e x c ite d p o s itro n can a b s tra c t an e le c tr o n fr o m su rrou n din g m a tte r and fo r m the bound state o f p o sitro n iu m (P s ). P s e x is ts in tw o ground sta te s , the s in g le t (p a ra ) P s w ith a n tip a r a lle l spin o rie n ta tio n ( 4.*), its s e lf-a n n ih ila tio n life t im e in fr e e sp ace is 1.25 X lO " 10 s and it d eca ys by tw o-ph oton e m is s io n , and the t r ip le t (o rth o ) P s w ith p a r a lle l spin o rie n ta tio n ( t t ) w ith a c o n s id e ra b ly lo n g e r in tr in s ic life t im e o f 1.4 X 10â&#x20AC;&#x2DC; 7 s . Its s e lf-a n n ih ila tio n o c c u rs v ia th ree-p h o to n e m is s io n . O rth o - and para P s a r e n o r m a lly fo rm e d in the r a tio 3 : 1 . Since th ese P s a tom s a re ge n e ra te d in a c c o rd a n c e w ith the O r e m o d el, w ith k in etic e n e rg ie s ra n gin g fro m 6 . 8 eV down to th e rm a l e n e r g ie s , it s e em s quite fe a s ib le that a c e r ta in fra c tio n o f them m a y undergo c h e m ic a l re a c tio n s w h ile s t ill p o s s e s s in g a p p r e c ia b le am ounts o f k in e tic e n e rg ie s w h e re a s o th er P s atom s m a y b e c o m e th e r m a liz e d b e fo r e th ey r e a c t . In fo rm a tio n about the e n e rg y at w hich a c h e m ic a l re a c tio n b etw een P s and rea cta n t o c c u rs can be obtained fr o m p o sitro n life t im e m e a s u re m e n ts . S ince the la r g e s t body o f e x p e rim e n ta l w o rk on P s re a c tio n s has been done in solu tion , this is d em o n stra ted by using as an ex a m p le the slo w in g down o f f r e e p o s itro n s and p o sitro n iu m atom s in condensed m a tte r . In solu tion one can e s tim a te that the v a rio u s even ts fo llo w a p p ro x im a te ly the tim e s c a le [ 2 ] shown on F ig . 1. F o r m a tio n o f P s , subsequent s e lf-a n n ih ila tio n o f p a r a - P s and the an n ih ilation o f fr e e p o s itro n s w hose a v e r a g e life t im e in condensed m a tte r is
84
AC H E
SCATTERING
(RAPID ANNIHILATION) FIG. 2.
ELECTRON TRANSFER
(RAPID ANNIHILATION)
Schematic presentation o f o-Ps interactions with matter.
about 0 . 1 - 0 . 5 ns, w ill o c c u r w ithin a r e la t iv e ly sh ort tim e a fte r the birth o f the p o s itro n . B eca u se o f the fin ite re s o lu tio n o f the tim in g equipm ent, p - P s s e lf-a n n ih ila tio n and fr e e p o sitro n an n ih ilation w ill g iv e r is e to a com ponent in the life t im e s p e c tra w ith a sh ort co m p o site life t im e , ti and in te n s ity Ip T h e e n e r g e tic o - P s , w hich is fo rm e d in the O r e gap [ 1] w ith k in etic e n e rg ie s v a r y in g betw een 6 . 8 eV and th e rm a l e n e r g ie s , has, as any oth er "h o t" atom , two a lte r n a tiv e s - n a m ely, it m a y undergo c h e m ic a l re a c tio n s w h ile s t ill hot, fo llo w e d by a ra p id annihilation o f the p ositron in the re s u ltin g re a c tio n prod u cts, o r lo s e its e x c e s s k in etic e n e rg y in m o d e ra tin g c o llis io n s , b eco m in g a th e rm a liz e d P s -a to m and r e a c tin g as such. T h e th e rm a liz a tio n tim e o f P s is around 0. 7 ns; thus, the p res e n c e and the re a c tio n s o f the th e r m a liz e d P s w hich lead to ra p id an n ih ilation o f the p o s itro n can be re c o g n iz e d by the ap p earan ce o f a second (lo n g - liv e d ) com ponent in the tim e s p e c tra , and the changes o f the a v e ra g e life tim e , t 2 , a s s o c ia te d w ith this com ponent. On the o th e r hand, "h o t" re a c tio n s betw een solu te and P s -a to m have to take p la ce s h o rtly a fte r the b irth o f the P s b e fo r e it b eco m es th e rm a liz e d . Thu s, the life t im e o f the p o s itro n s in c o rp o ra te d in p ositron iu m atom s taking p a rt in "h o t " re a c tio n s w ill b ecom e in d istin gu ish a b le fro m that o f the fr e e p o sitro n s o r p - P s , i . e . it w ill d rop out o f the second com ponent and appear as a p a rt o f the s h o r t-liv e d com ponent. T h e on ly o b s e rv a b le ev id e n c e fo r the o c c u rre n c e o f such a hot re a c tio n is then a red u ction o f the num ber of P s - a to m s re a c h in g th e rm a l e n e r g ie s . Since this num ber is r e la te d to the in te n s ity , I 2, o f the second com ponent in the tim e s p e c tra , hot re a c tio n can be d etected by a d e c r e a s e o f I 2.
IAEA-PL-615/6
FIG. 3.
85
Typical positron lifetim e distribution curve as acquired by fast-slow coincidence techniques.
R e a ctio n s o f th erm a l o - P s atom s Quantum m ech a n ics [ 1 ] p re d ic ts that the an n ih ilation life t im e o f the p o s itro n is b a s ic a lly d eterm in ed by the d e g r e e o f o v e rla p p in g o f p o sitro n and e le c tro n w ave functions, w hich lea d s fo r exa m p le to the in tr in s ic life t im e o f o - P s o f 1.4 X 10' 7 s. In a c o llis io n betw een o - P s and an oth er m o le c u le a m o r e o r le s s lo n g -liv e d c o llis io n co m p le x m ay be fo rm e d , in which the e le c tr o n d en sity at the p osition o f the p o sitro n w ill be d r a s tic a lly in c re a s e d ( F ig . 2 ). T h e a v e r a g e tim e that the P s spends in this c o m p lex w ill depend on the s ta b ility o f this c o m p le x . If on ly w eak (van d e r W a a ls ) fo r c e s a re o p e r a tiv e in h oldin g this c o m p lex to g e th e r, the P s w ill spend on ly v e r y lit t le tim e in this en viron m en t, the p o sitro n e x p e rie n c e s only fo r a sh o rt tim e the e ffe c t o f the in c re a s e d e le c tr o n d en sity, and the a v e r a g e life t im e o f the P s a p p ea rs on ly s lig h tly s h o r te r co m p a red with the in trin s ic life t im e o f the o - P s . On the o th e r hand, i f this P s c o llis io n co m p le x u n dergoes s ta b iliza tio n in v o lv in g genuine c h e m ic a l fo r c e s , e . g . bond fo rm a tio n , then the p ositron w ill fin d it s e lf fo r a p ro lo n ged p erio d in an en viron m en t o f high e le c tro n d en sity , and its life t im e w ill be su b stan tially red u ced . In o th e r ca ses this c o m p le x m ay be ju st a tra n s itio n state le a d in g to e le c tro n tr a n s fe r fro m P s to su b strate oxid ation o f P s . The product o f this la tte r p ro c e s s is a fr e e p o s itro n , w hose life t im e in condensed m a tte r is c o n s id e ra b ly s h o rte r (0 .1 - 0 . 5 ns) than that o f the o - P s . I f the su bstrate is p a ra m a g n etic the c o llis io n can r e s u lt in a spin c o n v e rs io n fr o m orth o to para P s , w hose in tr in s ic life t im e is on ly 1.25 X 10' 10 s. (B eca u se o f the e x tr e m e ly short in tr in s ic life t im e o f the para P s , re a c tio n s o f this s p e c ie s can be n e g le c te d .) Thu s, one can g e n e r a lly state that a ll in te ra c tio n s o f the o - P s with m a tte r lea d to a sh orten in g o f its apparent life t im e . T o obtain an a c c u ra te d e t e r m i nation o f the r e a c t iv it y o f th e rm a l P s to w a rd s v a rio u s su b strate as re q u ire d fo r the in v e s tig a tio n o f the tunnel e ffe c t th ese q u a lita tive p re d ic tio n s had to be d evelo p ed to a qu an titative m ethod fo r the ca lcu la tion o f the ch em ica l r a te constants fo r the re a c tio n s b etw een P s and su b stra te.
86
ACHE
T A B L E I . E X A M P L E S O F C H E M IC A L C O M PO U N D S SHOW ING S T R O N G / W E A K IN T E R A C T IO N S W IT H T H E R M A L P s A T O M S Strong interaction
Weak interaction
kobs> l O ' M - V 1
ko b s< 1 08 M -1 s’ 1
Nitroaromatics, Quinones, M aleic anhydride, Tetracyano ethylene,
Simple aliphatic or aromatic hydrocarbons: Alkanes, benzene, anthracene etc. , Aniline, phenol, haloalkanes, Halobenzenes, aliphatic nitro co m p .,
Halogens, Inorganic ions in solution (Eq> - 0.9 eV), Organic ions in Solution
Phthalic anhydride, benzonitrile, (Diamagnetic) inorganic ions in solution (Eo< - 0.9 eV)
T h is is now d em o n stra ted by using as an exa m p le the re a c tio n s o f th e rm a l P s in a d ilu te solu tion o f a d ia m a gn etic su b strate (M ): A n n ih ila tio n in so lv e n t p s + M 7p
2 ^
PsM
2
annihilation
(1)
K2
A c c o r d in g to the a b o ve re a c tio n sch em e the fo llo w in g re a c tio n s m ust be co n s id e re d : (1) R e a c tio n o f P s w ith su b stra te M to fo r m a P s - c o m p le x P s M (r a te constant K j); (2) D eco m p o sitio n o f P s M (r a te constant Кг); (3) P o s itr o n an n ih ilation in c o m p le x (d e c a y constant y c); (4) A n n ih ila tion o f P s in bulk s o lv e n t w ith r a te 7 p . (I f the c h e m ic a l re a c tio n is the oxid ation o f P s , it is assu m ed that the r e a c tio n o c c u rs v ia the c o m p le x P s M and that the sub sequent e le c tr o n tr a n s fe r is fast; i. e. co m p le x fo rm a tio n is c o n s id e re d to be the r a te d e te rm in in g step . ) B y se ttin g up a p p ro p ria te k in e tic equ ations and subsequent in teg ra tio n o f the re s u ltin g d iffe r e n tia l equations, the population o f the v a rio u s states in w h ich the p o s itro n s e x is t, o - P s and P s M , can be found as a function o f t im e . F r o m th ese v a lu e s and the p o s itro n an n ih ilation constants fo r these sta te s , the tim e -d e p e n d e n t tw o-ph oton an n ih ilation r a te can be ca lcu la ted . It is re p r e s e n te d by the fo llo w in g tw o -e x p o n e n tia l equation: R 2y = A exp( - y jt) + B e x p (- Y 2t) A and В a r e s c a lin g fa c to r s , r e la te d to the num ber o f p o s itro n s d isp la y in g an n ih ilation r a te y1 o r 7 2. 7 X is a co m p o s ite o f the d e c a y constants fo r fr e e p o s itro n s , s e lf an n ih ilation o f p - P s and in clu d es the hot re a c tio n s o f P s .
(2)
IA E A -P L-61 5/6
FIG. 4.
Electronic polarizabilities
87
o f simple hydrocarbons plotted against rate constants KQ^S for reactions
with thermal Ps.
y2 is g iven by the fo llo w in g e x p re s s io n :
T2 = 7p + E T i
rc
^ ob
[M ]
(3)
s
o r if Y e Âť K 2 : Y 2 = 7p + K i (M ) Y
ĐĄ
(4)
e s t. > 3 X 109 seconds
Th u s, the d e te rm in a tio n o f the c h e m ic a l re a c tio n ra te constant K i o r K 0bs o f the r e a c tio n o f th e rm a l P s and rea cta n t r e q u ir e s the m ea su rem en t o f the tw o-quantum an n ih ilation ra te , R.2 y, as a function o f tim e . T h is can be c o n ven ien tly a cc o m p lis h e d by d e la y e d c o in cid en ce tech n iqu es [ 1 ]. The p o s itro n e m itte r m o s t fre q u e n tly used is 22Na, w hich d eca y s under e m is s io n o f a p o s itro n to the e x c ite d state o f 22N e, which in turn u n d ergoes d e e x c i tation under e m is s io n o f a 1 .2 7 -M e V photon. T h e life t im e o f the ex c ite d 22N e is o n ly 3 ps, so that fo r a ll p r a c tic a l p u rposes the e m is s io n o f the p o s itro n and 1 .2 7 -M e V photon can be c o n s id e re d to o c c u r sim u lta n eou sly. T h u s, the p o s itro n life t im e d istrib u tio n can be d e te rm in e d by o b s e rv in g the tim e ela p s e d b etw een the g e n e ra tio n o f the 1 .2 7 -M e V photon and the
88
ACHE
0.5
-0 .5
-0.75
- 0.50
- 0.25
0.0
0.25
0.50
а (Р .т) FIG. 5. Relative rate constants (log (K/KNB)) for Ps reactions with substituted nitrobenzenes, plotted as a function o f Hammett's constants m j.
a p p ea ra n ce o f the 0. 51-M e V photons r e s u ltin g fr o m the an n ih ilation o f the p o s itro n . T h e s e tim in g m ea su rem en ts have been c a r r ie d out by con ven tion al fa s t - s lo w coin cid en ce tech n iq u es. A ty p ic a l life t im e d is trib u tio n cu rve is shown in F i g . 3 , It re p r e s e n ts the tw o-ph oton an n ih ilation ra te as given by E q . (2 ). 72. the slo p e o f the lo n g -liv e d com ponent in the tim e sp ectru m , can be a c c u r a te ly obtained by com p u ter a n a ly s is . B y d e te rm in in g yp , w hich is id e n tic a l (in dilu te so lu tio n s) with 7 2 m ea su red in the pure so lven t, and know ing the solu te con cen tration , M, the a pparen t constant fo r P s re a c tio n is g iven by
к
obs
- 72" t p [ M]
T h e r e a c t iv it y o f a la r g e num ber o f d ia m a gn etic com pounds tow a rd s P s was m e a s u re d by th is tech n iqu e. T h e re s u lts show that th ese com pounds can be g e n e r a lly d ivid ed into tw o c a te g o r ie s (T a b le I). T h e f ir s t one which shows on ly w eak in te ra c tio n with P s ( K ^ < 10s M " 1 s "1), is com p osed o f h y d r o carb on s [ 4 ] , h alogen ated h yd roca rb on s 1 4 ], a lip h a tic n itro compounds [5 ] and d ia m a g n etic in o rg a n ic ions [ 6 - 1 0 ] , w h ose standard re d o x p oten tials in aqueous solu tion a r e m o r e n e g a tiv e than -0 .9 e V . T h e secon d group en co m p a sses n itr o a r o m a tic s , quinones, h a lo g en es, conjugated an h yd rid es, te tra c y a n o e th y le n e , o rg a n ic ions and in o rg a n ic ions w ith a standard red o x p oten tial o f g r e a t e r than -0 .9 eV and d is p la y s a stro n g r e a c t iv it y tow ard s P s (K obs> 108 M ' V 1) [7 , 1 1 -1 5 ].
IA E A -P L-61 5/6
89
THE STERIC INTERACTION OF THE METHYL GROUP FORCES THE NITRO GROUP OUT OF THE PLANE OF THE RING. UOSS OF CONJU GATION RESULTS IN A REDUCTION OF THE RATE CONSTANTS.
СНз 2.2 x 10ю
2.1 x 10ю
0.85 x 10ю
0.039 x 10ю
FIG. 6. Demonstration of the steric effect on the Ps rate constants (M"1 s-1) by introducing a methyl group in ortho position to the nitro group.
P e rh a p s the s im p le s t explan ation fo r this b eh a vio u r, at le a s t as fa r as the n eu tra l m o le c u le s a r e c o n cern ed , can be p ro vid ed by d is c u s s in g the re s u lts s e m iq u a n tita tiv e ly in te r m s o f the e le c tr o n d o n o r-a c c e p to r o r m o le c u la r c o m p le x th e o ry [ 16] . A c c o r d in g to this m o d el the in te r m o le c u la r binding e n e r g y o f such a m o le c u la r c o m p le x is g iven by the sum o f the "v a n d e r W a a ls " e n e rg y (w hich a ls o contains the con trib u tion fro m e l e c t r o sta tic (d ip o l, induced d ip o l, qu adru pol), a ttra c tio n , London d is p e rs io n f o r c e s , and e le c tr o s ta tic re p u ls io n ), and the c h a r g e - tr a n s fe r reson a n ce e n e r g y . A s m entioned a b o ve, the life t im e o f the p o sitro n iu m atom w ill be d e te rm in e d by the tim e it spends on the a v e r a g e in a lo c a tio n o f in c re a s e d e le c tr o n d en sity , e . g . in a P s c o m p lex , and w ill thus be d ir e c t ly re la te d to the s ta b ility o f such a P s co m p le x . I f w e c o n s id e r f i r s t the n eu tral (d ia m a g n e tic ) m o le c u le s lis te d in T a b le I it can be seen that the grou p o f com pounds w hich show on ly w eak in t e r actio n s w ith P s a r e a ls o those known as w eak a c c e p to r m o le c u le s [1 6 ] . In the c a s e o f the s im p le a lip h a tic h yd ro ca rb o n s w hich have no d ip ole m om en ts one w ould ex p e c t that on ly London d is p e rs io n fo r c e s a r e o p e ra tiv e in h old in g the P s -s u b s tr a te co m p le x to g e th e r and the s ta b ility o f th ese c o m p le x e s should t h e r e fo r e be v e r y s m a ll and m a in ly re la te d to the e le c tr o n ic p o la r iz a b ilit y o f the su b stra te m o le c u le . Such a c o r r e la tio n can indeed be found i f the ra te constants K 0bs fo r P s re a c tio n a r e p lotted as a function of the e le c tr o n ic p o la r iz a b ilit ie s o f th ese m o le c u le s ( F i g . 4 ). F u r th e r m o r e , the p re s e n c e o f p o la r grou ps -O H , e t c . , w hich should in c r e a s e the s ta b ility o f th ese c o m p le x e s due to d ip ole in te ra c tio n s eem s to enhance s lig h tly the r e a c t iv it y o f these m o le c u le s tow a rd s P s [ 4 ] . Since p a ra c h o r and s u rfa ce ten sion a r e a ls o c lo s e ly r e la te d to the p o la r iz a b ility it is not s u rp ris in g that oth er in v e s tig a to r s found s im ila r c o r r e la tio n s betw een th e s e p a ra m e te rs and the P s r e a c t iv it ie s to w a rd s th ese m o le c u le s [1 7 - 1 8 ]. On the o th e r hand, the h ig h ly r e a c t iv e (d ia m a g n e tic ) n eu tral o rg a n ic substances shown in T a b le I a r e uncharged it e le c tro n com pounds and contain h ig h ly e le c tr o n e g a tiv e elem en ts such as oxygen and n itro g e n . A ll o f them a r e shown to be stro n g e le c tr o n a c c e p to rs and fo r m stable m o le c u la r c o m p le x e s [ 1 6 ] .
90
AC H E
SUCCINIC ANHYDRIDE
MALEIC ANHYDRIDE
BENZOQUINONE
0=C 0
3.1 x lO7 FIG. 7.
7.1 x lO9
5.0 x 10ю
Demonstration of the effect of conjugation on the Ps rate constants (M_1 s"1) in cyclic anhydrides.
T h e e ffe c t o f the nature o f the a c c e p to r s p e c ie s on the r e a c t iv it y tow a rd s P s fo llo w s b ro a d ly the o r d e r as ex p ected fr o m the s ta b ility o f th ese m o le c u la r c o m p le x e s with con ven tion al donors [ 1 9 ]. In the c a se o f substituted n itro b en zen es the r a te constants o f th e ir re a c tio n s with P s r e fle c t the e le c tro n -w ith d ra w in g p o w er o f the substituent group in the a c c e p to r m o le cu le . T h is is shown in F i g . 5 w h ere the o b s e r v e d ra te constants fo r the para and m e ta -su bstitu ted n itro b en zen es w e r e an alysed by a p p lyin g H a m m e tt's equation, c t j , the in d u ctive e ffe c t, and the fo u r te rm s fo r pi d e -lo c a liz a tio n e ffe c ts (ct£, ctr(BA) , and aj£) [1 9 ] . A s in d icated in F ig . 5, w h e re lo g (K / K NB) is plotted v s . o’, the b est fit is obtained using a blend o f and ctr v a lu e s w ith = 0 .4 8 and pf = 0 .3 4 . К is the o b s e rv e d r a te constant fo r substituted n itro b en zen e; K NB is the o b s e rv e d ra te constant fo r n itro b en zen e, 2 .7 ( ± 0 . 2 ) X 10 10 M ’ 1 s "1; cjj = PrCTr + Pi^i- T h e ra te constants fo r the c o rre s p o n d in g m e ta -com pounds can be fitte d w ith H a m m e tt's equation by settin g X m = P r /p™ = 0 .3 5 w h e re p™ = 0 .1 2 and pi = 0 .3 4 . T h is is con sisten t w ith the e x p ected tren d that reso n a n ce e ffe c ts a r e o f le s s im p o rta n ce in m e ta -su bstitu ted com pounds. On the o th e r hand, s t e r ic e ffe c ts a r e a ls o im p o rta n t in P s - c o m p le x fo rm a tio n . Thus substitution on a carbon adjacen t to a n itro grou p opposes the e ffe c t the n itro grou p [ 1 1 -1 3 ]. F o r e x a m p le, the r a te constants fo r o rth o n itro to lu en e is d e fin ite ly s m a lle r than that fo r m eta o r para n itr o tolu en e and tw o m eth yl grou ps adjacent to the n itro group d e c r e a s e the ra te constants by a lm o s t tw o o r d e r s o f m agnitude (F ig . 6 ) [ 2 1 ] . S im ila r s te r ic e ffe c ts a r e o b s e rv e d fo r ortho d in itrob en zen e and orth o c h lo r o - n it r o b en zen e [ 21 ] . In a ll th ese ca ses the o rth o substituent p re v e n ts the n itro group fr o m b ein g in the plane o f the a c c e p to r rin g , i . e . the c o -p la n a r ity and conju gation w ill be red u c e d . T h e im p o rta n ce o f the d e g r e e o f conjugation a v a ila b le in the a c c e p to r b e c o m e s a ls o c le a r ly v is ib le by com p a rin g the r a te constants fo r the fu lly conjugated m a le ic an h ydride and su ccin ic anhydride w h e re no conju gation e x is ts (F ig . 7) [ 2 1 ] . A l l th ese re s u lts p a r a lle l th ose p r e v io u s ly o b s e rv e d fo r the sam e a c c e p to r and con ven tion al c h e m ic a l don ors [ 1 6 ]. It s eem s in te r e s tin g to note that when the n itro grou p is se p a ra te d fro m the a ro m a tic r in g by a С Н г-grou p o r attached to an a lip h a tic group (F ig . 8 ) [ 2 1 ] , the r e a c t iv it y to w a rd s P s is g r e a t ly dim in ish ed , w hich again em p h a s iz e s that the com b in a tion o f n itro grou p and conjugated s y stem is re s p o n s ib le fo r the enhanced r e a c t iv it y tow a rd s P s .
91
1АЕА-РЬ615/6
ISO LA T IN G T H E N IT RO G RO U P FR O M T H E ТГ S Y S T E M L O W ER S T H E R A T E C O N ST AN T S NO,
NO, I 2 CH,
®0 N© 0 ® n N^
I
CH2
I
R
R; 2.7
X
10'10
1.46 x Ю8
-H
3.47 xIO 7 4.55 xIO7 -CH2CH3 5.17 xIO7
•CH3
FIG. 8.
Demonstration of the effect of conjugation on Ps rate constants (M-1 s"1) in various nitro compounds.
10"
I0 'w
S *
I09 „ °
I08
R.T.
10'
t 2.0
3.0
NITROBENZENE (9.67 mM) p-DINITROBENZENE (736mM) p-NITROANISOLE (16.12 mM) p-NITROBENZONITRILE (2.02mM) 2.6 OIMETHYLNITROBENZENE (206mM ) p-BENZOQUINONE (L3l5mM) 4.0
50
10 0 0 / T (°K)
FIG. 9. Observed rate constants (Kobs) for Ps reactions with various substrates in toluene solution as a function of the reciprocal temperature (in K).
ACHE
92
T A B L E I I . R A T E C O N S T A N T S F O R T H E R E A C T IO N S O F A V A R IE T Y O F IN O R G A N IC IO NS A N D C O M P L E X E S W IT H T H E R M A L P s A T O M S I N A Q U E O U S S O L U T IO N S Solute
Rate constants
(aq. solution)
(M'1 s'1) 3+
n+ ,(n" l) AG for A +e -* A aq. aq. (eV)
15.1 X 109
1.14
тт 3+ Fe
7.3X 109
0.77
Cu2+
3.2 X 10s
0.16
Fe(EDTA)" 3+ Co(CN)6
2.5 X 109
- 0.1
2. 0 X 108
- 0.8
Fe(phenanthroline)
Pb2+
<0. 5 X 10s
-1.5
Cd2+
<0.5 X 108
- 2.2
Ag+
<0.5 X 108
-2.3
Zn2+
<0.5 X 10s
- 2.8
Na+
<0. 5 X 108
-3.6
A fu rth e r c h a r a c te r iz a tio n o f P s (m o le c u la r ) c o m p le x e s and s im u lta n e ou sly su pp ortin g e v id e n c e fo r the P s c o m p le x m o d el has been a ch ie v e d by d e te rm in in g th e ir s ta b ility as function o f te m p e ra tu re [ 21 ] . F r o m F ig . 9, w h ere K obs is p lotted v e r s u s the r e c ip r o c a l o f the te m p e ra tu re (in K ), it can be seen that at lo w te m p e ra tu re s K obs in c r e a s e s w ith in c r e a s in g t e m p e r a tu res in a cc o rd a n c e with the A rrh e n iu s equation. (T h e q u a lity o f the e x p e rim e n ta l data points does not at p resen t a llo w ju dgem ent con cern in g w h eth er any slig h t c u rva tu re e x is ts in th is te m p e ra tu re re g io n which would be in d ic a tiv e o f a tu nnelling c o n trib u tio n .) A t h ig h e r te m p e ra tu re s K 0bs shows a m axim u m and d e c lin e s w ith a fu rth e r in c r e a s e o f te m p e ra tu re . T h is b eh a vio u r is ty p ic a l f o r s y s te m s w h ere an e q u ilib riu m e x is ts as in d ica ted in E q . ( l ) and su ggests that at th ese te m p e ra tu re s the d ecom p osition o f the P s co m p le x , even with th ese s tr o n g ly r e a c tin g com pounds, b ecom es im p o rta n t. I K 0bs is then not a n y m o re g iv e n s im p ly by K x but by the e x p re s s io n : K obs= KjXç/ (K 2 + Xc). ] W hat has been said so fa r about substances which a r e known as stro n g ж-a c c e p to rs s e e m s to ap p ly a ls o to a -a c c e p to r s such as the h alogen m o le c u le s which d is p la y the sam e high r e a c tiv ity to w a rd s P s . O th er stu dies on this to p ic by G old an sk ii et a l. [ 15] take into account d iffu sio n and v is c o s it y fa c t o r s . M o re r e c e n t e x p erim en ts in ou r la b o ra to ry [ 2 1 ], h o w e v e r, se e m to in d ica te that the con tribu tion o f d iffu sio n and v is c o s it y e ffe c ts to the o v e r a ll p r o c e s s is not o f any g r e a t con sequ en ce. In d ic a tio n fo r P s c o m p lex fo rm a tio n has a ls o been found in gas phase e x p e r im e n ts . G old an sk ii et a l. [ 22] postu late that the in te ra c tio n o f P s with com pounds such a s (C F 3)2N O and N2 0 4 in it ia lly lea d s to the fo rm a tio n o f e x c ite d c o m p le x e s w ith d e c o m p o sitio n life tim e s o f about 1 0 " 12 s. A s to the o th e r grou p o f s tro n g ly r e a c tin g com pounds, such as in o rg a n ic and o rg a n ic ion s in solu tion [ 9, 23-24], p resen t evid en ce based on the e x is tin g c o r r e la tio n betw een the o x id iz in g c a p a b ilitie s o f the ions and the o b s e rv e d
IAEA-PL-61 S/6
93
T IM E (Kinetic E n e rg y o f Particle)
T IM E (Kinetic En ergy
of Particle)
FIG. 10. A.(t), the time-dependent positron annihilation rate plotted as a function of time in argon gas (schematic presentation) (a) with chlorine additives present and (b) without.
ra te con stan ts, in d ica tes that h e re the m o s t lik e ly re a c tio n is the e le c tro n tr a n s fe r fro m P s to the ion when e n e r g e t ic a lly p o s s ib le . T e m p e ra tu re stu dies show again that the a c tiv a tio n e n e r g ie s a r e v e r y s m a ll (~ 0.1 e V ) and r e v e a l no evid e n c e fo r the p re s e n c e o f a lo n g - liv e d P s co m p le x [ 25] . S olvation e ffe c ts and (c h e m ic a l) co m p le x fo rm a tio n , w hich change the o x id iz in g c a p a b ility o f th ese ion s, d e fin ite ly a ffe c t the ra te constant [ 8 , 1 0 , 26] as shown in T a b le II.
H O T P O S IT R O N IU M C H E M IS T R Y T h e f ir s t in d ica tio n that P s atom s can r e a c t c h e m ic a lly at e n e rg ie s a b o ve th e rm a l to fo rm p o sitro n iu m com pounds w as obtained by T a o fr o m the study o f p o s itro n life t im e s in a r g o n -c h lo r in e gas m ix tu re s [ 2 7 ]. It has b een p r e v io u s ly o b s e rv e d that in in e r t g a s e s , such as a rgo n , the shape o f the o b s e rv e d p o s itro n life t im e d is trib u tio n c u rv e cannot be e x p re s s e d b y the m u lti-e x p o n e n tia l function d e r iv e d fo r p o s itro n annihilation in condensed m a tte r . In a d d itio n to the s h o r t- liv e d com ponent w hich in clu d es the gas
94
ACHE
phase, the d eca y o f para P s and p o s s ib ly p o s itro n addition com pounds, fo r m e d by h ig h ly e n e r g e tic p o s itro n s , and the lo n g - liv e d com ponent, which r e p r e s e n ts the an n ih ilation o f re a c tin g th e r m a liz e d orth o P s and o f th e r m a  liz e d o r n e a rly th e r m a liz e d fr e e p o sitro n , in th ese s p ectra a sh ou ld er section can be found lo c a te d b etw een the s h o rt- and lo n g - liv e d com ponent, w hich is due to the r e la t iv e ly lo n g slo w in g -d o w n tim e o f fr e e p o s itro n s in g a s e s . (In condensed m a tte r th is slo w in g -d o w n tim e is c o n s id e ra b ly s m a lle r and no sh ou ld er can be o b s e r v e d .) F r o m the e x p e rim e n ta l d is trib u tio n the t im e dependent an n ih ilation r a te A (t), w hich is due to the com bin ation o f e n e rg y depen den ce o f the an n ih ilation ra te and the n o n -n e g lig ib le slo w in g-d o w n tim e o f e ith e r f r e e p o s itro n o r P s , can be ob tain ed. A plot o f X(t) as a function o f tim e is shown in F ig . 10. In stead o f le v e llin g o ff a ft e r the in itia l r is e A (t) o s c illa t e s , which in d ica tes reso n a n c e s fo r p o s itro n an n ih ilation (at c e rta in tim e s a ft e r the b irth o f the p o s itro n ). A ssu m p tion s can be m ad e fo r the in itia l e n e r g y d istrib u tio n o f the p o sitro n , fro m w hich its v e lo c it y and k in e tic e n e r g y can be a s s e s s e d as a function o f tim e by using known o r ca lcu la ted s c a tte r in g c r o s s - s e c tio n s fo r p o s itro n -a rg o n c o llis io n s and the e n e rg y tr a n s fe r in v o lv e d in th ese c o llis io n s . F r o m the re s u ltin g t im e e n e r g y c o r r e la tio n it w as found [ 27] that reso n a n ce an n ih ilation o ccu rs in a rg o n w ith p o s itro n s p o s s e s s in g k in etic e n e r g y o f 0. 9 and 0. 7 eV . I f s m a ll am ounts o f c h lo rin e gas w e r e added to the gas m ix tu re a new sh ou ld er on top o f the f ir s t sh ou lder was o b s e rv e d . It w as a ls o o b s e rv e d that the sum o f the in te n s itie s o f this new sh ou lder and that o f the lo n g -liv e d com ponent re m a in e d constant w ith in c r e a s in g c h lo rin e co n cen tra tion , which w ould su g g est that the new sh ou ld er and the lo n g - liv e d com ponent have the sam e o r ig in , n a m e ly bein g the r e s u lt o f o rth o P s in te ra c tio n s with the su b s tra te . Thus the new reso n a n ce has been attrib u ted to P s compound fo rm u la tio n : P s + C l2 ^ P s C l + C l A c a lc u la tio n o f the s lo w in g -d o w n tim e o f the P s w hich can be fo rm e d with m a xim u m e n e r g ie s o f 6 . 8 eV is v e r y d iffic u lt. N e v e r th e le s s , a crude a p p ro x im a tio n has been m ade by assu m in g that, in v ie w o f the lo w e r m o b ility and h ig h e r s c a tte rin g m a s s , the P s r e q u ir e s much the sam e tim e a s a p o s itro n needs f o r slo w in g down o v e r the sam e e n e rg y ra n ge in a g iven s y s te m . W ith that a p p ro x im a tio n T a ó [ 27] found that the new reson a n ce an n ih ilation due to P s C l fo rm a tio n o ccu rs at P s e n e rg ie s o f 0 .5 eV with a m axim u m c r o s s - s e c t io n o f 10' 16 cm 2 (s e e a ls o F ig . 10). S im ila r o b s e rv a tio n s w e r e m ade in gaseou s a rg o n -o x y g e n s y s te m s , w hich w e r e in te r p r e te d b y P s O fo rm a tio n . G old an sk ii [2 8 ] d iscu ssed these re s u lts w ith in the fr a m e w o r k o f the th e o ry o f a "su b stitu tion re a c tio n (d i s  s o c ia tiv e attach m en t) p ro c e e d in g by the fo r w a r d m e c h a n is m " and tre a te d P s attachm ent to C l 2 and the subsequent d is s o c ia tio n to P s C l + Cl: P s + C l2 ^ P s C l + C l in a n a lo g y to the d is s o c ia tiv e e le c tr o n attachm ent p r o c e s s . T h e quantum m e c h a n ic a l tre a tm e n t o f th is m ech an ism lea d s to the fo llo w in g e x p re s s io n fo r the c r o s s - s e c t io n as a fu nction o f e n e rg y cr(E) ~ 10" 13 exp ( -2 E / w )c m 2
IA E A -P L-61 5/6
95
FIG. 11. I2, the intensity of the long-lived component plotted as a function of solute concentration in aqueous solutions of inorganic ions.
w h e re E ÂŁ E th and w is the v ib ra tio n a l quantum; in the c a se o f C l 2 = 0 .0 7 eV E th = th resh o ld e n e rg y . T h e d ir e c t m ech a n ism a lw a y s cau ses a sharp m axim u m in the e n e rg y depen den ce with a width ~ w/2 rig h t at the th resh o ld e n e r g y . I f this is a ls o tru e f o r P s re a c tio n s then, fr o m the o b s e rv e d reso n a n ce c r o s s - s e c t io n fo r P s C l fo rm a tio n o f 10" 16 c m 2, a th re s h o ld e n e rg y o f 0 .2 5 eV can be obtained f o r this p r o c e s s , w hich is on ly s lig h tly s m a lle r than that re p o rte d by T a o [2 7 ] (s e e p re c e d in g p a r a s ). F u r th e r m o r e , by assu m in g that the th resh old e n e rg y , E th, is r e la te d to the a ffin ity o f the c h lo rin e f o r p ositron iu m , â&#x201A;Ź psci, by the equation: = D - e Psci , w h e re D is the d is s o c ia tio n e n e rg y o f C l2, G old a n sk ii d e te rm in e d the p o sitro n iu m a ffin ity o f c h lo rin e as 2.25 e V . A s m en tion ed a b o v e , the p re s e n c e o f "h o t" P s re a c tio n s in condensed m a tte r can be r e c o g n iz e d by a red u ction in I 2 , the in te n s ity o f the lo n g liv e d com ponent in the p o s itro n life t im e s p e c tra as a function o f rea cta n t co n cen tra tio n . B y u sin g th is ap proach it was found that in aqueous solu tions o f in o rg a n ic ion s w h e re the r a te constant fo r the r e a c tio n betw een P s and solu te re m a in s b elow the d e te c ta b le lim it ( 1 07 1 / m o le -s e c o n d ), i . e . w h ere re a c tio n s o f th e rm a l P s a r e e x tr e m e ly slo w , I 2 e ven tu a lly a ssu m es a lo w e r lim it (o r satu ration v a lu e ), l|at f 0 , w hich is c h a r a c te r is tic fo r each in d ivid u a l com pound (F ig . 11) [ 2 9 ] . T h is re s u lt w as in te r p r e te d b y assu m in g that ( I 2 - I 2at) I 2 r e p re s e n ts the fr a c tio n o f a ll P s - a to m s fo rm e d which h ave s u ffic ie n t e n e r g y to o v e r c o m e the r e a c tio n b a r r i e r (I 2 = in te n s ity in pure s o lv e n t).
96
AC H E
I / [s
o l u t e
] U /M O L E )
FIG. 12. -l/ln(I2/I2 °) plotted as a function of the reciprocal solute concentration (mole/litre) for aqueous AgC104l CdCl2, НСЮ4, SnCl2and РЬ(СЮ4)г solution.
T A B L E III. A R B IT R A R Y V A L U E S O B S E R V E D F O R T H E R E A C T IV IT Y IN A Q U E O U S S O L U T IO N S O F Pb (СЮ 4) , SnCl2, H CIO 4 , C d C l 2 A N D A gC 10 4 Ion in solution
Slope 6a
Ag+
^react Â2
I (reactivity)a
1.0
17.3
0. 058
Pb++
0. 52
34.5
0.057
H+
1.0
Cd++
0.48
40.0
0.053
Sn++
0.45
51.8
0.045
a In arbitrary units
100
0. 01
97
IAEA-PL-615/6
5 -
4 -
3 -
о
(Л
i—iM 2
I \ \
25
-25
50
75
'f
-_ e * / P s
I
CuyCu
100
-A G'tKcal/ Mole ) 50
4.0
3.0
2.0
1.0
0.0
- E e(V ) FIG. 13.
1гЭ* vs - A G , the change of free energy in the process A^ + + e~-» A ^ ^
Thus it s e e m s that the re a c tio n s o f P s -a to m s in a solu tion can be tr e a te d in a n a logy to a s y s te m , w h e re r e c o il atom s r e a c t "h o t" to fo rm "h o t" r e a c tio n products o r b eco m e th e rm a liz e d in m o d e ra tin g c o llis io n s and a r e even tu a lly b ein g sca ven ged [ 3 0 ]. It is p la u sib le to assu m e that on ly the solu te s p e c ie s r e a c t c h e m ic a lly under the e x is tin g e x p e rim e n ta l conditions to any s ig n ific a n t extent w h erea s the so lv e n t m o le c u le s , e . g . w a te r, assum e the r o le o f an in e r t m o d e r a to r , m o d e ra tin g e n e r g e tic P s -a to m s down to th e rm a l e n e r g ie s . T h e r m a l P s - a to m s a r e even tu a lly "s c a v e n g e d " by the p ic k - o ff an n ih ilation p r o c e s s . T h e to ta l num ber o f o - P s atom s is re p re s e n te d by 12, the in ten sity o f the lo n g - liv e d com ponent, o b s e rv e d in the pure so lv e n t. T h e fra c tio n o f P s -a to m s r e a c tin g hot (at a g iven so lv e n t con cen tration ) is equal to P = (12 - 12 ) / 1 w h e r e I 2 is the o b s e rv e d in ten sity o f the lo n g - liv e d com ponent at that p a rtic u la r solute con cen tration . C on sequ en tly the e x p e rim e n ta l data can be tre a te d w ithin the f r a m e w o rk o f the E s tru p -W o lfg a n g k in etic th e o ry o f re a c tio n s [3 1 , 32] . T h e plot o f - l/ ln (1 - P ) as a fu nction o f the r e c ip r o c a l o f the solu te con cen tration (fo r v e r y dilu te solu tion s) re s u lts in s tra ig h t lin es as r e q u ire d by the th e o r y , F ig . 12. B y using the c la s s ic a l d ia m e te r o f the P s -a to m and the e ffe c t iv e d ia m e te rs o f the ion s in aqueous solution r e la t iv e r e a c t iv it ie s (in te r m s o f the E s tru p -W o lfg a n g k in e tic th e o r ie s ) w e re ob tain ed. The re s u lts su ggest that, w ith the excep tion o f the H CIO 4 s y s te m s , w h ere the
98
AC H E
FIG. 14.
Empirical correlation between D^g and I2.
e ffe c t iv e d ia m e te r fo r the H + in w a te r m igh t be c o n s id e ra b ly o v e re s tim a te d w ith 9 A , I re m a in s a p p ro x im a te ly constant w ithin the e x p e rim e n ta l e r r o r s in v o lv e d , which im p lie s that the c r o s s - s e c tio n s o r r e a c t iv it y fo r red o x re a c tio n s b etw een P s -a to m and ion (p e r c o llis io n ) a r e independent o f the nature o f the ion, as lon g as the P s has s u ffic ie n t (k in e tic ) e n e rg y a v a ila b le to o v e r c o m e the re a c tio n b a r r ie r . (S ee a ls o T a b le III. ) O ne o f the con dition s f o r the a p p lica tio n o f this th e o ry [ 33] w as that c o m p le te ra n d o m iza tio n o f hot atom e n e rg ie s at the top o f the re a c tio n re g io n could be assu m ed. W h eth er o r not this assu m ption can be m ade in the c a se o f the hot P s atom s w ill depend on the e n e r g y d istrib u tio n o f the P s -a to m s when they a re fo r m e d in the O r e gap. B y c o r r e la tin g the l|at in the v a rio u s s y s te m s to the fr e e e n e rg y changes in v o lv e d in the red u ction o f the in o rg a n ic ion (A^+ + e â&#x20AC;? -> A ^1" 1^ ) (s e e F ig . 13) by the P s the k in e tic d istrib u tio n o f the P s a^om w as rou gh ly a p p ro x im a te d [3 0 , 3 4 ]. A c c o r d in g to th is es tim a te m o s t o f the P s -a to m s should in itia lly be fo rm e d w ith k in e tic e n e rg ie s o f about 1. 25 - 1. 75 eV (in H 20 ) . One can fu rth e r assu m e that, unlike the c a se o f lo w e n e rg y hot a to m s , e . g . produced in p h o to ch em ica l re a c tio n s , which m igh t r e a c t on the f i r s t c o llis io n , hot P s - a to m s , becau se o f th e ir s m a ll m a s s , have to u ndergo a la r g e nu m ber o f c o llis io n s (m o s tly o f the e la s tic ty p e) b e fo r e they lo s e an a p p re c ia b le am ount o f k in e tic e n e rg y . T h is w ill m o s t lik e ly lead to a ra n d o m iza tio n o f e n e r g ie s b e fo r e th ey e n te r c h e m ic a l re a c tio n s which would s a tis fy the con dition fo r the a p p lic a b ility o f the k in e tic th e o ry o f hot re a c tio n s . S im ila r e x p e rim e n ts w e r e c a r r ie d out in solutions o f v a rio u s o rg a n ic com pounds such as the h alogen ated b en zen es, b e n z o n itrile e tc . in ben zen e. T h e y a ll show a ra p id d e c r e a s e fo llo w e d by a ty p ic a l le v e llin g o ff o f I 2 with
99
IAEA-P1^615/6
T A B L E IV .
BOND ENERGIES IN PO SITRO NIU M COMPOUNDS Compound
DpA <eV>
Ps-0
2.2 ± 0.5
Ps-OH
1.3 ± 0.5
Ps-Cl
2. 0 ± 0. 5
Ps-F
2.9 ± 0.5
Ps-CH3
&0. 0
Ps-NH2
«
0.0
in c r e a s in g solu te co n cen tra tion . H o w e v e r, a c le a r c o r r e la tio n b etw een the th re s h o ld e n e rg ie s and th erm od yn a m ic p r o p e r tie s o f th ese solu tes has yet to be e s ta b lish ed . E v id e n c e f o r "h o t" P s substitution re a c tio n s o f the type P s + H 2P 0 4 -» P s O + Н 2РО з w as found by T a o et a l. I 35] in aqueous solu tion s o f o x y a c id s , H 2O2 , H F , HC1 and N H 3 . T h e y a ls o o b s e rv e d that I 2 assu m es a plateau w ith in c r e a s in g solu te co n cen tra tio n . (One should, h o w e v e r, point out that th ese plateaus a r e re a c h e d at c o n s id e ra b ly h ig h e r solu te co n c e n tra tio n s than in the solu tion s o f in o rg a n ic ion s, which would im p ly that the c r o s s - s e c tio n s fo r "h o t" P s substitution re a c tio n s a r e su b sta n tia lly s m a lle r than th o se f o r the o xid a tion o f P s by hot m ech a n ism s. ) T h e sa m e authors postu late that, to fo r m a stab le P s compound in a re a c tio n : P s + A B ->■ P s A + В the m in im u m e n e rg y that m ust be su pplied by the p o sitro n iu m to enable the r e a c tio n to take p la ce is d eterm in ed by DAB - D pA, w h ere DAB and D pA a r e the d is s o c ia tio n e n e r g ie s o f compound A B and P s A , r e s p e c t iv e ly (d i s r e g a r d in g any a c tiv a tio n e n e rg y ); and that I 2 is a p p ro x im a te ly g iven by: I 2 ~ (Dab -Dpa)/Vab, w h ere VAb is the io n iza tio n p oten tial o f A B [ 3 6 ] . B y e m p ir ic a lly c o r r e la tin g I 2 w ith Dab and e x tra p o la tin g I 2 - 0 (F ig . 14) they obtain d is s o c ia tio n e n e r g ie s fo r the p o sitro n iu m com pounds P s - O , P s -O H e tc . (T a b le I V ). T h e s e v a lu e s , w hich show that the o b s e rv e d P s binding e n e r g y is c o n s is te n tly lo w e r than the c o rre s p o n d in g va lu es fo r the H binding e n e rg y , a r e in a cco rd a n ce w ith the th e o r e tic a l p re d ic tio n that any m o le c u le o f the type P s A should be le s s sta b le than H A , sin ce the e n e rg y o f z e r o point v ib ra tio n s fr o m w hich the d is s o c ia tio n e n e rg y is counted m ust in c r e a s e w ith the in c r e a s in g red u ced m a s s o f the m o le c u le .
M U O N IU M C H E M IS T R Y P a r it y n o n -c o n v e rs a tio n in the produ ction and d eca y o f the muon a r e the e s s e n tia l to o ls fo r a ll studies o f m uonium . T h e ju+ muon is produced in the d e c a y o f a p o s itiv e pi m e s o n c o m p le te ly p o la r iz e d w ith its spin orien ted p r e fe r e n t ia lly o p p o site to its d ir e c tio n o f m o tio n . T h e d e c a y o f a p o s itiv e
ACHE
100
FIG. 15.
Flow-chart showing the interactions of muons in a solution.
m eso n (ix+ - e + + v + v) is aga in a w eak in te ra c tio n and o ccu rs w ith an angular s y m m e tr y fa v o u rin g p o s itro n e m is s io n in the d ir e c tio n o f the muon spin, i . e . the nu m ber o f p o s itro n s fly in g alon g the in itia l d ir e c tio n o f flig h t o f the Ń&#x2020;+ turn out to be s m a lle r than that in the o p p o site d ire c tio n [ 3 7 ]. When p o la r iz e d p o s itiv e m uons a r e stopped in m a tte r, the "a p p a re n t" m agnitude and d ir e c tio n o f th e ir p o la riz a tio n can be m ea su red by o b s e r v in g th e ir a s y m m e tr ic d eca y as they p r e c e s s in a tr a n s v e r s e m a gn etic fie ld . It has been found that this "a p p a re n t" p o la riz a tio n depends s tro n g ly on the c h e m ic a l p r o p e r tie s o f the m ediu m , e . g . in in e r t g a ses such as a rgo n muons a r e s tro n g ly d e p o la r iz e d , w h e re a s in the p re s e n c e o f com pounds lik e C 2H 4 in a rg o n , the m uons e x p e r ie n c e le s s and le s s d e p o la riz a tio n as m o r e and m o r e re a g e n t is added [ 37 ] . T h e s e re s u lts have been exp lain ed by Iv a n te r and S m igla [ 37d, e ] by the s o - c a lle d "m u on iu m m e c h a n is m ". When p o la r iz e d m uons in te ra c t with m a tte r th ey cap tu re e le c tr o n s to fo rm m uonium , /Đť +Đľ~ o r Mu, in which
101
IAEA-PL-615/6
d e p o la riz a tio n o f the muon continues until the m uonium r e a c ts e ith e r in a hot o r a th e rm a l r e a c tio n . Should the r e a c tio n produ ct be a d ia m a gn etic M ucom pound, e . g . as the re s u lt o f a substitution re a c tio n , then d e p o la riz a tio n c e a s e s c o m p le te ly . On the oth er hand, i f the Mu b eco m es in c o rp o ra te d into a r a d ic a l, e . g . Mu addition to a C=C double bond, then the h y p e rfin e coupling o f the muon spin with that o f the unpaired e le c tr o n causes d e p o la riz a tio n in m uch the sam e m an n er as in fr e e Mu, though m o r e s lo w ly . (T h e ra d ic a l m a y r e a c t fu rth e r to fo r m even tu a lly a d ia m a gn etic com pound, whereupon muon d e p o la riz a tio n w ill s to p .) A flo w ch a rt show ing the in te ra c tio n s o f muons with a solu tion as p ostu lated by B r e w e r et a l. [ 38-39] is g iven in F ig . 15. (T h e p o s s ib ility o f re a c tio n s o f m uonium w ith the rea g en t to fo r m r a d ic a ls has been h e re exclu ded by the authors, although in p rin c ip le such a re a c tio n w ith a rea g en t should be p o s s ib le .) T h e em p lo yed e x p e rim e n ta l techniques [ 3 7 ] , w hich a r e ra th e r in v o lv e d , and cannot be d iscu ssed w ithin the scop e o f this r e p o r t, a llo w now a d e t e r m in ation o f the fo llo w in g p a ra m e te rs : h
= fra c tio n o f "h o t" m uonium atom s r e a c tin g hot to fo r m d ia m a gn etic com pounds r = fra c tio n re a c tin g hot to fo rm r a d ic a ls con tain in g m uonium k mr = c h e m ic a l r a te constant fo r the re a c tio n Mu + S olven t -* R a d ic a l k md = c h e m ic a l r a te constant fo r the re a c tio n Mu + R ea gen t -> D ia m a g n etic Compound k rd = c h e m ic a l r a te constant fo r the re a c tio n R a d ic a l + R eagen t -*■ D ia m a gn etic Compound F o r solu tion s o f B r 2 o r l 2 Ín b en zen e B r e w e r et a l. [3 8 -3 9 ] r e p o r t the fo llo w in g re s u lts (c h e m ic a l r a te constants in litr e s / m o le -s e c o n d X 1010):
г
0.13 ±0. 02
0± 0.1
kmr + 0.20 - 0. 05
kmd (k)
4
+4 -2
krd(I2)
°-8-0.3
kmd i + С П 00
h
krd(Br2) л „ +0-5 0-6 - Л0.3„
In o th e r substances the sam e authors [ 39] found c o n s id e ra b ly h igh er fra c tio n s o f Mu r e a c tin g as hot s p e c ie s . T h e r a te constants fo r Mu s u b s ti tution o r H -a b s tra c tio n and addition re a c tio n s in v a rio u s com pounds a re s u m m a rize d in T a b le V to g e th e r w ith th e ir a c tiv a tio n e n e r g ie s , i f known [4 0 ] . O f s p e c ia l in te r e s t is the c o m p a riso n o f the r a te constants fo r substitution o r H a b s tra c tio n v e rs u s addition to the it-e le c tr o n sy s te m in compounds con tain in g double bonds o r a ro m a tic rin g s [ 4 0 ] . In the c a se o f the m eth yla ted b en zen e, the sum o f the r a te constants fo r substitution o f H a b s tra c tio n and addition re m a in s fa ir ly constant, w h ile the p r o b a b ility fo r substitution o r H a b s tra c tio n in c r e a s e s m a rk e d ly , a ccom p an ied b y a s im ila r d e c r e a s e in the ra te constant fo r addition , i . e . on ly the m o le c u la r r e a c t iv it ie s a r e re d is trib u te d d iffe r e n tly b etw een v a rio u s channels. On the oth er hand, unbranched a lk y lb en zen es d is p la y a quite d iffe r e n t tren d . Mu substitution o r H a b s tra c tio n r a te constants in c r e a s e s te a d ily w ith g ro w in g chain length, without any s ig n ific a n t change in the r a te constant fo r
102
C O M PO U N D S
ACHE
T3 T3 (0
о
с о
10'
-M ü
kt and k2= ( 1.6 ± 0.3) x
с о и
ОТ да л
о тз с со с
о со
дэ 3 и
RATE
О
3 от
с
> S И hJ m <¡ 5
ж о
С
Ф (0
ь *ч &
в
о
ас
О
о ас
и
£ и
и X и
a0c. a,
В
V* ■ J-l U
a? 0
«-e* £ u
^4 £ и
4 £ 0 u
£
ас
«о
£ 3? u и u и
X
Й
J £ £ Ü U
í и
E
ас
и
С2Н4(Ú1 gas phase)
CONSTANTS
FOR
THE
Mu -REACTIONS
WITH
VARIOUS
и
IA E A -P L-61 5/6
103
Mu ad d ition . G old an sk ii and F ir s o v [4 0 ] in te r p r e t these la t t e r re s u lts by a ssu m in g that in te ra c tio n s o f Mu with a r o m a tic rin g s and with the sid e chain a r e independent o f each oth er p ro vid ed the chain len gth is s u ffic ie n tly long, i . e . in te ra c tio n p ro c e e d s not w ith the m o le c u le as a w hole but w ith its in d iv id u a l fra g m e n ts . O th e r in v e s tig a tio n s inclu de Mu re a c tio n s w ith in o rg a n ic ions such as Cu2+, F e 2+and F e 3+in aqueous solutions [4 1 - 4 2 ]. H e r e a gain m o r e than 50% o f the m uonium atom s fo rm e d seem to undergo c h e m ic a l re a c tio n s as hot s p e c ie s . In te r e s tin g ly enough the m uonium r a te constants w e r e found to be 50 to 100 tim e s fa s te r than p re d ic te d on the b a sis o f m ea su rem en ts o f h yd rogen atom s [4 2 ] which, a c c o rd in g to the au th ors, r a is e s som e questions about the ra te s d ete rm in e d fo r re a c tio n s o f a to m ic h yd rogen w ith th ese s p e c ie s .
C O N C LU S IO N S B e fo r e any attem pt could be m ade to u tiliz e P s o r Mu re a c tio n s fo r an in v e s tig a tio n o f the tunnel e ffe c t and the o th e r g o a ls outlined in the in t r o  duction it was im p e r a tiv e to obtain a re a s o n a b ly d e ta ile d p ic tu re o f the type o f c h e m ic a l re a c tio n s that P s o r Mu atom s undergo and the m ech an ism s in v o lv e d in th ese r e a c tio n s . W ith a r e la t iv e ly la r g e am ount o f e x p e rim e n ta l in fo rm a tio n now a v a ila b le , e s p e c ia lly in the c a se o f P s , a ll the e v id e n c e , as p re s e n te d in this r e p o r t, unam biguously p ro v e s that p a ra m e te rs w hich g o v e rn o r d in a r y c h e m ic a l re a c tio n s a ls o c o n tro l the ch e m ic a l re a c tio n s o f th e r m a l P s o r Mu a to m s. T h e r e fo r e , the study o f the tunnel e ffe c t by P s o r Mu re a c tio n s is a d e fin ite p o s s ib ility . T h e p h en o m en o lo gica l m a n ife s ta tio n o f the quantum m ech a n ica l tu nnelling in P s o r Mu re a c tio n s would be a c u rva tu re in the A rrh e n iu s p lo t. R e lia b le te m p e ra tu re stu dies o f th ese re a c tio n s a r e s till r a r e . In th ose ca ses w h ere the ra te constants w e r e d e te rm in e d o v e r a r e la t iv e ly la r g e te m p e ra tu re ra n g e, the e x p e rim e n ts w e re c a r r ie d out in so lu tio n s, w h ere any n o n -lin e a r ity in the A rrh e n iu s p lo ts, i f p resen t, m ay have been ob scu red by d iffu sio n and s o lv a tio n e ffe c t s . In addition , in m o s t o f th ese c a s e s the P s c o m p le x e s fo rm e d w e r e fa ir ly unstable at th ese te m p e ra tu re s , which added to the u n certa in ty o f the m ea su red p lo ts. Th u s, e x p e rim e n ts should be d e v is e d w h ere th ese c o m p lic a tin g fa c to rs can be a v o id ed . One lo g ic a l c h o ice would be the study o f P s o r Mu in g a ses as a function o f te m p e ra tu re and p re s s u re with s tro n g ly re a c tin g com pounds. It would d e fin ite ly be a g r e a t advantage i f th ese e x p e rim e n ts could be c a r r ie d out at low te m p e ra tu re s . In the c a se o f Mu re a c tio n s one should take fu rth e r advan tage o f the e x p e rim e n ta l p o s s ib ilitie s , which not on ly a llo w the r a te o f re a c tio n to be studied but a ls o can id e n tify the typ e o f Mu compound fo r m e d . It would be o f s p e c ia l in te r e s t to r e fin e th ese techniques in o r d e r to s e p a ra te H - a b s tr a c tion and su bstitu tion r e a c tio n in v o lv in g Mu a to m s. T h e s e re s u lts would p e r m it an in te r e s tin g co m p a ris o n w ith w id e ly studied T re a c tio n s and an a s s e s s m e n t o f the e ffe c t o f m ass and e n e r g y on the r e la t iv e p ro b a b ility o f substitution v e r s u s a b s tra c tio n . R e g a r d in g the P s hot r e a c tio n s , f i r s t p r io r it y should be g iv e n to the eva lu a tion o f a m o re p r e c is e re la tio n s h ip betw een the slo w in g -d o w n tim e and the k in e tic e n e r g y o f the P s in g a s e s , w hich in turn w ill le a d to an ex a ct
104
ACHE
d e te rm in a tio n o f the reso n a n ce e n e rg ie s at w hich P s r e a c t s . C on sid era tion should a ls o be g iv e n to the question o f w h eth er the th e o ry o f the "fo r w a r d r e a c tio n m ech a n ism " (s e e p re c e d in g p a ra s ), w hich would postulate a m a xim u m c r o s s - s e c t io n at th resh o ld e n e r g ie s , can be g e n e r a lly app lied fo r P s re a c tio n s . E x p e rim e n ts in solu tion s o r in the liqu id phase should be continued to enhance our kn ow ledge o f the re a c tio n m ech an ism s in v o lv e d in the re a c tio n s o f the s im p le s t r a d ic a ls a v a ila b le . In su m m a ry it can be said that the positron iu m and m uonium m ethod is im p o rta n t fo r the study o f m any p a ra m e te rs o f ch e m ic a l re a c tio n s : the va lu es o f the ab solu te r e a c tio n constants, the a c tiv a tio n e n e r g ie s o f p r o c e s s e s , the s te r ic fa c to rs ca lcu la ted fr o m th ese qu an tities, the p o s s ib ility o f id e n tify in g the p r im a r y products and r e fin in g the re a c tio n m ech an ism , o f d e te rm in in g the in flu en ce o f the a g g re g a te state o f m a tte r on the co u rse o f the p r o c e s s e s , e s tim a te s o f the r o le o f the tunnel e ffe c t in ch em ica l re a c tio n s and o f the p a ra m e te rs o f the a c tiv a tio n b a r r ie r , and e stim a tes o f the life t im e o f s h o r t- liv e d s p e c ie s .
REFERENCES [1]
[2] [3] [4] [5] [ 6] [7] [ 8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20]
(a) GREEN, J. , LEE, J. , Positronium Chemistry, Academic Press, NewYork,N,Y. (1964); (b) GOLDANSKII, V.I. , Atom. Energy Rev. £(1968) 3; (c) McGERVEY, J.D. , in Positron Annihilation, (STEWART, A .T ., ROELLIG, L. O. , Eds), Academic Press, New York, N. Y. (1967) 143; (d) MERRIGAN, J.A., TAO, S.J. , GREEN, J.H., in Physical Methods of Chemistry, (WEISSBERGER, A.. ROSSITER, B.W. , Eds) 2., Wiley, New York (1972) Part III D; (e) ACHE, H.J., Angew. Chem. (Int. Edn Eng. ) n_ (1972) 179; (f) GREEN, J.H. , MTP International Review of Science_ 8(Radiochemistry) (MADDOCK, A.G., Ed.) Butterworths, London (1972) 251; (g) GOLDANSKII, V.I. , FIRSOV, V. G., Ann. Rev. Phys. Chem. 22 (1971) 209. Time scale in the flow chart is based on data taken from Ref. [3 ]. TAO, S.J. , GREEN, J.H. , J. Chem. Soc. A (1968) 408. GRAY, P.E., COOK, C.F. , STURM, C. P. , J. Chem. Phys. 48 (1968) 1145. MADIA, W.J. , NICHOLS, A. L. , ACHE H.J. , J. Amer.Chem.Soc. 97 (1975) 5061. BARTAL, L.J. , NICHOLAS,J.B. , ACHE, H.J. , J. Phys. Chem. 76 (1972) 1124. BARTAL, L.J. , ACHE, H.J., Radiochim. Acta 17 (1972)205. NICHOLAS, J.B., WILD, R. E. , BARTAL, L.J., ACHE, H.J., J. Physic.Chem. 77 (1973) 178. BARTAL, L.J. , ACHE, H.J. , J. Inorg. Nucl. Chem. 36 (1974) 267. NICHOLS, A. L. , BARTAL, L.J. , WILD, R. E. , ACHE, H.J. , Appl. Phys. 4 (1974) 37. MADIA, W.J. ,NICHOLS, A. L. , ACHE, H.J. , J. Chem. Phys. 60 (1974) 335. MADIA, W.J. ,NICHOLS, A. L. , ACHE, H.J. , Appl. Phys. £(1974) 189. MADIA, W.J. ,NICHOLS, A. L. , ACHE, H.J. , Ber. Bunsenges. Phys. Chem. 78(1974) 179. SHANTAROVICH, V.P. , GOLDANSKII, V.I. , SHANTAROVICH, P. S. , KOLDAEVA, О. V. , Dokl. Akad. Nauk SSR 197 (1971) 1122. GOLDANSKII, V.I. , SHANTAROVICH, V.P. , Appl. Phys. 3 (1974) 335. See, e.g.: (a) BRIEGLEB, G ., Elektronen-Donator-Acceptor Komplexe, Springer Verlag, Berlin (1961); (b) ROSE, J. , Molecular Complexes, Pergamon Press, Oxford (1967); (c) Molecular Complexes (FOSTER, R. , Ed.), Elec. Science, London (1973). TAO, S.J. , J, Chem. Phys. 56 (1972) 5499. LEVAY, B. , VERLES, A. , Radiochem. Radioanal. Lett. 11 (1973) 227. MADIA, W.J. , NICHOLS, A. L. , ACHE, H.J. , J. Phys. Chem. (in press). EHRENSON, S. , BROWNLEE, R.T.C. , TAFT, R.W., Physical Organic Chemistry (STREITWEISER, A. , TAFT, R.W. , Eds)_10, Wiley, New York (1973) 1.
IA E A -P L-6 1 5/6
[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
[34] [35] [36] [37]
[38] [39] [40] [41] [42]
105
MADIA, W.J. ,NICHOLS, A. L. , ACHE, H.J. , see Ref. [5 ]. GOLDANSKH, V. I. , MOKRUSHIN, A.D. , TATUR, A.O. , HighEnergy Chem. (Engl, transi. ) ^ (1969) 22. NICHOLAS, J.B. , BARTAL, L.J. , WILD, R.E. , ACHE, H. J. , J. Phys. Chem. 77 (1972) 178. BARTAL, L.J. , ACHE, H.J. , J. Phys. Chem. 77 (1973) 2060. WILLIAMS, T. L. , ACHE, H.J. , J. Chem. Phys. 50 (1969) 4493. GOLDANSKII, V.I. , ZUSMAN, R. I. , MOLIN, Y .N ., SHANTAROVICH, V.P. ,Dokl.Akad. Nauk SSR_188 (1969) 1079. TAO, S.J. , Phys. Rev. Lett. 14 (1965) 935. GOLDANSKH, V. I. , DALIDCHIK, F.I., IVANOV, F. I. , High Energy Chem. 3 (1969) 142. English version. BARTAL, L.J. , NICHOLAS, J.B. , ACHE, H.J. , J. Phys. Chem. 76 (1972) 1124. BARTAL, L.J. , ACHE, H.J. , Radiochim. Acta 19 (1973) 49. ESTRUP, P.J. , WOLFGANG, R. , J. Amer. Chem. Soc. 82 (1960) 2665. WOLFGANG, R.J. , J. Chem. Phys. 39 (1963) 2983. For recent reviews on hot atom chemistry see: (a) WOLF, A.P. , Advan. Phys. Org. Chem. 2 (1964) 202. (b) WOLFGANG, R. , Prog. React. Kinet. 3 (1965) 124. (c) WOLFGANG, R., Annu. Rev. Phys. Chem. 16 (1965) 15. (d) STOCKLIN, G. , in Chemie heisser Atome, Verlag Chemie, Weinheim (1969). BARTAL, L.J. , ACHE, H.J. , J. Inorg. Nucl. Chem. 36 (1974) 922. TAO, S.J. , GREEN, J. H. , J. Phys. Chem. 73 (1969) 882. TAO, S.J. , GREEN, J.H. , J. Chem. Soc. (1968) 408. For general papers on muon and muonium interactions see, for example: (a) HUGHES, V.W. , Physics Today (Dec. 1967) 29; (b) HUGHES, V.W. , Annu. Rev. Nucl. Sci. 16 (1966) 445. (c) HUGHES, V.W. , McCOLM, D.W., ZIOCK, K. , PREPOST, R. , Phys. Rev. A¿(1970) 595; (d) IVANTER, I. G. , SMIGLA, V.P. , Sov. Phys. JETP 27 (1968) 301; (e) IVANTER, I.G. , SMIGLA, V.P. , Sov. Phys. JETP 28 (1969) 796; (f) FIRSOV, V.G. , BIAKOV, V. M. , Sov. Phys. JETP 20 (1965) 719. BREWER, J.H. , CROWE, K.M. , JOHNSON, R. F. , SCHENCK, A. , LBL-595 and LBL-593. BREWER, J. H. , CROWE, K.M. ,private communication. Data were taken from the compilation of results found in Ref. [1(g)] . The authors do notspecify whether the measured rage constants are for hot or thermal Mu reactions. BABAEV, I., BALATS, M.Y. , MYASISHCHEVA, G . C . , OBUKHOV, Y. V. , ROGANOV, V.S. , FIRSOV, V.G. , Sov. Phys. JETP 23 (1966) 583. BREWER, J.H. , CROWE, K.M. , JOHNSON, R. F. , SCHENCK, A. , WILLIAMS, R.W. , Phys. Rev. Lett. 27 (1971) 297.
IA E A -P L-61 5/7
IMPACT OF THEORETICAL CHEMISTRY ON ELUCIDATION OF HOT ATOM REACTION MECHANISMS M .D . N E W T O N B r o o k h a v e n N a tio n a l L a b o r a to r y , U p to n , L o n g Isla n d , N .Y ., U n ite d S ta te s o f A m e r ic a Abstract
IMPACT OF THEORETICAL CHEMISTRY ON ELUCIDATION OF HOT ATOM REACTION MECHANISMS. The current impact of the various areas of theoretical chemistry is briefly examined with regard to the elucidation of hot atom reaction mechanisms. The particular relationship between theory and experiment and improved understanding of energy transfer and chemical kinetics is discussed from the point of view of the theorists with some emphasis on the experimental contributions to improving theoretical understanding and testing theoretical models. Applications of modem theory to hot atom chemistry are suggested.
T h is p ap er b r ie fly exa m in es the c u rre n t im p a ct o f the v a rio u s a re a s o f th e o r e tic a l c h e m is try on our understanding o f the fundam ental a sp ects o f hot atom c h e m is try . W h ile g r e a t advan ces have been m ade in the scop e and so p h is tic a tio n o f hot atom la b o r a to r y e x p e rim e n ts in the past decad e, so have t h e o r e tic a l c a p a b ilitie s m ade en orm ou s s tr id e s , both in te r m s o f d e ta ile d e m p ir ic a l and ab in itio p o ten tia l e n e rg y s u rfa c e s , and in te r m s o f a b ility to p r e d ic t the m ic r o s c o p ic d y n a m ica l d e ta ils o f a v a r ie t y o f r e a c tiv e and nonr e a c t iv e p r o c e s s e s . T h e fa c t that th ese th e o r e tic a l tech n iqu es, im p lem en ted by h ig h -s p e e d d ig ita l co m p u ters, have been brought to b e a r so in te n s iv e ly on the p ro b le m s o f hot atom c h e m is try , and w ith such en cou ragin g p r e lim in a r y re s u lts , is (in the m ind o f the p re s e n t au thor) one o f the m o st s o lid in d ication s o f the continuing v ita lity o f hot atom c h e m is tr y — ju dged as an a re a o f p r im a r y im p o rta n ce in understanding e n e rg y tr a n s fe r and c h e m ic a l k in etics. T h e in c re a s in g a b ility o f hot atom c h e m is try to stim u la te a re s p o n s e fr o m th e o r is ts , and the m utual in te rp la y betw een th eo ry and e x p e rim e n t w hich has re s u lte d , a re o f c o u rs e not at a ll s u rp ris in g , sin ce by the v e r y fa c t it c o v e r s such a b roa d e n e rg y ra n g e , hot atom c h e m is try o ffe r s a p a r tic u la r ly c o m p re h e n s iv e ch a llen ge to the th e o r is t. R e a c tio n s which o ccu r r e a d ily at high e n e r g ie s (i.e . r e la t iv e k in etic e n e r g ie s ), m ay at lo w e r o r th e rm a l e n e r g ie s , p r o c e e d e ith e r not at a ll, o r by d iffe r e n t m ech a n ism s. T h e ra n g e of e n e rg y a ls o f o r c e s one to b road en on e 1 s th e o r e tic a l p e r s p e c tiv e s in a v a r ie t y o f w ays. A s opposed to the usual situ ation fo r th e rm a l r e a c tio n s , a v a r ie t y o f e le c tro n ic sta tes m ay b eco m e a c c e s s ib le to hot re a c ta n ts , and the im p lic a tio n s o f c u rv e c r o s s in g s , a vo id ed c r o s s in g s , and n on -a d ia b a tic beh aviou r m ust be c lo s e ly s c ru tin iz e d . A n im p o rta n t subset o f e x c ite d sta te s , i.e . io n ic sta tes, is a lw a ys a p o ten tia l c o m p lic a tin g fa c to r ; w h ile the s o - c a lle d "re s o n a n c e r u le " o r "a d ia b a tic p r in c ip le " has often been co n v in c in g ly in vok ed to ru le out the im p o rta n c e o f ion ic sta te s , the g e n e r a l situ ation is qu ite dependent on the nature and co n cen tra tio n o f su b stra tes and m o d e ra tin g s p e c ie s . A n im p o rta n t upshot o f m uch re c e n t th e o r e tic a l w o rk is that co m p lete e x c ita tio n fu nctions (c r o s s - s e c t io n s as a fu nction o f r e la t iv e k in etic e n e r g y )
107
108
NEW TON
b ased on e x te n s iv e c la s s ic a l t r a je c t o r y ca lcu la tio n s and r e a lis t ic poten tial e n e rg y s u rfa c e s a r e now a v a ila b le , not only fo r the is o to p ic v a ria n ts of the p ro to ty p e re a c tio n , H + H2, but a ls o fo r the 18F + HD and T + C H 4 (and is o to p ic v a r ia n ts ) s y s te m s . In addition, ex c ita tio n functions fo r a v a r ie t y o f re a c tio n s h ave been obtained using m o r e a p p ro x im a te k in em a tic and hard sph ere m o d e ls . T h e s e e x c ita tio n functions a re b ein g e x p lo ite d in a num ber o f ways: e .g ., in te rp re ta tio n o f p re v io u s p h en o m en o lo g ica l c o r r e la tio n s such as bond e n e rg y e ffe c ts ; te s tin g and g e n e r a liz in g s im p le k in etic th e o r ie s , p re d ic tin g produ ct y ie ld s , is o to p e e ffe c ts , and s te r e o c h e m ic a l d e ta ils o f re a c tio n path w ays, and a s s e s s in g the r e la t iv e im p o rta n ce o f r e a c t iv e and n o n -re a c tiv e , in e la s tic even ts. T h e p oten tia l im p o rta n c e o f in e la s tic c o llis io n s , e s p e c ia lly when p o ly a to m ic m o le c u le s a r e in v o lv e d , is now ra th e r w id e ly re c o g n iz e d , and so m e p r e lim in a r y re s u lts fr o m the T + C H 4 study, in d ica te qu a n tita tively that in e la s tic e n e rg y lo s s m ay be a s ig n ific a n t e ffe c t. A n addition al re s u lt of t r a je c t o r y ca lc u la tio n s w ith im p orta n t b e a rin g on a ccu ra te p re d ic tio n of produ ct y ie ld , is the re c o g n itio n o f the fa c t that the fa te o f "p ro d u c ts " is h igh ly contingent on subsequent c o llis io n s and oth er d yn a m ica l fa c to r s ; i.e . in te rn a lly hot produ cts a re su b ject to e ith e r s ta b iliz a tio n o r u n im olecu lar decay; and som e produ cts w hich a re both tr a n s la tio n a lly and in te rn a lly hot, m a y perh aps d is s o c ia te upon c o llis io n . Thus, e n e rg y lo s s is a m u ltifa c e te d p ro b le m , in v o lv in g both rea cta n ts and to som e d e g r e e , the fa te o f produ cts. A s w e sh a ll e m p h a size b elow , in d iscu ssin g carb on atom c h e m is try , a b ility to e ffe c t a change in the e le c tr o n ic state o f the hot rea cta n t, is another im p ortan t r o le o f the " in e r t " m ediu m , which m ust be u n derstood in g r e a te r d eta il. W h ile the p re s e n t com m en ts focu s on gas phase r e s u lts , it should be noted that som e p ro m is in g com p u ter sim u la tion studies have been c a r r ie d out fo r r e c o i l atom re a c tio n s in c r y s ta llin e s o lid s (tra n s itio n m e ta l h exa-h alo c o m p le x e s ), w ith in d ica tio n s that product d istrib u tion s in such s y s te m s can be accounted fo r by a to m ic c o llis io n dyn am ics and s im p le re a c tio n s o f a w e ll-d e fin e d num ber o f d e fe c ts . O f c o u rs e , in a condensed phase, m any events absent in the ga s phase m ay assu m e im p o rta n c e , such as re p e a te d attack b etw een a p a r tic u la r p a ir o f rea cta n ts. Up to now we have dw elt on the im p a ct o f fu ll d y n a m ica l ca lcu la tion s. Th e c h e m ic a l dyn am ics is , o f c o u rs e , to ta lly d e te rm in e d by the p o ten tia l e n e rg y s u rfa c e (once in itia l con dition s have been s p e c ifie d ). In the re m a in d e r o f the d iscu ssio n w e sh a ll focu s on the p oten tia l e n e rg y s u rfa c e s — how they a re obtained and the in fe r e n c e s that can be draw n fr o m them — em p h asizin g im p o rta n t d iffe r e n c e s betw een s im p le m on ovalen t rea c ta n ts , such as H and F , and the im p orta n t p o ly v a le n t s p e c ie s , carbon . T o the extent that the system s H + H 2, F + H 2, and H + C H 4 a re ty p ic a l, re c e n t e x p e rie n c e in d ica tes that sim p le s e m i- e m p ir ic a l p oten tia l e n e rg y s u rfa c e s a re qu ite adequate in accounting fo r m any o f the d e ta ils o f the re a c tio n s , inclu ding the e n e rg y d istrib u tio n of the p rod u cts. T h e s e p oten tia l e n e rg y s u rfa c e s a re based p r im a r ily on r e a d ily a v a ila b le th e rm o c h e m ic a l and s p e c tro s c o p ic data, which a re in c o rp o ra te d into a m o d e l fo r c e fie ld , such as that obtained fr o m s im p le v a le n c e bond th e o ry (e .g ., the fa m ilia r L E P S m o d e l). T o som e extent the p o ten tia l e n e rg y s u rfa c e s a ls o m ake use o f the lo c a tio n and m agn itu des o f th resh o ld s or b a r r ie r s , as in fe r r e d fr o m e x p e rim e n ta l data o r a ccu ra te ab in itio ca lcu la tion s. T h e approach v i s - à - v i s ab in itio ca lcu la tio n s has been to m ake op tim a l use o f them when a v a ila b le , but o th e rw is e to p ro c e e d on the b a sis of e m p ir ic a l data o r s e m i- e m p ir ic a l c a lcu la tio n s. In p r a c tic e , this ap proach has been em in en tly s u c c e s s fu l in studying m any re a c tio n s o f m on ovalen t s p e c ie s : the
IA E A -P L-61 5/7
10 9
H + H 2 and F + H 2 d y n a m ica l stu dies w e r e not at a ll dependent on ab in itio e n e r g ie s ; the s e m i- e m p ir ic a l p oten tia l e n e rg y s u rfa c e s which th ey w e re b ased on w e r e su pported by subsequent ab in itio c a lcu la tio n s. In the c a s e o f the m o re co m p le x T + C H 4 s y s te m , ab in itio e n e r g ie s w e re a lre a d y a v a ila b le , and th ey p la yed an im p o rta n t, but s t ill a r e la t iv e ly m in o r r o le in p a ra m e t e r iz in g the to ta l p oten tia l e n e rg y s u rfa c e s . W e now turn to an in h eren tly m o r e co m p le x situ ation — the r e a c t iv it y o f a to m ic carb on , w hich w e ll e x e m p lifie s the unique p ro b le m s that a r is e in the c a se o f a p o ly v a le n t rea cta n t. One finds h e re that exten sion s o f sim p le v a le n c e bond th e o ry a re not n e a rly so s tr a ig h tfo rw a rd as in the c a se of m on ovalen t atom s becau se one m ust d eal w ith a m u ltip lic ity of sta tes. W e sh a ll f ir s t outline som e o f the c o m p lic a tio n s , and then d iscu ss m o d e l ab in itio ca lcu la tio n s on the С + H 2 s y s te m , and the r o le th ey p lay in in te rp re tin g the re a c tio n s of hot carb on atom s and alkan es. W e note in p a ssin g that a m u lti p lic ity o f sta tes is not unique to m u ltiva len t atom s. F o r e x a m p le, th e re a re r e a lly s ix independent e le c tr o n ic sta tes a s s o c ia te d w ith the 2P ground state o f the flu o rin e atom . W h ile th e re m ay th e r e fo r e be so m e in te re s tin g nonad iabatic e ffe c ts , it n e v e rth e le s s appears that m o st o f the c h e m is tr y can be u nderstood in te r m s o f the lo w e s t ad iab atic p oten tia l e n e rg y s u rfa c e . T h e q u a d riva len t carb on atom u n d ergoes a r ic h v a r ie t y o f re a c tio n s with h yd ro ca rb o n su b stra tes, even when one lim it s th ese to sa tu ra ted s p e c ie s . P a r t o f the fa s c in a tio n o f th e s e re a c tio n s is that w h ile m any of th em a re thought to be "h o t" r e a c tio n s , r e a c t iv it y p e r s is ts even under con dition s of high m o d era tio n , but with in d ica tion s that the m ech a n ism (and the p ro d u cts) of re a c tio n m ay be quite s e n s itiv e to the k in etic e n e rg y . W h ile r e a c t iv it y o f th e rm a l carbon atom s with unsaturated h yd roca rb on s is not at a ll s u rp ris in g , the d e g re e and nature o f r e a c t iv it y w ith satu rated s p e c ie s is not so e a s ily u n derstood. A t any r a te th e re is a v a s t body o f e x p e rim e n ta l data b ea rin g on the r e a c t iv it y o f both th e rm a l and hot carb on atom s, and th ese data p ro v id e f o r am p le te s tin g o f any c o m p re h e n s iv e th e o r e tic a l m o d el. T h e c o m p le x ity o f carb on atom c h e m is tr y is exp ected in v ie w o f the fa ct that the carb on atom in its lo w e s t e le c tr o n ic co n figu ra tio n (s 2 p2) g iv e s r is e to tw o lo w - ly in g e x c ite d sta tes (the 1D and ’■S sta tes a re , r e s p e c t iv e ly , 1.26 and 2.68 eV above the 3P ground sta te ), and the tw o lo w e s t sta tes (3P and 1 D) have sp a tia l and/or spin d e g e n e ra c y . A s re a c tio n p ro c e e d s , th is d e g e n e ra c y is c o n v e rte d into a m a n ifo ld o f lo w - ly in g e x c ite d m o le c u la r sta tes. The c o m p le x ity fo r ty p ic a l pathw ays in the С + H 2 s y s te m is illu s tr a te d in F i g . l . T h e p o ten tia l e n e rg y s u rfa c e s shown s c h e m a tic a lly , a r e based on th e rm o m ic a l data, and ab in itio con fig u ra tio n in te ra c tio n ca lcu la tio n s. One sa lien t fe a tu re o f F i g . l is to e m p h a size that, in addition to the usual a b stra ctio n pathway, the carb on atom , becau se o f its p o ly v a le n t c h a ra c te r, m ay in s e rt into a s in g le bond (h e re H -H ) to g iv e at le a s t a te m p o r a r y re a c tio n c o m p lex . S om e s p e c ific addition al fe a tu re s o f the c o r r e la tio n d ia g ra m a re: (1) B a r r ie r s w hich d is c rim in a te betw een s in g le t and t r ip le t sta tes; (2) the e x is te n c e o f s iz a b le b a r r ie r s fo r re a c tio n s w hich a re n e v e r th e le s s s tro n g ly e x o th e rm ic ; (3 ) a stro n g dependence o f b a r r ie r m agnitude on an gle o f approach (fo r the 3P sta te); th is is a m a n ife s ta tio n o f an o r b ita l s y m m e try e ffe c t, which d icta tes that p erp e n d ic u la r (C2v) ap proach o f 3P carb on to the H 2 m o le c u le is "fo r b id d e n " th e r m a lly (i.e . a b a r r ie r o f the o r d e r o f eV e x is ts ) w h e re a s a lo w e r b a r r ie r e x is ts fo r a m o re obliqu e (C s) approach; (4) the p o s s ib ility o f tw o d iffe r e n t, but lo w - ly in g , sta te s fo r the a b stra ctio n produ ct — 27t o r 4E CH. T h e o c c u rre n c e o f the lo w - ly in g 4 E* sta te o f CH
110
NEW TON
is e s p e c ia lly in te r e s tin g inasm uch as it has n e v e r been o b s e rv e d s p e c tr o 足 s c o p ic a lly . T h is is b eca u se it is not con n ected by a llo w ed tra n s itio n s with the known m a n ifo ld o f doublet sta tes. A v a r ie t y o f ab in itio ca lcu la tio n s, how足 e v e r , in d ica tes that the state is w ithin an e V o f the ground 2?r sta te, and its p o ten tia l im p o rta n c e in s o rtin g out carb on atom re a c tio n m ech a n ism s w ill be r e f e r r e d to in the fo llo w in g d iscu ssion . T h is ex a m p le illu s tr a te s the in c r e a s in g ly c r u c ia l r o le w hich can be p layed by ab in itio ca lcu la tio n s in c o n s tru c tin g u sefu l c o r r e la tio n d ia g ra m s with w hich to an alyse r e a c tiv ity . A n o th e r a sp ect o f carb on atom c h e m is tr y , w hich has re s u lte d in much sp ecu lation , is the a b ility o f the 1S state to u ndergo r e a c t iv e c o llis io n s . E x p e r im e n ta l ev id e n c e fo r th e rm a l 1S carb on atom s is at b est am biguous. It is , h o w e v e r, p la u sib le to exp ect an a p p re c ia b le amount o f n u cleogen ic ca rb on atom s to e n ter the r e a c tiv e k in etic e n e rg y ra n g e ( ~ 10 e V ) in the 1S state. T h e ab in itio c a lcu la tio n s re p re s e n te d in F i g . l su ggest that the c o llis io n o f *S atom s w ith s in g le bonds would lea d e ith e r to no re a c tio n o r to d is s o c ia tio n . W e m ay s u m m a riz e th is g e n e r a l s u rv e y o f e le c tro n ic sta te s by sa yin g that the p o ly va len t nature o f carb on show s up as a c o m p li足 ca ted m a n ifo ld o f s ta te s , m any o f w hich a r e a c c e s s ib le to r e c o il carbon a tom s in the r e a c tiv e e n e rg y zon e. T h is is in s tro n g co n tra st to the situation f o r tritiu m , and c o rre s p o n d in g to the in c re a s e d c o m p le x ity we fin d a g r e a te r dependence on ab in itio ca lcu la tio n o f c r u c ia l fe a tu re s o f the p oten tia l e n e rg y s u rfa c e s . F u r th e r m o r e , as the num ber and d en sity o f ad iabatic su rfa c e s g o e s up, so a ls o do the p o s s ib ilitie s fo r n on -ad iab atic e ffe c ts a lso in c re a s e : e .g ., sw itch in g fr o m one p oten tia l s u rfa c e to another v ia s p in -o rb it coupling, v ib r o n ic cou plin g, o r oth er m ech a n ism s. Our com m en t above with re g a r d to the r e a c t iv it y o f the 1S state was based on exam in ation o f the adiabatic s u rfa c e w hich a r is e s fr o m it. S ince th is s u rfa c e ( F i g . l ) is f a ir ly w e ll s e p a ra te d fr o m oth er s u rfa c e s , our r e lia n c e on a s in g le ad iabatic su rfa ce s e e m s ju s tifie d . H a vin g docum ented the function o f ab in itio ca lcu la tio n s in m apping out the a c c e s s ib le sta tes fo r p o ly va len t sy s te m s such as the carb on atom , w e a ls o note that v a rio u s g e n e ra liz a tio n s o f the s im p le tra d itio n a l va le n c e bond (o r L E P S ) ap p roach es a re c u rre n tly bein g d eve lo p e d (e .g ., the d ia to m ic s in -m o le c u le s m eth od ), and a r e b ein g app lied to p o ly va len t situ ation s. N e e d 足 le s s to say, a p ra g m a tic com bin ation o f ab in itio and s e m i- e m p ir ic a l m ethods w ill continue to be the optim um approach fo r obtaining the p oten tia l en erg y in fo rm a tio n n e c e s s a r y to c a r r y out d e fin itiv e d yn am ical stu dies o f hot atom re a c tio n s . O u r fin a l c o n s id e ra tio n in th is con tribu tion w ill be to a s s e s s the re le v a n c e o f the data re p r e s e n te d by F i g . l to w a rd s s o rtin g out the d e ta ile d m ech an ism o f a c e ty le n e and eth ylen e fo rm a tio n fr o m r e c o il carbon atom s and alkanes. T h e evo lu tio n o f v a rio u s p ro p o sed m ech a n ism s o v e r the past decade has b een d iscu ssed in another con tribu tion fr o m th is pan el (D r. A .P . W o lf)1. W e sh a ll focu s on the h ypoth esis that a c ety len e a r is e s fr o m the in s e rtio n into a C H bond, fo llo w e d by the s p littin g o ff o f tw o ra d ic a ls , w h erea s ethylene a r is e s in a seco n d a ry in s e rtio n r e a c tio n o f m ethyne (C H ), the m ethyne being the p r im a r y product o f an a b s tra c tio n o r in s e rtio n -d e c o m p o s itio n re a c tio n o f ca rb on w ith the alkane. One can ask: (1) Do the d e ta ils o f the ab in itio p oten tia l e n e rg y s u rfa c e s ( F i g . l ) a llo w the re s o lu tio n o f th is m o d el to be sh arpen ed (i.e ., w hich sta tes o f carb on a re e x p ected to le a d to which states
1 IAEA-PL-615/12,
these Proceedings.
111
IAEA-PL-615/7
^
[за , (1Пи )]
.
- ' S
+ 2 H ( 3 S)
(D IS S O C IA T IO N )
t
^ V
- D
+
2 H
(2 S )
/
FIG.l. Pertinent energy levels are displayed for various states of the C + H2 reaction. Dissociation limits (C + H+H) are indicated in parentheses. The single dashed line refers to a non-symmetrical (Cs) insertion path.
o f w hich p ro d u c ts ? ); and ( 2 ) what p re d ic tio n s a r e im p lie d by the p o ten tia l e n e rg y s u rfa c e s , which a re am en able to e x p e rim e n ta l te s t? In a n sw erin g the above qu estion s one m ust f ir s t c o n s id e r the re le v a n c e o f the С +Hg sy s te m as a m o d e l fo r the p r im a r y attack o f a r e c o il atom on a C H bond. A t le a s t at th e rm a l e n e r g ie s , s e v e r a l ex a m p les can be found w h ere H 2 and C H bonds d isp la y s im ila r r e a c t iv it ie s . F o r e x a m p le, H a b s tra c tio n by t r ip le t C H 2 is v e r y in e ffic ie n t, and bond in s e rtio n by s in g le t C H 2 a p p ea rs to be quite fa c ile . R a te constants fo r the d isa p p ea ra n ce o f C H in the p re s e n c e o f H 2 and C H 4 a re found to be quite s im ila r . F la s h p h o to ly s is e x p e rim e n ts on ca rb on suboxide (C 30 2) in the p re s e n c e o f H2 o r CH 4 have in d ica ted that *D ca rb on atom s (p resu m a b ly th e r m a l) r e a c t r e a d ily w ith both H 2 and CH 4 . H o w e v e r, we m ust note that the r a te o f d isa p p ea ra n ce o f 3P atom s s eem s to im p ly a ra p id in s e rtio n re a c tio n w ith H 2 (show ing so m e p re s s u re depen den ce), w h erea s CH 4 is e s s e n tia lly u n re a c tiv e . In co n tra st, our c a lc u la tio n s ( F i g . l ) r e s u lt in a v e r y la r g e b a r r ie r f o r p erp e n d ic u la r in s e rtio n (~ 4 e V ), a s c r ib e d ab ove to o r b ita l s y m m e try e ffe c ts , and a thorough exam in ation o f the p oten tia l e n e rg y s u rfa c e fo r m o r e ob liqu e ap p roach es, in d ica tes that any in s e rtio n pathway w ill in v o lv e a b a r r ie r o f at le a s t 1 eV . W e a re at p re s e n t continuing th ese stu dies so as to pinpoint the lo c a tio n o f the b a r r ie r . Although g r e a t c a r e was taken in the e x p e rim e n ta l w o rk to e lim in a te extran eou s r e a c tio n s , perh aps fu rth e r study would be w o rth w h ile to d e te r m in e in m o r e d e fin itiv e fash ion the p r im a r y m ech a n ism fo r lo s s o f 3P carbon in the p re s e n c e o f and СзОг . A p r io r i, it s e e m u n lik ely that H 2 would p e r m it fa c ile in s e rtio n , sin ce la r g e r C H 4 m o le c u le , w hich cou ld m o r e e ffe c t iv e ly s ta b iliz e an in s e rtio n c o m p le x , is found to be u n re a c tiv e . A t any
112
NE W T O N
ra te ou r ca lcu la ted e n e rg y b a r r ie r s fo r 3P ca rb on atom in s e rtio n a re q u a lita tiv e ly con sisten t w ith the la c k o f r e a c t iv it y betw een th e rm a l 3P carbon atom s and alkan es, and we f e e l that the С + H2 ca lcu la tio n s p ro v id e a u sefu l m o d e l fo r alkane r e a c tiv ity . W e p ro c e e d then, on the b a sis o f the С + H 2 m o d el, to discu ss the exp ected fate o f e n e r g e tic carb on a tom s, lim itin g o u r s e lv e s to the 3P and XD sta te s , s in c e the 1S state has been d ealt with above. V e r y s im p ly , we in fe r fr o m F i g . l that: (a) S u ffic ie n tly e n e r g e tic 3P carb on atom s could pass o v e r the b a r r ie r and y ie ld a te m p o ra ry in s e rtio n co m p lex ; n ea r the th resh o ld re g io n , at le a s t, this co m p le x cou ld be exp ected to have a su ffic ie n t life t im e fo r in te rn a l e n e rg y tr a n s fe r and bond b re a k a g e lea d in g to a cetylen e
(b) S im ila r s ta b iliz a tio n o f the in s e rtio n c o m p le x fr o m e n e rg e tic *D carb on atom s would be h igh ly u n lik ely, sin ce th e re is no apparent en ergy b a r r ie r . T h e in s e rtio n co m p le x would thus d ecom p ose to y ie ld m ethyne (C H ) in the 2 7t state. (c ) B oth e n e r g e tic 3P and ^-D carb on atom s could y ie ld CH d ir e c tly by a b stra ctio n , but a s ig n ific a n t p o rtio n o f the C H a ris in g fr o m 3P carbon would p ro b a b ly be in the qu artet sta te ( 4 l f ) , a state which would not in s e r t into a C H bond so as to y ie ld eth ylen e. In su m m a ry, w e would thus attrib u te the a c ety len e y ie ld fr o m hot carbon atom s e x c lu s iv e ly to the 3P state, w h erea s the ^ m ethyne (the p rop osed eth ylen e p r e c u r s o r ) would com e p r im a r ily fr o m the in s e rtio n -d e c o m p o s itio n pathway o f e n e r g e tic -t) atom s. Som e 2тг m ethyne could a ls o co m e fro m d ir e c t a b stra ctio n by e n e r g e tic !D atom s. T o co m p le te the p ic tu re , we note that in the n ea r th e rm a l ra n g e , s ta b iliz a tio n o f aD in s e rtio n m igh t be su fficien t to a llo w a c e ty le n e fo rm a tio n , as ou tlined above, w h erea s a ll channels fo r 3P carb on would b e c o m e c lo s e d . Thus, an in te re s tin g , en ergy-d ep en d en t sw itch in the r o le o f 1D carbon atom s is postulated. S in ce our th e o r e tic a l a n a ly s is has a llo w ed ra th e r distin ct m ech an istic r o le s to be a ssign ed to the lo w e s t s in g le t and t r ip le t sta tes o f carb on , the m o s t obviou s e x p e rim e n ta l te s t would be to add to the usual r e c o il carbon e x p e rim e n t, an " in e r t " substance which would e ffic ie n tly c o n v e rt 1D atom s to 3P atom s by c o llis io n a l d e a ctiva tio n , without s tro n g ly a ffe c tin g the kin etic e n e rg y d istrib u tio n . Xenon is such a substance, and unpublished w o rk in the la b o r a to r y o f A . P . W o lf shows that X e attenuates eth ylen e produ ction m uch m o r e s tro n g ly than good k in etic e n e rg y m o d e ra to rs lik e N e. On the oth er hand, attenuation o f a c e ty le n e by X e is in acco rd a n ce with expectation s based s im p ly on k in etic e n e rg y m o d era tio n . T h e s e e x p e rim e n ta l re s u lts a re co n sisten t w ith the above th e o r e tic a l p re d ic tio n s : the sharp ethylene red u ctio n a r is e s fr o m d ep letio n o f *D atom s; the m ild red u ctio n in the a c e ty le n e y ie ld re s u lts fr o m a balan ce b etw een c o n v e rs io n o f 1D to 3P atom s and n o rm a l k in etic e n e rg y m o d era tio n , with the la tte r e ffe c t apparently pred om in an t. In addition, the above con clu sion (fr o m th e o r e tic a l c o n s id e r a tio n s) that the JD in s e rtio n -d e c o m p o s itio n m ode is a m a jo r s o u rce of
IAE A -P L-61 5/7
113
m ethyne is g iv e n stro n g support by iso to p e e ffe c ts o b s e rv e d in ethylene prod u ction (W o lf et a l.), which im p ly enhanced fo rm a tio n of CD r e la t iv e to CH (this argu m en t is , o f c o u rs e , b ased on the assu m ption that m ethyne is the ethylene p r e c u r s o r ). T h e o v e r a ll p ictu re which e m e r g e s fr o m the com b in ed a p p lica tio n of th e o ry and e x p e rim e n t ju st outlined, su ggests that d e ta ile d m ech a n istic a n a ly s is , is p ro m is in g and fe a s ib le , even under con dition s o f c o m p le x ity ty p ifie d by the carb on atom . T o the extent that e x p e rim e n ta l hot atom c h e m is tr y , in conjunction with th e o r e tic a l tech n iqu es, can thus s o r t out the r e la t iv e r e a c t iv it ie s o f d iffe r e n t e le c tr o n ic sta tes o v e r a w ide ra n ge of e n e r g ie s , it is p ro v in g it s e lf as a potent and fundam ental p rob e o f c h e m ic a l in te ra c tio n s .
FU TU R E W ORK W e have com m en ted on the cu rre n t c a p a b ilitie s o f th e o r e tic a l c h e m is try in d ea lin g with im portan t e n e r g e tic and dynam ic a sp ects of hot atom ch em istry. T h e r e a re obviou s exten sion s of the w ork d e s c rib e d which a re w orth y o f su bstan tial e ffo r t. In the a r e a o f r e c o il h yd rogen and flu o rin e c h e m is try , w h ere s e v e r a l re a c tio n e x c ita tio n fu nctions a re a lre a d y known, fu rth e r d y n a m ica l stu dies o f e n e r g y - lo s s and d eco m p o sitio n p r o c e s s e s should p ro v id e d e fin itiv e an sw ers to cu rre n t sp ecu lation about the r e la t iv e im p o rta n ce o f th ese p r o c e s s e s , and thus a llo w d e ta ile d c o m p a ris o n o f th e o r e tic a l and e x p e rim e n ta l produ ct y ie ld s , through the use o f a v a r ie ty o f stoch a stic tech n iqu es. When this sta ge has been attained, we sh a ll have a ch ie v e d b a sic understanding not only o f r e a c t iv e p r o c e s s e s , and th e ir v a r ia tio n with en e rg y , but a ls o o f m any n o n -r e a c tiv e e n e rg y tr a n s fe r p r o c e s s e s , which in flu en ce c o llis io n d e n s itie s and produ ct s ta b iliza tio n . In the r e a lm o f p o ly va len t ato m ic re a c tio n s , m uch addition al e ffo r t must be d evoted to tech n iqu es fo r g e n e ra tin g ad iabatic p o ten tia l e n e rg y s u rfa c e s , and the v a lid ity o f using such su rfa c e s in the fra m e w o rk of the B orn O p p en h eim er a p p ro x im a tio n m ust be c r it ic a lly evalu ated. M o d el d yn am ical stu dies in v o lv in g p o ly v a le n t atom s can be exp ected in the n ear fu tu re, and such e ffo r t s se e m e s p e c ia lly a ttr a c tiv e , in sp ite o f the addition al c o m p lic a  tion s r e la t iv e to m on ovalen t atom s, becau se o f the in trig u in g re la tio n s h ip s b etw een the m ech a n ism s g o v e rn in g r e a c tiv ity in the th e rm a l and high en erg y re g io n s .
BIBLIOGRAPHY I.
Calculation of complete excitation functions A. Full 3-dimensional classical trajectory calculations: T + H2, D2KARPLUS, Đ&#x153;., PORTER, R.N., SHARMA, R.D., J. Chem. Phys. 45 (1966) 3871. (F + HD) MUCKERMAN, J.T., J.Chem.Phys. 57_(1972) 3388. (T+CH4) VALENCICH, T ., BUNKER, D.L., Chem.Phys.Lett. 20(1973) 50; RAFF, L.M., J.Chem.Phys. 60 (1974) 2220. B.
Computer simulation studies using kinematic and hard sphere models: SUPPLINSKAS, R., J.Chem.Phys. 49 (1968) 5046. MALCOLME-LAWES, D.J., J.Chem. Soc. (Faraday II) 68(1972) 1613, 2051.
114
NEW TON
II.
Computer simulation of recoil events in crystalline solids ROBINSON, M.T., ROSSLER, K., TORRENS, I.M., J.Chem.Phys. 60 (1974) 680. ROSSLER, K., ROBINSON, M.T., Proc.V.Int.Conf. on Atomic Collisions in Solids, Gatlinburg, Tenn.(1973).
III.
Direct calculation of integral reaction probabilities: PORTER, R.N., J.Chem.Phys. 45 (1966) 2284; PORTER, R.N., KUNT, S., ibid. 52 (1970) 3240; ADAMS, J.T., PORTER, R.N., ibid. 59 (1973) 4105.
IV.
General review of high energy reactions: DUBREM, J., Annu.Rev.Phys.Chem. (1973) 97.
V.
Ab initio calculations on C + H2 system: BLINT, R.J., NEWTON, M.D., manuscript in preparation
VI.
Recent nucleogenic recoil carbon work: WOLF, A.P., etal., manuscript in preparation.
DISCUSSION A .G . M A D D O C K : It is r e fr e s h in g to find a th e o re tic ia n who w ill stick his neck so fa r out that he a ctu a lly su ggests things lia b le to e x p e rim e n ta l test. M . N E W T O N : W ill it e v e r be p o s s ib le to g e t a beam of carbon atom s? It would be good to p ick up the lo o s e end co n cern in g the r e a c tiv ity of C ( 3P ). T h e a v a ila b le data com e fr o m the p h o to ly sis of carb on suboxide, and th ere a re co m p lic a tio n s in understanding what the r e a l p r im a r y steps a re . A .G . M A D D O C K : In the carbon suboxide w ork , the m ethod o f d etection is s p e c ific fo r the p a rtic u la r state o f the carbon atom , is n 't it? Is n 't it D avid H u sain 's w o rk ? I a g r e e that one d oesn 't n e c e s s a r ily know e x a c tly what is happening to the carb on atom — you g e n e ra te a m ix tu re o f sta tes and lo o k s p e c ific a lly at the d isa p p ea ra n ce o f the in d ivid u al state you p r e fe r to stu dy. M . N E W T O N : T h e r e a re s e v e r a l s p e c ie s in the s y stem , and the question is , " I s he r e a lly o b s e rv in g the re a c tio n o f carb on atom s with h ydrogen m o le c u le s , o r is it with so m e oth er s p e c ie s ? " Y . L E E : T h e c la s s ic a l t r a je c t o r y m ethod has been shown to be v e r y u sefu l, e s p e c ia lly in the high e n e rg y sch em es. H o w e v e r, it is by no m eans s u ffic ie n t, is it? E s p e c ia lly n ear the th resh old . A t p resen t the quantum m ech a n ica l ca lcu la tio n s se e m to be v e r y d iffic u lt, but it s e em s c r u c ia l to t r y to understand what the lim it o f c la s s ic a l m ech a n ics is . N e a r the th resh o ld w e have to w o r r y about a d ia b a ticity , z e r o -p o in t e n e rg y , and a ll o f th ose things. M . N E W T O N : That p ro b le m is a lre a d y b ein g fa c e d in the F + H 2 case. It is a lw a ys an in te re s tin g p ro b le m in the c la s s ic a l t r a je c t o r y calcu lation , and even a ro s e with H + H2. In the c la s s ic a l t r a je c t o r y ca lcu la tio n e v e ry th in g fo llo w s d e te r m in is tic a lly fr o m the in itia l con d ition s. But, in a c la s s ic a l ca lcu la tio n , should one put in the z e r o -p o in t e n e rg y f o r the H 2 m o le c u le ? I f you say, " C e r t a in ly it sh ou ld," the tro u b le is that it m ay then be used in a "fra u d u le n t" way — it m ay be used to get o v e r a b a r r ie r with the fo rm a tio n o f p rodu cts which do not have the r e q u is ite z e r o -p o in t e n e rg y le ft to them . T h e ca lcu la tio n s o f B u n k e r-V a le n c ic h and R a ff have done the obvious — they h ave t r ie d it both w ays, with and without the z e r o -p o in t e n e rg y . W ith m ethane th is is a n o n - tr iv ia l p ro b le m — m ethane has about 25 k cal/ m ole
IAE A -P L-61 5/7
115
o f z e r o -p o in t e n e rg y . What th ey c la im with a w ave o f th e ir hands is that th ey g e t about the sam e re s u lts w hether o r not th ey include the z e r o -p o in t e n e rg y . C e r ta in ly , at high e n e rg ie s the c la s s ic a l re s u lts a re fin e. F .S . R O W L A N D : In rea d in g what you have w ritte n , I see that you have d e s c rib e d the H + C H 4 ca lcu la tio n s as s e m i- e m p ir ic a l, although the B unkerV a le n c ic h s u rfa c e s c le a r ly a re not. T h e ir s u rfa c e s a re ju st s tra ig h tfo rw a rd e m p ir ic a l s u rfa c e s . M . N E W T O N : I ' l l be happy to con ced e anything about d istin ctio n s b etw een e m p ir ic a l and s e m i- e m p ir ic a l s u rfa c e s . Bunker and V a le n c ic h did m ake use o f som e ab in itio ca lcu la tio n s, h o w e v e r. One o f the m o s t c r u c ia l r o le s o f ab in itio ca lcu la tio n s is the id e n tific a tio n o f height and lo ca tio n of re a c tio n b a r r ie r s . F o r in stan ce, in the a b stra ctio n re a c tio n o f H + C H 4 the qu estion o ccu rs o f how fa r the o r ig in a l C -H bond has been stre tc h e d at the b a r r ie r point. I think Don Bunker would a g r e e that th is was a c r u c ia l a sp ect o f his p oten tia l e n e rg y s u rfa c e . I would a g r e e , h o w e v e r, that his s u rfa c e was ra th e r e m p ir ic a l. F .S . R O W L A N D : T h e y had about 20-25 p rop osed s u rfa c e s , and they ch eck ed them u n til they found one that a g re e d w ith som e p a rtic u la r e x p e rim e n ta l data, and they then used that su rfa c e . A c tu a lly the only one which fit these data was the la s t s u rfa c e o f the whole group — the o th ers a ll fa ile d . D .J. M A L C O L M E -L A W E S : I f th ey had taken 500 s u rfa c e s , they m ight have obtained another e n tire ly d iffe re n t s u rfa c e that a g re e d w ith the sam e e x p e rim e n ta l data. F .S . R O W L A N D : Bunker and V a le n c ic h c la im that th e re a r e no u n exp lored g e n e r a l c la s s ific a tio n s o f s u rfa c e . I suppose o th ers could attem pt to v e r ify th is statem en t too. M . N E W T O N : T h e B u n k e r-V a le n c ic h s u rfa ce m igh t b e tte r be c a lle d e m p ir ic a l. But the point is that p eop le such as Bunker and R a ff a r e able to m ake the b est use o f a ll e x p e rim e n ta l data, and they m a y o r m ay not ch oose to use k in etic data as p a rt o f th e ir p ro c e d u re fo r c a lib ra tio n o f the s u rfa c e s . T h e y do m ake v e r y p ertin en t use o f a fe w v e r y a ccu ra te e n e rg ie s in the c o n stru ctio n o f the s u rfa c e s . A . P . W O L F : What about the d is c re p a n c y in d iffe re n t p o in ts -o f-v ie w r e la t iv e to the qu estion o f in v e r s io n v e rs u s re te n tio n in the substitution re a c tio n o f tritiu m with m ethane? M . N E W T O N : W hich d iffe r e n c e s ? A . P . W O L F : W e ll, som e of the e x p e rim e n ta lis ts c la im that in v e rs io n is the predom in an t m od e, and o th ers c la im ... M . N E W T O N : W hich e x p e rim e n ta lis ts ? A . P . W O L F : I am thinking o f the kind o f w o rk which O lah has done in w hich they su ggest a lte rn a te m ech an ism s in the substitution re a c tio n . O f c o u rs e , they a r e not using the sam e s y stem , but a ll th ese e x p e rim e n ts a r e tie d to g e th e r w h eth er they use p roton s o r h yd rogen atom s. M . N E W T O N : T h a t's a n o n - tr iv ia l d iffe re n c e . A .P . W O L F : I know the d iffe r e n c e is n o n -tr iv ia l. I 'm r e a lly asking about the p re d ic tio n s fo r the substitution re a c tio n with tr itiu m plus m ethane. M . N E W T O N : Y ou can fo r g e t about the proton e x p e rim e n ts in this connection. T h e p roton addition lea d s to a v e r y stron g, stab le CH 5 in t e r m ed ia te , w h erea s th e re is not a stable en tity with C H S. The in te re s tin g con clu sio n is that both the ca lcu la tio n s o f R a ff and o f B u n k er/ V alen cich — the c a lcu la tio n s a re r e a lly qu ite d iffe re n t — turn out to in d ica te that up to about
116
NEW TON
1 0 0 k c a l/ m o le o f en erg y m o st of the substitution is by the in v e rs io n m ode.
Th at con clu sion is a lit t le bit in d ire c t — you e x tra c t th is fr o m R a ff's c a lc u la tio n s fr o m the fa c t that he u n d erestim a tes the substitution c r o s s - s e c tio n by a fa c to r o f 2 o r 3 in the low e n e rg y re g io n b elow 100 k ca l/ m o le. In his p a p er he sa ys the only lo g ic a l con clu sion is that the in v e r s io n m ode has been exclu ded fr o m the ca lcu la tio n . H e a g r e e s that th e re is a la r g e amount o f in v e rs io n , at le a s t up to 100 k ca l/ m o le. B u n k e r's c a lcu la tio n s quite e x p lic itly y ie ld th is r e s u lt, w h ile above about 130 k ca l/ m o le reten tio n b eco m es the dominant substitution m ode. F .S . R O W L A N D : I think that you a re e x a g g e ra tin g the a ch ievem en t of th e o ry in th is s y stem at th is point, when you say that it accounts s a tis fa c to r ily f o r the e x p e rim e n ta l re s u lts . Stated another way, at low e n e rg ie s Bunker/ V a le n c ic h s e e a h ig h e r c r o s s - s e c tio n with m o s tly in v e rs io n , w h ile R a ff s e e s a lo w e r c r o s s - s e c t io n w ith no in v e rs io n . M . N E W T O N : R a ff adm its that he c a n 't fit y o u r p h otoch em ica l data — h e 's o ff by a fa c to r o f th re e in fittin g you r y ie ld s . F .S . R O W L A N D : A l l rig h t. But B u n k er/ V a len cich fit th ese y ie ld s because th ey took m o r e than 2 0 p oten tia l s u rfa c e s and e m p ir ic a lly ch ose the one which g a v e the b est fit. T h e y 'v e fitte d much of the p h otoch em ical data as part o f the p ro c e d u re fo r ch oosin g the su rfa c e in the fir s t p lace. M . N E W T O N : But fo r c e d it to be in v e rs io n . F .S . R O W L A N D : N o, they d idn 't fo r c e it to be in v e rs io n , but I think one s t ill needs to w o r r y about how unique th e ir p o ten tia l s u rfa c e is . How much v a r ia tio n in th is s u rfa c e can one to le r a te , s t ill fittin g the p h otoch em ica l data, and what w ill the e ffe c t o f this v a r ia tio n be on the in v e rs io n / re te n tio n p ro b le m ? A . P . W O L F : Since th is is a v e r y fundam ental point in tritiu m hot atom c h e m is try , and w e 'r e ta lk in g about e x p e rim e n ta l v e r ific a t io n o f th e o ry , can you go to a s lig h tly m o r e co m p le x m o le c u le to te s t th is? O r does the c a lc u la tio n then b e c o m e so e m p ir ic a l as to be qu estion a b le? F o r ex a m p le, i f you went to e ith e r m eth y l flu o rid e o r ethane, then you could c o n c e iv a b ly do an e x p e rim e n t to te s t the p re d ic tio n s . M . N E W T O N : Y ou would have to have a p p ro p ria te substitution. A . P . W O L F : R ig h t, you would have to u se d eu teriu m and tritiu m substitution. M . N E W T O N : I t 's p la u sib le. H o w e v e r, the situ ation is not to ta lly r o s y — I d on 't want to o v e r s e ll the c a se. F .S . R O W L A N D : Y ou s t ill see som e w o rk f o r the th e o ris t to do? A .P . W O L F : T h e e x p e rim e n ta lis t s t ill has w o rk as w e ll — he hasn't d em on stra ted the la ck o f in v e r s io n in m ethane. T h e r e is no e x p e rim e n ta l e vid en ce that d e m o n stra tes its absence. M . N E W T O N : T h e situ ation is that we h ave, alm o st sim u ltan eou sly, tw o qu ite independent, ra th e r d e ta ile d d yn a m ica l studies w hich y ie ld s im ila r r e s u lts in m any r e s p e c ts . T h e r e is no c o m p e llin g ev id e n c e that in v e rs io n is not a m a jo r channel fo r the low e n e rg y substitution in m ethane. F .S . R O W L A N D : I 'v e been tr y in g to persu ad e T r in a V a le n c ic h to run the sam e s y s te m with h yd rogen atom s o f m a ss 19, le a v in g e v e ry th in g e ls e the sam e — ju st put in d iffe r e n t m a s s e s . M . N E W T O N : T h e y did do CD4. F .S . R O W L A N D : W hat I would lik e to s e e is som e v e r y d iffe re n t m a s s e s w hich then b egin to be m o r e lik e the r e a l e x p e rim e n ta l s y s te m s fo r which in v e rs io n / re te n tio n has a lre a d y been run. She is ra th e r relu ctan t to use
IAE A -P L-61 5/7
117
h yd rogen of m ass 19 as a substitute fo r flu o rin e — she would ra th e r w o r r y about the C - F bond in the b egin n in g, and do the w hole ca lcu la tio n c o r r e c t ly . M . N E W T O N : One o f the oth er r e s u lts that c o m es out o f th ese ca lcu la tio n s is the n o n -r e a c tiv e c r o s s - s e c tio n s . F o r in stan ce, th e re is a t a b le in T r in a V a le n c ic h 's th e s is which shows that ty p ic a lly 40-50% of the k in etic e n e rg y o f the tr itiu m is lo s t in in e la s tic c o llis io n s . T h is kind o f qu an titative in fo rm a tio n is b eco m in g a v a ila b le , w h erea s people used to id ly specu late about how im p orta n t in e la s tic c o llis io n s w e re . F .S . R O W L A N D : It don't think it was id le sp ecu lation . T h e r e a re e x p e rim e n ta l m ea su rem en ts fr o m ten y e a r s ago that in d ica te that the e n e rg y lo s s e s in m ethane a r e g r e a t e r than the e n e rg y lo s s e s in r a r e g a s e s — that m ethane is a b e tte r m o d e ra to r than h eliu m , and v e r y m uch b e tte r than the oth er r a r e g a s e s — it had to be in e la s tic c o llis io n a l e n e rg y lo s s . M . N E W T O N : But now th e re is qu an titative in fo rm a tio n a v a ila b le , and you have to have qu an titative in fo rm a tio n to ca lcu la te the y ie ld s . T h a t's the im p o rta n t p art — you c a n 't c a lcu la te the y ie ld s fo r the r e a c t iv e p r o c e s s e s u n less you a ls o know the e n e rg y lo s s e s in the n o n -r e a c tiv e c o llis io n s . F .S . R O W L A N D : It should be p o s s ib le to use the B u n k er/ V a len cich data to c a lc u la te and c o m p a re with the e x p e rim e n ta l m ea su rem en ts o f in e la s tic e n e rg y lo s s , which a r e a v e ra g e d o v e r a v e r y b road ra n ge o f e n e rg ie s . M . N E W T O N : A n o th er im p orta n t re s u lt fr o m such ca lcu la tio n s is the a v e r a g e im p act p a ra m e te r, and the a v e ra g e angle in the a b stra ctio n re a c tio n . T h is is quite p ertin en t to the d iscu ssio n y e s te rd a y o f the m ech a n ism o f a b stra ctio n . N a tu ra lly , in a b stra ctio n as the e n e rg y g o e s up, you tend to approach the fo r w a r d s c a tte rin g , la r g e im p act c o llis io n that you would exp ect. T h is a ll co m e s out v e r y qu a n tita tively in th ese ca lcu la tio n s. D .J. M A L C O L M E -L A W E S : W e lik e to lo o k at th e o ry as having tw o r o le s : one is the r a tio n a liz a tio n o f known e x p e rim e n ta l fa c ts , and the secon d is the p re d ic tio n o f new fe a tu re s which w ill be am enable to e x p e rim e n ta l v e r ific a tio n . I would lik e to m en tion tw o typ es of ca lcu la tio n s which we have been c a r r y in g out. T h e f ir s t is the h a rd -s p h e re ca lcu la tio n , which we have now extended to s ix b o d ies with fou r d iffe re n t sy s te m s : F ir s t , T + C H 4; secon d, T + C F 4 , which shows a v e r y la r g e T F a b stra ctio n function, and a v e r y s m a ll substitution function; third,, F + CH4, w hich shows a la r g e H F a b stra ctio n function; and fou rth , F + C F4 , which shows an a b stra ctio n function fo r F 2, and a substitu tion function which g o es out to v e r y high e n e rg ie s . T h e m ean e n e rg y in the substitution produ ct in the f ir s t th re e s y s te m s is betw een 4 and 5 eV , w h ile the m ean e n e rg y in the F + C F 4 s y s te m is 7-8 e V with a s ig n ific a n t num ber of even ts in v o lv in g m o r e than 10 eV . T h e s e high e n e rg y even ts w ould p r e sum ably g iv e r is e to CF 2 as a product. T h e g e n e r a l ru le s e e m s to be that a la r g e amount o f e n e rg y is dep osited only when a heavy s p e c ie s hits a m o le c u le which contains tw o o r m o re h eavy atom s. In the c a s e o f F + C H 4, the m o le c u le contains only one heavy atom and th e re is no p o s s ib lity fo r d ep osition o f a la r g e amount o f e n e rg y . In this c a s e , the in te rn a l e n e rg y o f the substitution product w ill be quite s im ila r to that found in the T + C H 4 c a se. N o w , in the m o r e ex a ct ca lcu la tio n s w hich we have done, we have b ased them on a p e rtu rb ed m o le c u la r o r b ita l p oten tia l e n e rg y s u rfa c e , and have m ade th ese ca lcu la tio n s fo r e v e r y p o s s ib le v a r ia tio n o f h yd rogen is o to p ic exchange, and have ju st begun them fo r oxygen and flu o rin e . T h e p a rtic u la r tre a tm e n t can be used fo r d iva len t and p o ly v a le n t s p e c ie s in g e n e ra l.
118
NEW TON
F i r s t w e did H + H2, and obtained the ty p ic a l shape you would expect fr o m the K a rp lu s ca lcu la tion . Th en w e did T + H 2, and found som eth in g v e r y in te re s tin g . In addition to a peak at about 2 eV , th ere was a la r g e reson an ce at the bond stren gth — at about 4.5 eV . Since K w e i has r e c e n tly m ea su red H + T 2, we did the is o to p ic r e v e r s e ca lcu la tio n as w e ll, and w e found again both a hump and a reso n a n ce, although both w e r e m uch s m a lle r than in the T + H 2 c a se. A p a rt fr o m the ra th e r obvious fa c t that w e would lik e to te s t e x p e rim e n ta lly fo r w hether th ese reso n a n ces r e a lly do e x is t, I would lik e to m ention: e x a ctly whey we think that the h a rd -s p h e re ca lcu la tio n s do ra tio n a liz e the e x p e rim e n ta l data. I 'm su re that S h e rry R ow land w ill be able to com e up with som eth in g they don't a g r e e with. A p a r t fr o m the in te rn a l e n e rg y d ep osition , the low y ie ld o f F substitution in T + CF 4 is one exa m p le; the high en erg y t a il to the ab stra ctio n function in the T + CH 4 s y s te m is another exam p le. N ow , e x a c tly what is the e x p e rim e n ta l data with which this la tte r a g r e e s ? F i r s t the H T / R T r a tio is v ir tu a lly id e n tic a l in c e rta in m o d e ra to rs such as CF 4 , N 2 and N e, and y e t СБ^ is an e x tr e m e ly e ffic ie n t m o d e ra to r w h ile N e is not. T h e old id ea o f the m o d e ra to r changing the r a tio has to go, and it is c le a r that the m o d e ra to r p la ys som e oth er r o le as w e ll. N ow , oth er m o d e ra t o r s such as h eliu m , which is a lm o s t as good a m o d e ra to r as C F4 , change the H T /parent r a tio — by in c re a s in g it. P o o r e r m o d e ra to rs such as xenon d e c r e a s e the H T / p a ren t ra tio . I f the re s u lts o f R o o t and R ow land a re taken — in which D2 is added to CH 4, the H T / p a ren t r a tio in c r e a s e s in v e r y m uch the sam e way as i f He is added to T + C H 4. W e b e lie v e th is a ll re s u lts fr o m the c o llis io n a l d is s o c ia tio n o f tra n s la tio n a lly - e x c ite d H T , which has the e ffe c t o f m aking an e ffe c tiv e e x c ita tio n function w hich is d iffe re n t fr o m that o f the p r im a r y event excita tio n function. One way in w hich we b e lie v e this can be te sted is by adding som e s c a v e n g e r such as b ro m in e. B ro m in e r e a c ts v e r y e ffic ie n tly at lo w e n e rg ie s w ith tr itiu m atom s,and the e ffe c t that b ro m in e w ill have upon both a b stra ction and substitution produ cts m ust be co n s id e re d . Substitution w ill be constant b eca u se only th is is ab ove the e n e rg ie s fo r the b ro m in e co m p etitio n and is u n affected by ex tra p o la tio n to in fin ite b ro m in e con cen tration . H o w e v e r, the a b s tra c tio n function w ill not be constant, and you would exp ect to get a v e r y m uch la r g e r s c a v e n g e r e ffe c t in the p re s e n c e o f xenon than we get in helium . T h is , o f c o u rs e , is the e x p e rim e n ta l re s u lt. L . L IN D N E R : Can you m ake som e com m en ts about the fu tu re? And p erh ap s s tic k y o u r neck out som e m o re about the polyvanent atom s in the fu tu re — such as sulphur atom s, oxygen , phosphorus? M . N E W T O N : Sulphur and oxygen b a s ic a lly have the sam e m a n ifo ld of e le c tr o n ic sta tes. D id you ch oose th ose ex a m p les in you r qu estion becau se th ey have the sam e m a n ifo ld , o r is th e re som e independent rea so n ? P h o s phorus is a lit t le d iffe re n t but w ill a lso have tw o lo w -ly in g states of d iffe re n t spin m u ltip lic ity . L . L IN D N E R : T h e r e was another re a s o n . T h e hot atom ch e m is ts , of c o u rs e , know about the d iffic u ltie s w ith oxygen — you can 't do much with it b eca u se o f th e la c k o f a lo n g e r - liv e d is o to p e . But w ith sulphur, th e re a re e x p e rim e n ta l p o s s ib ilitie s . T h e r e a re a lso lo ts of d iffic u ltie s , but the opening is th e re . M . N E W T O N : It is o f p a rtic u la r in te r e s t to m e that th ere is so much high qu ality data in the th e rm a l ra n g e , which is v e r y u sefu l in c a lib ra tin g the c o m p re h e n s iv e th e o r ie s .
IA E A -P L-61 5/7
119
A . G. M A D D O C K : Strange as it m ay seem , th e re is now a lm o s t as much data on th e r m a l ca rb on atom s as th e re is on th e rm a l sulphur atom s. T h e amount o f c la s s ic a l data on the beh aviou r of th e r m a liz e d sulphur atom s is in c r e d ib ly s m a ll. Y . L E E : It is p o s s ib le to produ ce a beam o f carb on atom s — nothing is im p o s s ib le . W e a re tr y in g som e d ivalen t s p e c ie s — an oxygen atom s o u rc e with 0 ( 3 P ) o r 0 ( 1 D ). S im ila r ly , one can produ ce carb on atom s by p y r o ly s is o r p h oto d isso cia tio n in the beam . You w ould have to do it by m ix in g it into a r a te gas. O f c o u rs e , if you have a la s e r you can shoot the la s e r d ir e c t ly into the n o z z le and d is s o c ia te during the expansion. When you s et up a p o ly v a le n t beam s o u rc e , you have to c h a r a c te r iz e the beam , and t h e r e a re num erous c o m p lic a tio n s in v o lv e d . Y o u need m uch m o r e m on ey and m an p ow er. M . N E W T O N : T h a t's m o r e fa v o u ra b le than I had exp ected . The p ro b le m w hich I m en tion ed e a r lie r w ith r e g a r d to th e rm a l carb on atom s m igh t also apply to th e rm a l oxygen atom s — the p ro b le m o f w h eth er the ground state in s e r ts . It s e e m s to m e v e r y stra n ge that C ( 3P ) would in s e r t v e r y e ffic ie n tly into H 2 and not into the C -H in m ethane. In te r m s o f u n im o lecu la r sta b iliz a tio n , m ethane should be ab le to accom m od ate the in s e r tio n m o r e r e a d ily than H 2 could. T h e p eo p le who have done th is carb on atom e x p e r i m ent a re v a g u e ly a w a re o f th is c o n s id e ra tio n , and perhaps a beam e x p e rim e n t m igh t be the only d e fin itiv e way to s o rt th is out. A lthou gh this is a p ro b le m , it is s t ill qu ite p ertin en t to pinning down the a b ility to c h a r a c te r iz e an e n tire e n e rg y sp ectru m . D .J. M A L C O M E - L A WES: W e 'v e done so m e ca lcu la tio n s on the 0 + H 2 re a c tio n , and, c o n v e rtin g to th e rm a l r a te constants, our re s u lts a re w ithin 2 0 % o f the m o s t re c e n t e x p e rim e n ta l gas phase w ork , d ir e c tly goin g to OH +H . M . N E W T O N : T h a t's an e x o th e rm ic re a c tio n . W ith carbon atom s, the a b s tra c tio n re a c tio n is en d oth erm ie. F .S . R O W L A N D : I w ish to com m en t on the question o f the e ffe c ts o f b ro m in e s c a v e n g e r on the T + C H 4 and s im ila r s y s te m s with r e s p e c t to the c o m p e titio n b etw een b ro m in e and the v a rio u s re a c tio n s w ith m ethane. M y fe e lin g is as I e x p re s s e d it y e s te rd a y . Y o u can find in the lite r a tu r e e x p e rim e n ta l data e ith e r w ay, to fit w h a tever you want. It is n 't that ea sy to m ake the m ea su rem en t it s e lf. I would lik e to illu s tr a te som e o f the d iffi c u ltie s that w e have had w ith data fr o m the th e s is o f F r e d S tein k ru g er, one of m y students who r e c e n tly fin ish ed h is Ph .D . Our m e a su rem en ts o f H T / H T fr o m e q u im o la r m ix tu re s o f CD 4 and C2 Hg, m o d era ted in neon — a r e c o il tr itiu m e x p e rim e n t in the gas phase — a re in co m p le te d is a g re e m e n t with the p re v io u s m ea su rem en t by B a k er and W olfga n g. T h e slo p in g c u rv e in the e a r lie r e x p e rim e n t was in te rp re te d as p r o o f o f "h ig h e n e rg y s trip p in g " — but the s lo p e in our e x p e rim e n ta l c u rv e g o e s the oth er d ire c tio n . Our m e a su rem en ts w e re m ad e with b ro m in e s c a v e n g e r at 1.2%. In this p a rtic u la r s y s te m , it is ju st a b a s ic d is a g re e m e n t as to how one m e a s u re s what is a ctu a lly goin g on. It is n 't a qu estion o f in te rp re ta tio n , it is a qu estion of e x p e rim e n t. In a s im ila r w ay ... A .G . M A D D O C K : The tw o e x p e rim e n ts w e r e id e n tic a l? Both neon m o d e ra te d and b ro m in e -s c a v e n g e d ? D .J. M A L C O L M E - L A W E S : N o , th ey a re not, becau se th e re was c e r ta in ly c o n s id e ra b ly m o r e b ro m in e th e re in the B a k e r e x p e rim e n ts . A l l B a k e r 's
120
NEW TON
e x p e rim e n ts w e re done with o v e r 3% b ro m in e and it does m ake a d iffe re n c e , b ecau se you g e t r id o f som e o f the lo w e n e rg y ab stra ction . F .S . R O W L A N D : I f the b ro m in e co n cen tra tion w e re o v e r 3% then they should not have d e s c rib e d it as 1.7% in the pu blication . W e have c a r r ie d out e x p e rim e n ts o v e r a w id e r ra n ge o f va lu es f o r b ro m in e con cen tration , but th ese a re our data w hich m o s t c lo s e ly a p p roxim a te the B a k e r data. W e have a lso co m p a red our re s u lts with those taken fr o m D ick H a ll's th e s is at Y a le , in v o lv in g H T / C H 3T v e rs u s p e r cent b ro m in e. H is re s u lts v a r y one way with m o d e ra tio n b etw een 80 and 92% fo r C H 4, and the r e v e r s e p ic tu re with C D 4 fo r the sam e p e rc e n ta g e s o f m o d e ra to r. When w e c a r r ie d out our e x p e rim e n ts at 80 and 90%, we found that a ll our re s u lts f e l l c lo s e ly to g e th e r — d is a g re e in g with both o f H a ll's lin es,a n d a g re e in g with each o th e r s u ffic ie n tly c lo s e ly fo r it to be hard to say w hether th e re is a tren d e ith e r way. A g a in , when w e tr y to re p ro d u c e e x a c tly the sam e exp erim en t, we don't get the sam e e x p e rim e n ta l resu lt. T h e d iffic u lty h e re r e q u ir e s c o m p a ris o n s beyond those in the published lite r a tu r e . In th ese e x p e rim e n ts , th e re is alw ays the qu estion of knowing ex a c tly what was m ea su red — do you m e a s u re the amount o f H T which show s up in the cou n ter? O r do you m ake c o r r e c tio n s to that num ber? F in d in g out what the c o r r e c tio n s a re in an yon e's e x p e rim e n t is u su ally v e r y d iffic u lt — taking the data rig h t fr o m the la b o r a to r y and fo llo w in g it down through a ll the p ro c e s s in g to the fin a l published num ber is not som eth in g you v e r y often fin d in the lite r a tu r e . W e a re not su re e x a c tly how to explain th e s e d iffic u ltie s — a ll w e can say is that we a r e unable to re p ro d u ce th e ir ex p e rim e n ta l re s u lts . D .J. M A L C O L M E -L A W E S : T h e r e is one m a jo r d iffe r e n c e b etw een the w ay th ese tw o grou ps w ork . T h e R ow land group u ses 1720 P y r e x g la s s , and we use qu artz. F .S . R O W L A N D : R igh t. D .J. M A L C O L M E -L A W E S : Y ou assum e that anything that is fo rm e d on the w a ll g o es into the w a ll and stays th e re . W e assum e that if H T is fo rm e d by re a c tio n at the w a ll, it w ill com e out again, and t h e r e fo r e m ake a c o r r e c tion fo r th is. N ow , I don't think that we do the c a lib ra tio n w e ll, and in fact I think w e 'v e got it w ron g, becau se I think the m a jo r con trib u tion to things com in g fr o m the w a lls is com in g fr o m carbon fra g m e n ts stuck on the w a ll, r a th e r than fro m h y d ro x y l in the w a ll. But, on the oth er hand, I don't think you do the c o r r e c tio n at a ll. F .S . R O W L A N D : A s you know, U rch has pu blished re s u lts in which he show ed th e re was nothing d iffu sin g fr o m the w a ll with 1720 P y r e x . D .J. M A L C O L M E -L A W E S : But, you ca n 't do the D avid U rch type o f e x p e rim e n t and then say that nothing is com in g fr o m the w a lls . What he did was put CD 4 into a bulb, and look fo r H T — and with P y r e x 1720 th e re was none. But the point is that th e re is s t ill p ro b a b ly quite a lo t of D T which would have happened had you had CD 3 r a d ic a ls sittin g on the w all. F .S . R O W L A N D : What we know is that th e re is a b a sic d is c re p a n c y in the e x p e rim e n ts . F r o m m y point o f v ie w , it is n e v e r p o s s ib le to find out what the c o r r e c tio n is in the e x p e rim e n ts with q u a rtz v e s s e ls . I t 's n e v e r published in such a way that one can re a d that "w e s ta rte d out with this o b s e rv e d re s u lt, and m ade a 3% c o r r e c tio n fo r H T com in g fr o m the w a ll..." D .J. M A L C O M E - L A WES: Y ou can alw ays ca lcu la te it. Y o u 'r e alw ays g iv e n the p r e s s u r e , and the re a c tio n v e s s e l, and if you know the com p o sitio n
IA E A -P L-61 5/7
121
o f the re a c tio n v e s s e l, and you can w o rk out the r e c o il ra n ge and D avid U rch pu blished the c a lib ra tio n c u rv e fo r r e c o il ran ge again st w a ll H T . F .S . R O W L A N D : One is n e v e r su re that you have r e a lly re a d e v e ry th in g o ff in e x a c tly the sam e way. It would be v e r y u sefu l i f th e re w e re an ex p e rim e n t d e s c rib e d in which it said, " T h e s e w e re our n u m bers. We m e a s u re d so m uch H T ; we m ade the fo llo w in g c o r r e c tio n s " — not by sayin g, "u s in g this ta b le " but by actu ally g iv in g the num ber. B e fo r e we a rgu e any m o r e about what con clu sion s should be draw n, we have to a g r e e about the e x p e rim e n ta l re s u lts . D .J. M A L C O L M E -L A W E S : T o be fa ir when you m en tion D ic k H a ll's r e s u lts , th e re a re at le a s t 1 2 d iffe re n t runs in which he c o n s is te n tly finds v a r ia tio n s in the H T / R T r a tio with B r 2 con cen tration . It s e e m s to m e that th e re is no grounds fo r assu m in g that it is ju st a c a r e le s s e r r o r . F .S . R O W L A N D : N o, I don't think that it is c a r e le s s . I f you want m y explan ation , I think th e re is an e r r o r in the c o r r e c tio n b ein g m ade, and that you a r e m aking it c o n s is te n tly , lea d in g to a s y s te m a tic e r r o r . D .J. M A L C O L M E -L A W E S : T h e r e is r e a lly v e r y lit t le d iffe r e n c e in the c o r r e c t io n fr o m one o f those runs to an oth er. I f you m ust change the p e rc e n ta g e o f b ro m in e fro m 1.0 to 1.5%, the actu al change in the w a ll- H T c o r r e c t io n is m inute. F .S . R O W L A N D : T h e e x p e rim e n ta l d is c re p a n c y is not v e r y b ig e ith e r — w e 'r e ta lk in g about nu m bers that d is a g r e e by r e la t iv e ly s m a ll am ounts. In any even t, b e fo r e we a re goin g to get an yw h ere in try in g to a g re e on exp lan ation s, th e re has to be a g re e m e n t that with a p a rtic u la r exp erim en t you g e t a p a rtic u la r re s u lt. And th e re is n 't any a g reem en t now, as fa r as I can see. D .J. M A L C O L M E -L A W E S : W ould you a g r e e about the d iffe re n t e ffe c ts o f d iffe re n t m o d e ra to rs , fo r ex a m p le? T h at, fo r in stan ce, the H T / p aren t r a tio is d iffe re n t in xenon than it is in h eliu m ? F .S . R O W L A N D : In h eliu m you have the p ro b le m o f the ion con tribu tion . D .J. M A L C O L M E -L A W E S : M ake it C F4 . F .S . R O W L A N D : I would have to see what the c o r r e c tio n s a re that a re m ade in each s y s te m b e fo r e I could say. I don't think that the c o rre c tio n s a re n o n -tr iv ia l. K . R O S S LE R : I want to m ake a com m en t and put a question. Y ou m en tion ed that we a re doing th e o r e tic a l com p u ter sim u la tio n s, e s p e c ia lly in in o rg a n ic sy s te m s . T h e s e a re not in fa ct th e o r e tic a l. T h e y a re only c o m pu ter sim u la tion s, and th e re is a b solu tely no th e o ry excep t the c o llis io n p oten tia ls in it. F r o m this point of v ie w , le t m e put m y question. W hat do you think would change in y o u r ca lcu la tio n s in goin g fro m the gas phase to a condensed phase — to a liq u id , o r even the s o lid s ta te? W e tr ie d to sep arate the pure c o llis io n dyn am ics fr o m the secon d step o f c h e m ic a l recom b in a tion . M . N E W T O N : In you r s y s te m , you tre a te d it as an io n ic la ttic e , and used a s c re e n e d coulom b p oten tia l fo r b in a ry encounter coulom b s c a tte rin g , which is quite w e ll-d e fin e d . K . R O S S LE R : I keep putting th is qu estion to the o rg a n ic p eo p le. Is th e re not a high chance fo r a d ir e c t, s im p le substitution m ech a n ism ? It 's not d ir e c tly co m p a ra b le to the gas phase w h ere th ere is a lo t o f a b stra ctio n , which would lea d to a c o n tra d ictio n with our con clu sion o f 6 6 % d ire c t substitution. M . N E W T O N : What do you m ean by d ir e c t in this situ ation?
122
NEW TON
K . R O S S LE R : I f you shoot co p p er atom s into a co p p er la ttic e , o r c h lo rin e atom s into a pure c h lo rin e la ttic e , you would have not 50% as L ib b y has said, but 6 6 % as the exact fig u re in our ca lcu la tio n s. T h is in v o lv e s the c o llis io n o f one atom , re p la c in g another, and stayin g in its p ositio n until it is cap tu red by so m e c h e m ic a l bond. S in ce th is w ork s in the condensed phase w ith in o rg a n ic s o lid s , I w on der w hether this s im p le m ech an ism m ay not a lso be w ork in g with o rg a n ic s p e c ie s in condensed phases â&#x20AC;&#x201D; liq u id s and s o lid s ? M . N E W T O N : W h ile I think that the s o lid -s ta te m o d els a re quite w e lld efin ed , I don't b e lie v e that they a re n e c e s s a r ily a p p lica b le to the liqu id phase. Just the dyn am ics o f any o rd in a ry liq u id , a sid e fr o m r e a c t iv it y â&#x20AC;&#x201D; this is a w h olly d iffe re n t p ro b le m . T h ey can be tr e a te d by com p u ter sim u lation , but I don't s e e any obvious exten sion o f the s o lid - la ttic e w o rk to liq u id s. A . P . W O L F : You 'r e try in g to co m p a re peanuts and elephants. What R o s s le r is r e f e r r in g to is the c la s s ic a l b illia r d - b a ll m o d e l used in the e a r ly days o f hot atom c h e m is tr y to exp lain s o - c a lle d "r e te n tio n " as p a rtic u la r prod u cts. I don't think the kinds o f ca lcu la tion s N ew ton does have any a p p li c a b ility to b illia r d - b a ll c o llis io n s w h a ts o e v e r. The qu estion in a b illia r d b a ll c o llis io n is , "H o w m any o f the even ts w ill re s u lt in c o m p lete m om entum tr a n s fe r with e je c tio n o f the atom , with the oth er atom com in g to r e s t , and then re -b o n d in g ? T h a t's quite d iffe re n t fr o m doing a th e o r e tic a l calcu lation in som e r e la t iv e ly s im p le s y stem askin g qu estion s about the heights o f the v a rio u s p oten tia l b a r r ie r s . M . N E W T O N : R o s s le r is asking about the a p p lica tion o f dyn am ical ca lcu la tio n s to the liqu id . In a s o lid , you have a la ttic e . In a liq u id , you have a d istrib u tio n function fo r the m o le c u le s in a liqu id . But, a sid e fr o m that, you can use the sam e b asic m o d e l fo r calcu lation . K . R O S S L E R : But the f ir s t " f r e e " m ovem en t o f the m o le c u le s is h in dered so that i f you have a v e r y ra p id hot p ro c e s s , it m ight do som e of these hard c o llis io n s .
IA E A -P L-61 5/8
RĂ&#x201D;LE OF ATOMIC BEAMS IN POT ATOM CHEMISTRY* Y. LE E** J a m e s F r a n c k I n s titu te and D e p a r tm e n t o f C h e m is tr y , T h e U n iv e rs ity o f C h ic a g o , C h ic a g o , 111., U n ite d S ta te s o f A m e r ic a
Abstract
ROLE OF ATOMIC BEAMS IN HOT ATOM CHEMISTRY. Many experimental techniques exist which can provide experimental data needed to understand the dynamics of a chemical reaction. These are briefly identified with some of the limitations suggested. The present status of the investigation of the chemical reactions of energetic atoms by the crossed nuclear beam method is reviewed in detail. Anticipated future developments and fruitful areas of research are identified.
An u n derstan din g of hot atom c h e m is try , as in the u n derstan din g of m any oth er m a c r o s c o p ic phenom ena, in v o lv e s e lu cid a tin g and u n derstanding the e le m e n ta r y a to m ic and m o le c u la r p r o c e s s e s w hich c o m p ris e the m ech an ism o f the hot atom c h e m is tr y under con sid era tio n . T h e develop m en t o f the th e o ry o f hot atom c h e m is tr y is thus tie d v e r y c lo s e ly to the g e n e r a l advancem ent o f the th e o r y o f e le m e n ta r y c h e m ic a l re a c tio n s and e n e rg y tr a n s fe r p ro c e s s e s . T h e th e o r e tic a l understanding o f the dyn am ics o f c h e m ic a l re a c tio n s has m ade som e s ig n ific a n t p r o g r e s s in re c e n t y e a r s . E s p e c ia lly w ith the advent o f the h ig h -s p e e d com p u ter, it has now b eco m e p o s s ib le to c a r r y out r e lia b le ca lcu la tio n s o f p o ten tia l e n e r g y h y p e rs u rfa c e s o f sim p le sy s te m s enabling us to a cq u ire a th e o r e tic a l understanding o f the fa c to r s w hich dom inate the dyn am ics o f a c h e m ic a l re a c tio n through m o le c u la r s c a tte rin g calcu lation s. T h e e ffo r t s o f e x p e r im e n ta lis ts in obtain in g d e ta ile d d y n a m ica l in fo rm a tio n a re e s s e n tia l, not on ly fo r u n derstanding the im p o rta n t c h e m ic a l and k in e  m a tic fe a tu re s w hich dom inate the o b s e rv e d m a c r o s c o p ic phenom ena, but a ls o f o r p ro v id in g te s ts f o r the e x is tin g th e o r ie s and a p p roxim a tion s r e le v e n t to the e le m e n ta ry c h e m ic a l re a c tio n s . Such te s ts should d em on stra te which m ethods a re fr u itfu l and w hich should be abandoned and should m ake our understanding o f dynam ics o f e le m e n ta r y c h e m ic a l p r o c e s s e s c lo s e ly and f i r m l y b ased on f ir s t p rin c ip le s . T o understand the dyn am ics o f a c h e m ic a l re a c tio n fr o m e x p e rim e n ta l data it is n e c e s s a r y to obtain s u ffic ie n t d e ta ile d m ic r o s c o p ic in fo rm a tio n . Id e a lly , one w ould lik e to obtain fo r g iv e n s p e c ifie d in itia l rea cta n t conditions such as m o le c u la r quantum state, r e la t iv e v e lo c it y o r b ita l angular m om entum (o r im p a ct p a r a m e te r ), and the o rie n ta tio n o f rea cta n t m o le c u le s at the m om en t o f " c o llis io n " , the d e ta ile d in fo rm a tio n on the d istrib u tio n o f quantum sta tes o f produ ct m o le c u le s , th e ir an gu lar d istrib u tio n s, the life t im e s of any c o llis io n " in t e r m e d ia t e s ," and the exten t o f the m ix in g o f the a v a ila b le e n e r g y am ong v a rio u s d e g r e e s o f fr e e d o m o f the com p lex. * **
Work supported by the USAEC. Present address: Department of Chemistry, University of California, Berkeley, Calif., USA.
123
124
LEE
M any e x p e rim e n ta l techniqu es have been d evelo p ed in re c e n t y e a rs , w hich w ill s a tis fy m ost of the re q u ire m e n ts m en tion ed above. F o r the s p e c i fic a tio n o f the in itia l state o f a c h e m ic a l re a c tio n , one could em p lo y a c ro s s e d m o le c u la r beam a rra n g e m e n t s ta rtin g with beam s o f v e lo c it y s e le c te d m o le cu les. R ota tio n a l state s e le c tio n o f the m o le c u le s in the beam has been a ch ie v e d with inhom ogeneous e le c t r ic o r m a gn etic fie ld s fo r m o le c u le s such as th a lliu m flu o r id e [ 1 ], a lk a li h a lid es [ 2 - 4], h yd rogen h a lid es [ 5], NO. [ 6 , 7 ] and H2 [ 8 , 9]. M o le c u le s with s y m m e tr ic top con figu ra tion s such as CH3I and C F 3I [ 1 0 - 1 6 ] , have been o rie n te d w ith a h exapole e le c t r ic fie ld fo r use in c o llis io n stu dies. The re c e n t d evelop m en ts in la s e r te ch n o lo g y have now m ade it p o s s ib le to e x c ite a beam o f m o le c u le s to a g iven v ib r a tio n a l-r o ta tio n a l state [ 17 ]. A g r e a t d eal o f d e ta ile d in fo rm a tio n about the d istrib u tion of product quantum sta tes has been obtained fr o m the a n a ly s is o f ch em ilu m in escen ce s p e c tra [1 8 ], o r fr o m the e q u a l-g a in [1 9 ] o r z e r o - g a in techniqu es [ 20] o f the c h e m ic a l la s e r m ethod. M o re r e c e n tly , m o le c u la r beam reson an ce s p e c tro s c o p y [21 - 23], d e fle c tio n o r fo c u s in g b y inhom ogeneous e le c t r ic fie ld [2 4 , 25], and la s e r flu o r e s c e n c e tech n iqu es [ 26, 27] have been fr u it fu lly a p p lied in the d eterm in a tio n o f produ ct state d istrib u tion s fo r som e r e a c tio n s f o r w hich such in fo rm a tio n cannot be obtained by ch em ilu m in escen ce o r c h e m ic a l la s e r m ethods. In c r o s s e d m o le c u la r b ea m e x p e rim e n ts , the m ea su rem en t o f product angular d istrib u tio n s often p ro v id e s in fo rm a tio n on the p r e fe r r e d o rie n ta tio n of the m o le c u le during the re a c tio n , the life tim e of c o llis io n c o m p le x e s , the co n fo rm a tio n o f the co m p le x and the d isp o sa l of an gu lar m om entum . T h e m ea su rem en t o f v e lo c it y d istrib u tion s o f p r o duct m o le c u le s u su a lly o n ly r e v e a ls the p a rtitio n in g o f the to ta l e n e rg y b etw een in te rn a l d e g r e e s o f fr e e d o m and tra n s la tio n a l e n e rg y , but in som e fa v o u ra b le c a s e s product v ib ra tio n a l states have been id e n tifie d d ir e c t ly w ith the aid of the la w s o f co n s e rv a tio n o f to ta l e n e r g y and angular m om entum [2 8 , 29]. A lthou gh th ese advanced techniques have been used on m any o cca sio n s in the past to obtain valu ab le d yn a m ica l in fo rm a tio n , w e have not y e t rea ch ed the stage w h ere we can put a ll the m ethods to g e th e r and c a r r y out an e x p e r i m ent w ith v e lo c it y and quantum state s e le c te d rea cta n t m o le c u le s and r e v e a l a ll the d e ta ile d in fo rm a tio n on quantum sta tes and an gu lar d istrib u tion s of the product m o le c u le s . A t p resen t th e re a re tw o ap p roach es to obtaining in fo rm a tio n on product m o le c u le s . T h e f i r s t m ethod depends upon the m ea su rem en ts of an gu lar d istrib u tion s and v e lo c it y d istrib u tion s o f product m o le c u le s [ 30]. T h e oth er approach r e lie s on s p e c tr o s c o p ic m ethods such as in fr a - r e d ch em ilu m in escen ce [ 18], the c h e m ic a l la s e r technique [ 19, 20], the la s e r flu o r e s c e n c e m ethod [ 26, 27], o r m o le c u la r b eam reson an ce s p e c tro s c o p y [ 2 1 - 2 3 ] . In th ese la tte r m ethods, although s in g le c o llis io n conditions a re often a ch ieved and a c r o s s e d b eam a rra n g e m e n t is a lso often used, in g e n e r a l the an gu lar d istrib u tio n s o f product m o le c u le s a re not m e a s u re d and the in itia l r e la t iv e v e lo c it ie s a re not v e r y w e ll defined. In what fo llo w s , the p re s e n t status o f the in v e s tig a tio n o f the c h e m ic a l re a c tio n s o f e n e r g e tic atom s b y the c r o s s e d m o le c u la r b ea m m ethod a re r e v ie w e d . T h e p r o g r e s s o f the m o le c u la r b eam m ethod in g e n e r a l has been r e v ie w e d quite fre q u e n tly in re c e n t y e a r s [ 30 - 33]. In a c r o s s e d m o le c u la r b eam e x p e rim e n t two w e ll- c o llim a t e d b ea m s of atom s and m o le c u le s a re c r o s s e d in a vacuum ch am b er and angular and v e lo c it y d istrib u tio n s o f id e n tifie d product m o le c u le s a re m ea su red in d etail. T h e advan tages o f d ir e c t o b s e rv a tio n o f the con sequ en ces o f sin g le c o llis io n s
IA E A -P L-61 5/8
125
in the u n derstanding o f dynam ics o f c h e m ic a l re a c tio n s a re obviou s. H o w e v e r, the c r o s s e d m o le c u la r beam e x p e rim e n t is ra th e r in v o lv e d , it r e q u ir e s a high le v e l of te c h n o lo g ic a l sop h istica tio n , and sin ce m ost of the equipm ent has to be fa b r ic a te d fo r s p e c ia l p u rp oses, it a lso r e q u ir e s the stro n g support o f an e x c e lle n t p r e c is io n w orkshop. In a ty p ic a l e x p e rim e n t with adequate pumping equ ipm ent, one can a ch ieve an e x p e rim e n ta l con dition such that, in the c o llis io n r e g io n of a s c a tte rin g ch am b er m ain tain ed at 1 0 ~ 7 t o r r , the num ber d en sity o f v e lo c it y s e le c te d a to m ic and m o le c u la r beam s is 3 X 1010 m o le c u le s / c m 3 and 3 X 1012 m o le cu les/cm 3, r e s p e c tiv e ly . W ith a r e la t iv e v e lo c it y o f 10 5 cm /s and a c r o s s se c tio n o f 1 A 2, the num ber o f product m o le c u le s prod u ced in a c o llis io n vo lu m e o f 0.01 cm 3 is a p p ro x im e ta ly nin2vcrV = 1010 m o le c u le s / s . A t a fa v o u ra b le s c a tte r in g an gle, one could ex p ect about l/lOOO o f th ese m o le c u le s to e n te r the d e te c to r w ith I o an gu lar reso lu tio n . H en ce, one needs a m o le c u la r beam d e te c to r w hich is capable o f d e tectin g at le a s t 1 0 7 m olecu les/s e n te rin g the a p ertu re o f the d ete c to r. The e ffic ie n t y o f the ion so u rce of the m ass s p e c tr o m e te r is ty p ic a lly 0 . 1 % and i f the tra n s m is s io n o f ion s through the m ass s p e c tr o m e te r is 1 0 %, 1 0 7 m o le c u le s / s a r r iv in g at the d e te c to r w ill y ie ld 10 3 ion s/s. T h is is a v e r y high ion le v e l fo r m od ern p a r tic le cou n ters, if the background ion counts prod u ced in the io n iz e r o f the d e te c to r is n e g lig ib le . But in fa c t, 107 m o le c u le s / s a r r iv in g at a d e te c to r with an opening o f 0 .1 cm 2 m ovin g at 1 0 ° cm /s w ill o n ly g iv e a s te a d y -s ta te num ber d en sity o f 10 3 m o le c u le s / c m 3 c o rre s p o n d in g to a p a r tia l p re s s u re o f 3 X 1014 t o r r . It is obviou s that the d etection of product m o le c u le s at th is le v e l is not p o s s ib le u n less the p a r tia l p re s s u re o f the background gas w hich in t e r f e r e s w ith m a ss s p e c tr o m e tr ic d etection o f produ ct m o le c u le is at a s im ila r o r lo w e r le v e l. T h is condition can be a ch ie v e d b y e x te n s iv e d iffe r e n tia l pum ping o f the d ete c to r. It is v e r y com m on to use th re e sta g es o f d iffe r e n tia l pum ping in v o lv in g a com bin ation o f a su blim ation pump, ion pumps, and a liq u id h eliu m c ry o g e n ic pump to red u ce the to ta l p re s s u re fr o m 1 0 7 t o r r in the m ain ch am b er t o ~ 1 0 'n t o r r in the io n iza tio n ch am b er [ 34]. P ro d u c tio n o f a m o le c u la r b ea m a lso r e q u ir e s a la r g e pum ping capacity. T h r e e sta g es o f the d iffe r e n tia l pum ping is often used fo r p rodu cin g a s u p erso n ic beam . T y p ic a lly , the p re s s u re behind the n o z z le is ~ 1 atm , the gas is expanded through a 0 . 1 -m m -d ia m . n o z z le in to a ch am b er m aintained at a p re s s u re o f 3 X 10 ”4 t o r r by a 10-in d iffu sion pump w ith a pumping speed o f 4500 litr e s / s . W ith a 4 -in d iffu sion pump the p re s s u re in the bu tter ch am b er is red u ced to 5 X 10 "6 t o r r and the p re s s u re in the c o llis io n ch am b er is m aintained at 10 " 7 t o r r b y a 10-in d iffu sion pump. A ty p ic a l e x p e rim e n ta l a rra n g em en t w ith tw o fix e d beam s and a ro ta ta b le d e te c to r is shown in F i g . l . T h e produ ction o f high in te n s ity m o n o e n e rg e tic high e n e r g y a to m ic o r m o le c u la r beam s is n e c e s s a r y fo r the study o f hot atom c h e m ic a l re a c tio n s b y the c r o s s e d m o le c u la r beam m ethod. T h e r e a re th re e m ethods which a re co m m o n ly used in the produ ction o f high e n e r g y n eu tra l a to m ic beam s: reso n a n ce c h a rg e exchange o f e n e r g e tic ion s; the sp u tterin g o f n eu tral p a r tic le s fr o m a s o lid s u rfa c e b y a high e n e r g y ion beam , and high p re s s u re h y p e rs o n ic expan sion o f a gas m ix tu re through a s m a ll n o z z le at high te m p e ra tu re . T h e in te n s ity o f an a to m ic b eam p rodu ced fr o m a sp u tterin g b ea m s o u rce on a ch a rge exchange beam s o u rce is about one thousand to one m illio n tim e s le s s than that prod u ced b y the expansion of a gas m ix tu re containing a s m a ll amount o f h ea vy atom s o r m o le c u le s in a ligh t c a r r ie r gas at high te m p e ra tu re . T h is "s e e d e d " beam s o u rce w ill g iv e a num ber
LEE
126
F I G . l.
D ia g ra m o f m o le c u la r b e a m apparatus.
density of high energy atom ic species of approxim ately 3 X IO 10 a to m s/c m 3 in the c o llis io n region and is the only beam source which gives a sufficiently intense beam of atom s to make crossed m o le cu lar beam studies of hot atom c h em ical reactions possible at this tim e. In a supersonic beam source, the p ressure of the gas is m aintained at near 1 atm behind a nozzle of 0.1 m m in diam eter and the gas is expanded into a vacuum cham ber at a flow rate of about 2 cm 3/s . The p ressure of the vacuum cham ber can be m ain tained at 4 X 1 0 ' 4 to r r with a 1 0 -in diffusion pump. The tem p erature of a m onoatom ic gas w ill drop fro m 3 0 0 to ~ 5 K through ise n tro p ic expansion and esse ntially a ll the enthalpy of the gas, ( 5 / 2 )kT, w ill be converted to the kinetic energy of the gas m oving along the s tre a m line with v e ry s m a ll velocity spread. If the gas is composed of diatom ic m o le cules, the ro ta tio n a l degrees of freedom w ill also relax except fo r hydrogen m olecules and the velocity of the beam w ill reach ( 7 / 2 )kT. If a beam is produced fro m a m ixture of 9 9 % H2 and 1 % Xe, the Xe w ill have the sam e velo city as H ^ if the p ressure behind the nozzle is sufficiently high such that the expansion through the nozzle is m a in ly a hydrodynam ic flow. The average m ass of the gas m ixture, M, is 3 . 3 2 (= 0 . 0 1 MXe+ 0 . 9 9 M H^ and the velo city of the beam v is c h aracterize d by the average kinetic energy of ( 5 / 2 )kT = ( l / 2 )Mv2, since the ro ta tio n a l re lax ation of H2 is ra th e r in e ffi cient and can be considered as m onoatom ic gas. However, the kinetic energy of Xe is ( l / 2 )MXev 2 = (M Xe/ M ) ( l / 2 )M v2 = (MXe/M )( 5 / 2 )kT. The kinetic energy contains a gain fa c to r of MXe/M over the energy obtained by expanding pure Xe gas. F o r a gas m ixture containing 1 % of Xe and 9 9 % of H2, the gain factor is 4 0 , and at 3 0 0 0 K, the kinetic energy of Xe is 2 6 eV, fa r beyond
IAEA-PL-615/8
Scattering Chomber
127
Nozzle Chomber 5 x l0 '4 torr 4 5 0 0 litre /s
3 *1 0 6 torr 5 4 0 0 litre /s
Mirror-^ , .. . \¿?to optica I / i pyrometer Retarding Energy Analyser г
Electron to Electron 5
Multiplier
щ ш //М
'
-
I
Repeller Retarding Piote
Thermocouple Water Cooled Water Cooled Copper Copper Radiation Electric Coniaci Shield
Collimation Chomber 3 X I 0 ' 5 torr 1200 litre /s M
I cm F IG .2
D ia g ra m o f tungsten o v e n in je c t io n unit.
Retarding F IG .3 .
Potentiol (Volts)
Is e n tro p ic lim it s versus pressure fo r 1 °}о Xe-99<7o H 2 m ix tu re .
the average bond energy of 5 eV. A tungsten oven that can be re is tiv e ly heated to 3 0 0 0 K is shown in F ig . 2 . F ig u re 3 shows the p ressure re qu ired to achieve the ise n tro p ic lim it at tem p e ratures of 2 7 8 and 6 1 5 K fo r a l% X e + 9 9 % H2 m ixture. It is possible to produce high energy F , Cl, B r and I atom ic beam s by seeding F2, C l2, Br2 and I 2 in He at a high tem perature. If the p a rtia l
LEE
128
T A B L E I. THE U P P E R L IM IT O F KIN ET IC E N E R G Y O F BEAM S O F ATOMS O R M O L E C U L E S P R O D U C E D B Y "S E E D E D " SUPERSONIC EXPANSION A to m or
C a rrie r
O ven
O ven
K in e t ic
m o le c u le
gas (99.5<ft)
m a te r ia l
te m p e ra tu re
e n e r g y (k c a l/ m o le )
(° c )
850
26.2
G ra p h ite
1500
74.6
He
Tu n gsten
2500
2 53.2
I
H
Tu n gsten
2500
381.6
Ar
H2
Tu n gsten
2500
138.9
Kr
H2
Tu n gsten
2500
464.1
2500
690.6
F
He
N ic k e l
Cl
He
Br
Xe
H2
Tu n gsten
C H 3Br
H2
Stain less s te e l
300
110.4
C H 3I
H2
Stain less s teel
300
150.7
H2
Stain less s teel
300
93.9
H2
Stainless s te e l
300
107.6
B en zen e T o lu e n e
p ressure of halogen m olecules is kept low, e sse ntially a ll the m olecules w ill dissociate and re m a in as atom s through the ise ntrop ic expansion. The upper lim it of the kinetic energy of a "seeded" beam is lim ite d by the m ass of atom s which determ ine the gain factor and the m a x im u m tem perature at which the oven can be operated fo r a prolonged period. The upper lim it of the kinetic energies of v ario u s atom s and m olecules are estim ated and liste d in Table I. The oven m a te ria l and oven tem peratures are also listed. The gas m ix tu res are assum ed to contain 0 . 5 % of heavier gas in 9 5 % of ligh t c a r r ie r gas. N um erous crossed m o le cu lar beam experim ents have been p erform ed in the h yp e rth e rm al energy range by using "seeded" m o le cu lar beam source. By seeding CH3B r or CH3I in H2 m olecule, the kinetic energy dependence of an exoergic reaction К + CH 3I -»• KI + CH3 [ 3 5 ] and an endoergic reaction I + CH3B r -*■IB r + C H 3 [ 3 6 ] has been investigated recently. By seeding F atom s in He, reactions of F + C2H4-> C2H3F + H and F + CH3I ->• IF + CH3 have been studied at energies up to 0 . 5 eV [ 3 7 ]. In the reaction of F + C 2H 4, although the re action interm ediate C2H4F lives longer than its ro tational period, the ex perim ental re su lts indicate that the available energy is not com pletely rando m ize d before the decom position of the complex. M olecule-m olecule reactions in the h y p erth erm al energy range have also been investigated fo r F2 + C2H4->C2H4F + F and CH3I + F2 -*■CH3IF + Fg [ 3 7 ]. The re action of CH3I + F2 -► CH3IF + F is p a rtic u la r ly worth noting. F ro m the threshold energy fo r the appearance of CH3IF , the dissociation energy was found to be at least 2 0 k c a l/m o le for CH3IF -*■CH3I + F. This is just one of the exam ples that shows that the power of the crossed m o le c u la r beam technique is not just lim ite d to the investigation of the
IAEA-PL-615/8
129
/
F IG .4 .
A n g u la r and v e l o c i t y d istribu tion s fo r s c a tte re d ions fro m Rbl.
dynam ics of a well-known chem ical reaction. The a b ility of observing the consequences of single c o llision s gives a great potential fo r the exploration of new chem ical reactions and new c h em ical compounds. The fo rm a tio n and the p rop e rties of the transien t species produced in high energy c o llision s of m olecules could be investigated fr u itfu lly by the crossed m o le cu lar beam method. In addition to the atom tra n sfe r reactions, beam s of Xe and K r seeded in H2 have been used in studies of c o llis io n induced dissociatio n of a lk a li halides [ 3 8 ] and th a liu m halides [ 3 9 ]. F ig ure 4 shows the angular and velo city d istribution s of Rb+ and I ' produced in Xe + R b l dissociative c o llisio n [ 3 8 ]. The near c o llin e a r c o llisio n was found to be the m ost efficient con fig u ra tio n and the c o llis io n of Xe with I has higher dissociatio n p ro b ab ility than with Rb. F o r m any charge tra n sfe r processes such as К + B r2 -» K ++ B r2’ [ 4 0 ] and К + B r -» K++ B r" [ 4 1 ] at high co llisio n energy, a beam of a lk a li atom s produced by the charge exchange m ethod or fro m a sputtering beam source is often quite adequate to c a rry out p recise m easurem ents of angular d istribution s of ions produced, since the cross-sections are quite large and the detection of an io nic atom is m uch e a sie r than a ne u tra l atom.
LEE
130
There have been some attem pts at using seeded radioisotopes of brom ine and iodine [ 4 2 ] in carry in g out m o le cu lar beam experim ents. The high se n sitivity of detecting and identifying com plicated product m olecules by ra d io gas chrom atography can be a great advantage in an experim ent in which a beam of seeded radioisotope is injected into the gas cham ber fo r the investigation of the controlled hot atom reactions. On the other hand, if both angular and velocity distribution s were to be m easured in the study of the dynam ics of a chem ical reactions, the advantage of using a ra d io isotope is not v e ry clear. The m o le c u la r be a m m ethod is a ra p id ly developing system . Continuing the development of beam sources and detection methods w ill no doubt make this method m o re u sefu l and m ore im po rtan t. The recent development of la s e r fluorescence methods fo r the detection of scattered diatom ic m olecules [ 4 3 ] and the selective excitation of m olecules in the beam by a las e r are som e of the exam ples of new developments. F o r the next few years, the hot atom c h em ical reactions w hich w ill be investigated extensively are the reactions of halogen atoms and halogen m olecules. At the present stage of development of the m o le cu lar beam method, these re actions can be studied in great detail.
REFERENCES [1 ]
B E N N E W IT Z ,
H .G .,
K R A M E R , K . H . , P A U L , W ., T O E N N IS , J .P ., Z . Phys. 177
[2 ]
B E N N E W IT Z ,
H .G .,
H A E R T E N , R ., M U LLE R , G .. Z . Phys. 226 (1 9 6 9 ) 136.
[3 ]
B E N N E W IT Z ,
H .G .,
G E N G E N B A C H , R ., H A E R T E N , R ., M Ü LLE R , G ., ib id . , p.
[4 ]
B E N N E W IT Z ,
H .G ..
H A E R T E N . R ., Z . Phys. 227 (1 9 6 9 ) 399.
(1 9 6 4 ) 84.
279.
[5 ]
K A IS E R , E .W ., J. C h e m . Phys. 53 (1 9 7 0 ) 1686.
[6 ]
S T O L T E , S ., REUSS, J ., S C H W A R T Z , H .L ., A b str. V I I IC P E A C , N o r th - H o lla n d , A m s te rd a m (1 9 7 1 ) 658.
[7 ]
S T O L T E , S ., R E U S S .J ., S C H W A R T Z , H . L . , P h ysica 57 (1 9 7 2 ) 254.
[8 ]
M O ER K E R K E N , H . , PRIO R, М ., REUSS, J ., P h ysica 50 (1 9 7 0 ) 49 9 .
[9 ]
M O ER K E R K E N . H . , REUSS. J ., A b str. V I I IC P E A C , 66 3 , N o r th -H o lla n d , A m s te rd a m (1 9 7 1 ) 663.
[1 0 ]
BEUHLER, R .J., Jr., B E R N S T E IN , R .B ., K R A M E R , K . H . , J. A m . C h e m . S o c . 88 (1 9 6 6 )
[1 1 ]
BEUHLER, R .J., Jr., B E R N STE IN , R .B ., C h e m . Phys. L ett. 2 (1 9 6 8 ) 166.
5331.
.
[1 2 ]
BEUHLER, R .J., Jr., B E R N STE IN , R .B ., J. C h e m . Phys. 5Л_(1 9 6 9 ) 5305.
[1 3 ]
B RO O KS, P .R ., JO NES, E .M ..J . C h e m . Phys. 45 (1 9 6 6 ) 3449.
[1 4 ]
B RO O KS. P .R ., J. C h e m . Phys. 50 (1 9 6 9 ) 5031.
[1 5 ]
B RO O KS, P .R ., Discuss. Farad ay S o c . 55 (1 9 7 3 ). In press.
[1 6 ]
M A R C E L IN . G ., B RO O KS, P .R ., J. A m T c h e m . S o c . 95 (1 9 7 3 ) 7885.
[1 7 ]
M O O R E , C .B ., Z I T T E L , P .F ., S c ie n c e 182 (1 9 7 3 ) 541;
[1 8 ]
C A R R IN G T O N . T . , P O L A N Y I, J .C .. In t. R ev. o f S c ie n c e : C h e m ic a l K in e t ic s , O x fo rd : M T P . (1 9 7 2 ) ch . 5.
M O O R E , C .B ., A n n . R e v . Phys. C h e m . 22 (1 9 7 1 ) 387.
[1 9 ]
PARKER, J .H ., P IM E N T E L , G . C . . J . C h e m . Phys. 51 (1 9 6 9 ) 91.
[2 0 ]
M O L IN A , M .J ., P IM E N T E L , G . C ., IEEE J. Q uant. E lectron 9^ (1 9 7 3 ) 64.
[2 1 ]
FR E U N D , S .M ., F IS K , G . A . , H E R S C H B A C H , D .R ., KLEM PERER, W .. J. C h e m . Phys. 54 (1 9 7 1 ) 2510.
[2 2 ]
M A R IE L L A , R .P ., J r., H E R S C H B A C H , D .R .. KLEM PERER, W . , J. C h e m . Phys. 58 (1 9 7 3 ) 3785.
[2 3 ]
B E N N E W IT Z , H .G ., H A E R TE R R ., M U LLE R , G ., C h e m . Phys. L e tt. 12 (1 9 7 1 ) 335.
[2 4 ]
G R IC E , R ., M O S C H , J.E ., S A F R O N , S . A . , T O E N N IE S , J .P ., J. C h e m . Phys. 53 (1 9 7 0 ) 3376.
[2 5 ]
M A L T Z . C . , W E IN S T E IN , N . D . , H E R S C H B A C H , D .R ., M o l. Phys. 2 4 (1 9 7 2 ) 133.
[2 6 ]
S C H U L T Z , A . , CRUSE, H .W ., Z A R E . R . N . . J . C h e m . Phys. 57 (1 9 7 2 ) 1354.
[2 7 ]
CRUSE, H .W ., D A G D IG IA N , P .J., Z A R E , R .N ., Discuss. F arad ay S o c . 55 (1 9 7 3 ). In press.
[2 8 ]
S C H A F E R , T . P . , S IS K A , P .E ., P A R S O N , J .M ., T U L L Y , F .P ., W O N G , Y . C . , LEE, Y . T . , J. C h e m . Phys. 53 (1 9 7 0 ) 3385.
[2 9 ]
S C H A F E R , T . P . , Ph. D. T h e s is , T h e U n iv e rs ity o f C h ic a g o , C h ic a g o (1 9 7 2 ).
[3 0 ]
K IN S E Y , J .L ., S e e R e f. [ 1 8 ] C h ap. 6.
1АЕА-РЬ615/8
[ 31 ]
131
Т О EN N4 S , J. P . , P h y sica l C h e m is try , A n A d v a n c e d T r e a t is e , K in e tic s o f Gas R ea ctio n s 4 , A c a d e m ic Press N e w Y o r k (1 9 7 3 ) C h.3.
[3 2 ]
M o le c u la r B eam S c a tte r in g , Discuss. Farad ay S o c . 55 (1 9 7 3 ). In press.
[3 3 ]
B E R N S TE IN , R .B ., L E V IN E , R .D ., M o le c u la r B e a m , O x fo rd U n iv e rs ity Press, in press.
[3 4 ]
LEE, Y . T . , M C D O N A L D , J .D ., L eB R E T O N , P .R ., H E R S C H B A C H , D .R ., R ev. S c i. Instr. 40 (1 9 6 9 ) 1402;
[3 5 ]
G ERSCH , М . E ., B E R N STE IN , R .B ., J. C h e m . Phys. 56 (1 9 7 2 ) 6131;
P A R S O N , J .M ., LEE, Y . T . , J. C h e m . Phys. 56 (1 9 7 2 ) 4658.
R U L IS , A . M . , B E R N STE IN , R.B ., Discuss. F araday S o c . 55 (1 9 7 3 ). [ 36]
W O N G , Y . C . , LEE, Y . T . U npu blished w ork.
[3 7 ]
FAR R AR , J .M ., Ph. D. T h e s is , T h e U n iv e rs ity o f C h ic a g o , C h ic a g o (1 9 7 4 ); FA R R A R , J .M ., LEE, Y . T . , t o b e pu b lish ed in J. C h e m . Phys. (1 9 7 4 ).
[3 8 ]
T U L L Y , F .P ., Ph. D. T h e s is , T h e U n iv e rs ity o f C h ic a g o , C h ic a g o (1 9 7 3 ); T U L L Y , F . P . , C H E U N G . N .H ., LEE, Y . T . . to b e p u b lish ed in ] . C h e m . Phys.
[3 9 ] [4 0 ]
P A R K S , E .K ., H A N S E N , N .J ., W EXLER, S ., J. C h e m . Phys. 58 (1 9 7 3 ) 5489. BAEDE, A . P . M . , A U E R B A C H , D .J ., LO S, J., P h ysica 64 (1 9 7 3 )1 3 4 ; A U E R B A C H , D .J ., HUBERS, M . M . , BAEDE, A . P . M . , L O S , J ., C h e m . Phys. 2 (1 9 7 3 ) 107.
[4 1 ]
DEL V IG N E , G . A .L ., L O S , J ., Ph ysica 67 (1 9 7 3 ) 166.
[4 2 ]
W orks in progress in K F A J iilic h F e d e ra l R ep u b lic o f G erm a n y .
[4 3 ]
S C H U L T Z . A . , C R U S E ,H .W ., Z A R E , R .N ., J. C h e m . Phys. 57 (1 9 7 2 ) 1354; CRUSE, H .W ., D A G D IG IA N , P .J., Z A R E , R .N ., Discuss. F araday S o c . 55 (1 9 7 3 ). In press.
DISCUSSION Y. L E E : M olecu lar beam experim ents are quite pow erful in the sense that you can le a rn the detailed dynam ics of these reactions. Today, I only showed some of the re sults of the reactive scattering experim ents, but one can also study energy tra n sfe r processes — fo r exam ple, the tra n s fe r of energy into bending v ib ra tio n s, etc. The key point is that we try to connect the dynam ics to the fir s t p rin cip le s in such a way that one day w e 'll be able to understand what is going on fro m fir s t p rin c ip le s alone. G. STOCKLIN: E very tim e I hear Lee talk I alm ost feel like giving up because the gap is so great between the in fo rm a tio n he is able to obtain fro m his experim ents and what we can obtain with ours. But, let me pour a little water into the wine. I have gathered together on this slide (see Table A) the various m o le cu lar beam studies so fa r which re fer to substitution reactions. To date no endo th e rm ie substitution re action has been studied by crossed beam experim ents. Lee has described sev eral exotherm ic substitution reactions which have fa ir ly high cross-sections, and m any of which occur in pi-electron system s. Then, there have been studies of the m ethyl brom ide system with iodine atom s, an endotherm ie system in which Lee observed the abstraction product IB r , but did not observe the substitution reaction. We have also done some other crossed beam experim ents at J ü lic h , again without observing any substitution reactions. In each case, we were able to observe only upper lim its fo r the cross-section. In some cases we had a dark re actio nogoing on, while in others we were able to establish an upper lim it of 0 . 0 2 A. F in a lly , we have the W exler experim ent which started with T2. This whole experim ent is highly questionable because the cross-section was m uch too large. He probably had m u ltip le c o llis io n processes, m aking the in te r pretation of the re su lts very difficult. And then we come to the experim ents with solid targets, the MenzingerW olfgang and A dloff-P aulus ex perim ents, w hich also have th e ir shortcom ings. So fa r, is it not correct that there are no crossed beam studies available
132
LEE
Н
Е
Е
M«
DÛjy
00 g
U
o
X
3?
u
Q U
V +
+
t
t Ü
Q
G
X u
+ X u
u
t
o
Q
X
X
X
(J?
u
u*
o
ШНЭЯ P9SS0J3
133
IAEA-PL-615/8
cd £
Й O £
’X
c E £ 2 M J=
с
° л U -£
o 5
LT ° <л 5 D c « . 00 c oo « c «J _ a Щ 2 XI Ü “ C DO c (U C uT. 4) 2 o ra TD
<
л
со >>
A. (cont.).
Н Н ПС Ж ПС
X
и
t? U
и
и
t t Î X
TABLE
о
s ia S ie j p i p s
хГ
t t
134
LEE
of an endoergic hot substitution reaction? I am quite sure fro m the experience in our lab o ra to ry that we have a long way to go before we can study the kinds of hot substitution processes that Rowland was discussing e a rlie r. Y. L E E : I think we need to distinguish "have not trie d " fro m "have not been able to observe". F ro m the the oretical point of view, if you look at the reaction of b rom in e atom with fluorobenzene, this substitution reaction is going to be very, v e ry slow, fo r the reason that it is endoergic. When you fo rm the complex, you w ill see that energy rando m ization, while it m ight not be perfect, w ill occur. You are going to see very different volum es of phase space available fo r the two reaction channels, and the brom ine- loss channel is going too dom inate. I w ouldn't be su rp rise d if this cross-section is s m a lle r than 1 A2, and aligo that in the corresponding saturated case it m ight be sm a lle r than 1 A2, as w ell, so that we m ight not be able to see it. P robably if I were to do the B r plus CH3I experim ent, I should be able to see that it is an exotherm ic substitution reaction. C le a rly we can see the flu orine m olecule plus ethylene re action fo rm in g the fluoroethyl ra d ic a l (F 2 + C244) in the crossed beam experim ent. We can also do H B r with iodine in an endotherm ie re action fo rm in g IB r. But you are rig h t, we haven't trie d any endoergic substitution reactions yet. F.S. ROW LAN D: W hat is the cross-section fo r the reaction of fluorine atom s with m ethyl iodide? Y. LEE: About 5 to 1 0 Â 2. F.S. ROW LAN D: The reaction of flu orine atom s with m ethyl iodide is one which we.have looked at with th e rm a l 18F atom s, and which Yuan Lee has looked at in the crossed beam s. He sees the abstraction reaction with a high cross-section. What we see is the fo rm a tio n of CH 318F as one of the reactions o c c u rrin g in this th e rm a l system , with a y ie ld of 1 %. It is a sm a ll yield, but it is definitely there in the th e rm a l system s, and has a s till higher yield in the hot system s. O u r experim ent involves the m easurem ent of the yield of CH318F versus mole frac tio n of CH3I in m ixtures with gaseous SF6. If the observed CH318F yield were a ll the re sult of hot substitution, then the yield would extrapolate to zero y ie ld at zero m ole frac tio n of CH3I, but it a ctu ally extrapolates to 1 %. This is a substitution reaction which follow s that pathway in s m a ll yield in the th e rm a l system in com petition with the high yield th e rm a l a bstraction reaction. If you want a substitution reaction with a high yield in the crossed beam apparatus, then the re action to try is flu orine atom plus (CH3) 4Pb. In our experim ents with 18F , we find a y ie ld of about 2 5 % fo r CH31SF versus the com peting re action fo r the fo rm a tio n of H 18F which presum ably accounts fo r the other 7 5 %. A .P . W O LF: In a nozzle beam at 6 0 0 °C, you are going to have Pb a ll over the place v ery rapidly. Y. L E E : You don't need to heat it to 6 0 0 °C. You can seed it at room tem p erature in m o le c u la r hydrogen. G. STOCKLIN: A re you sure that this re s id u a l 1 % y ie ld is not a th e rm a l w a ll reaction? Some kind of th e rm a l w all, dark reaction, and not re a lly a th e rm a l substitution re action in the gas phase? There is always a p roblem w ith these halides. This is fluorine-for-iodine and you m ay have some kind of other reaction. F.S. ROW LA N D: C e rta in ly this is an exotherm ic reaction. However, the yields are h ig h ly reproducible as a function of the m ole fra c tio n of m ethyl iodide in the experim ent, and that doesn't look to me the way you
IAEA-PL-615/8
135
would expect fro m a w all reaction. The behaviour with the p a ra m e te r that we are v arying is quite consistent. Y. L E E : It is true that you won't be able to see the ra re event in the beam experim ent if the p ro b ab ility of reaction is v ery sm a ll. The beam experim ent is not very sensitive. On the other hand, the beam experim ent can give you v ery detailed in fo rm a tio n on the reactions you do see. We are doing experim ents that involve orienting the m olecule, and we can study the coupling between bond breakage and bond fo rm a tio n and try to understand the dynam ics as a function of different o rb ita l angular m om enta. Now I think it is also possible to produce the m olecules in a given ro tational state. W e've done some experim ents with H C l in the J = 1 state and com pared them with the n o rm a l experim ent with J = 0 . You can even do M quantum dependence, w ith H C l (J =l, M=l) fo r exam ple, and spin the m olecule perp en dicular to the re la tiv e velo city and m easure the c h em ical effect. One of m y students is also try in g to use c h em ical la s e r excitation, m aking a C W -chem ical las e r pum ping the H F m olecule in the beam . We can then pum p the m olecule into a given v ib ra tio n a l, ro ta tio n a l state and study energy tra n s fe r in chem ical reactions. I think that there is not a difference in philosophy. We have only one m achine, and what I am stre ssing now is try in g to connect the dynam ics with fir s t p rin cip le s . On the other hand, if we tooled up tom orrow to study hot atom c h e m istry shooting in brom ine- or iodine-seeded beam s, we can produce tons and tons of data. You can do one chem ical re action each day, and in 3 6 5 days you could have 3 6 5 system s. But who's going to analyse it? D .M . RICHM AN: Is it w orth doing one o r two reactions in the hot atom field? Y. L E E : Yes, w e're re a lly com ing back to some of the ch em ical reactions. F o r instance, m any com bustion processes involving free ra d ic a ls can be iso lated in the crossed beam apparatus, one by one. J .P . A D L O F F : I have a v ery p ra c tic a l question. W hat is the cost of such a m achine? Y. L E E : The cost of the m achine depends on whether you make a ll of the m e ch a nic a l draw ings and do lots of the soldering y ourself. Then it costs less than $ 1 0 0 0 0 0 to get the m achine operating. I spent about a year. G. STÔCKLIN: D id you count the sa la rie s of the people involved? Y. L E E : Yes, but I didn't have an engineer or any technicians. I made 3 0 0 draw ings m yself. G. STOCKLIN: I think $ 1 0 0 0 0 0 is on the low side. J .P . A D L O F F : How m any m achines do you have? Y. L E E : One "u n iv e rs a l" m achine only. F.S. ROW LAND: I have another question. As you m entioned, the re action of 18F w ith ethylene as a scavenger was c a rrie d out quite some tim e ago. We have now done m ore experim ents on this system , and the life tim e fo r the fluoroethyl-18F ra d ic a l, as determ ined by c o llisio n with SF6, or NF3, or with CF4 , is v e ry m uch longer than 1 0 "lz s. It is m ore like 1 0 "9 s. Does it make sense to you that energy fa ils to rando m ize in a ra d ic a l which lives as long as 1 0 "9 s? If the life tim e were 1 0 ”12s, then one can ea sily see why it m ight not be random ized. But if the life tim e is 1 0 ' 9 s, it seem s strange that the energy is n 't better rando m ize d in that tim e period than your c a l culations have indicated. Y. LE E: Here I'm talk ing about an iso lated m olecule prepared in a vacuum , w hich w ill keep the in itia l phase re la tio n of the in itia l excitation.
136
LEE
You are talk ing about an experim ent done in a homogeneous phase in which subsequent c o llisio n s can random ize the phase, and also m ight tra nsfe r some of the energy of the m olecule. I think the phase rando m ization might play a v ery im po rtan t role in the rando m ization of energy — that is , soft co llisio n s can help in in tra m o le c u la r energy transfe r. F.S. ROW LAN D: But in our system there are n 't any collision s. If you drop the p re s su re s down to p re ssure s of the order of 200 to r r , there is a long tim e before there are any co llisio n s, let alone soft or hard collisions. It s till seems to me inconsistent why this ra d ic a l lasts so long that its decay is com petitive with c o llision s at this pressure and s till fa ils to rando m ize the energy. Y. L E E : In the system F plus ethylene, we obtained a ll of our energy ra nd o m iza tio n in fo rm a tio n fro m the energy dependence — how the ra d ic a l dumped energy. In this experim ent, we didn't crank up the energy of the ra d ic a l u n til its life tim e was only one ro ta tio n a l period. If the life tim e were that short, then we can get direct in fo rm a tio n about the life tim e of the ra d ic a l in te rm s of the ro ta tio n a l period. F.S. ROW LAN D: You can m easure life tim e s at around 1 0 "12 s, while our upper lim it on life tim e m easurem ents is about 1 0 ' 10 s. But with the fluoro ethyl ra d ic a ls , 10"10s is m uch too fast — on that tim e scale we're a lready sta b iliz in g everything. The fluoroethyl ra d ic a ls sim p ly don't decompose in that tim e period, and that seems to me a very long tim e for the fluoro ethyl ra d ic a l not to random ize its energy. In your paper you indicated that the reaction was about 1 5 k ca l/m o le exotherm ic. Y. L E E : About 1 2 k ca l/m o le . The 1 0 "12s tim e just came out of m y head; it w asn't calculated. But 1 0 "10 or 1 0 ' 9s is a long tim e. D .J. M A LC O LM E-LA W E S: In your experim ents involving the d is sociation of the a lk a li halide by c o llisio n with xenon, I am interested in your calculations — the ones you described as "im p u ls e " calculations. Was it just a b illia rd - b a ll m odel? Y. LE E : No. Those were re a l tra je c to ry calculations taking the re a l potentials fo r the a lk a li halides. D .J. M A LC O LM E - LA W E S : And these " r e a l" calculations give the correct answer? D .J. L E E : Yes. However, if you take the hard-sphere potential, you get alm ost the same answer. This m eans fo r hot atom c h em istry that if you know the potential surface ra the r closely, you can alm ost the treat the atom as a b illia r d b a ll in high energy processes. D .J. M A LC O LM E -LA W E S: But if I understood you correctly , you said that if you took xenon with ru b id iu m iodide or with caesium brom ide the hard-sphere p icture would give you effectively the wrong end of the m olecule. Y. L E E : Not the hard-sphere potential. The im pu lse approxim ation. The hard-sphere lim it of the im pu lse approxim ation, but the im pu lse ap prox i m ation only takes care of the in itia l energy tra n sfe r. It doesn't take care of the m ass ra tio , and that's where the e r r o r is introduced. Not the h a rd ness of the potential; th a t's a secondary effect. The m ass ra tio is the m ost im po rtant. D .J. M A LC O LM E -LA W E S: The reason that this is p a rtic u la rly in teresting to m e is that the e a rly hard-sphere calculations on t r itiu m atoms re acting with HD tended to give the wrong isotope effect — m ore HT yield than DT, whereas e x p erim entally the DT yield is observed to be larg e r.
IAEA-PL-615/8
137
We have just done some c la s s ic a l tra je c to ry calculations fro m which we get the correct isotope effect. A nalysis of these tra je c to rie s suggests that it is the velo city at which the "breaking-bond" can break which is con tr o llin g the reaction. C le a rly if you have the configuration T-D-H, the fastest bond to break is the D-H bond, and that breaks m ore re a d ily and you are left with m ore DT as product. Y. L E E : The situation is quite different with the xenon-rubidium iodide system. D .J. M A LC O LM E-LA W E S: But you also have a heavy species hittin g a m olecule which is heavy at one end and light at the other, so that the con tr o llin g facto r is not re a lly the in itia l im p u lse , but the frequency with which the breaking bond w ill fir s t break. That w ill have to occur when the lightest end of the m olecule is away fro m the point of im pact; while the in itia l im p u lse leads to the opposite conclusion. Y. L E E : It's not re a lly that sim ple. If you follow our tra je c to rie s and v ary the m asses, you w ill see the secondary energy tra n s fe r, and in this case, the b rom in e w ill tra n sfe r best the energy of the co llid in g xenon. But there could have been a trip le co llisio n in between, and the energy tra n sfe r can v ary quite widely. This is the case when the ru b id iu m m ass ra tio is such that secondary tra n sfe r b ring s energy back to the xenon. But if you have one m ore c o llisio n , lots of the energy w ill again be tra n s fe rre d back to the struck m olecule. D .J. M A LC O LM E-LA W E S: However, during the tim e it takes fo r the energy to be passed between the two heavy atom s, the light atom has already started to move away, and because it is lig h te r it moves away fa ste r, and then we don't have to bother about these secondary collision s. F.S. ROW LAND: I think that what M alcolm e-Law es is saying is that your tra je c to rie s in which you had a heavy atom hittin g a light atom attached to a heavy atom â&#x20AC;&#x201D; in these, the distant heavy could not get away fast enough, and you therefore had m u ltip le c o llision s. Y. L E E : If the light p a rticle is re a lly very lig h t, then it w ill be like the B orn-O ppenheim er approxim ation. F.S. ROW LAND: But M alcom e-Law es is talk ing about H, D and T, so nothing is re a lly very light re la tive to anything else in the way that electrons are lig h te r than atom s. C rudely his re lative m asses are about the same as yours. Y. L E E : But he's try in g to generalize that the re action occurs because the light p a rticle comes out easily , and I'm saying that it is not that sim ple. The tra je c to ry calculations depend upon the c la s s ic a l turning-point fo r the three-body in teractions and the subsequent phase re la tio n s of the two-body interactions play a v ery im po rtan t role. So it 's not re a lly the sim ple case that the light p a rticle moves away m ore rapidly. It's re a lly a very comlic ate d situation. D .J. M A LC O LM E-LA W E S: I ' l l agree that the situation is not quite as sim p le as I'm try in g to make it sound, but the point is that in a ll of the tra je c to ry calculations which we have done, it seem s to be the case that it is the velocity of the leaving group which determ ines what the favoured products of the re action w ill be. Y. L E E : T his is fo r the T + HD reaction, right? D .J. M A LC O LM E -LA W E S: W e've also done it fo r six-body c ollision s. Y. L E E : If we change the ru b id iu m to som ething with a m ass num ber around 1 0 0 , the secondary c o llisio n s w ill disappear. If you increase the
138
LEE
ru b id iu m m ass to a little bit la rg e r, but not as large as iodine, then heavylight-heavy w ill tra n s fe r m ore energy. You can elim in a te secondary c o llision s and the im pu lse approx im ation w ill then give the right answer. M. NEWTON: A ll this discussion is concerned with co llin e a r c o l lis io n s , is n 't it? Y. L E E : Yes. M. NEWTON: P a r tic u la r ly with HD you can get com plications such as its rotation around the centre of m ass in which the H atom covers m ore space because it is fu rth e r fro m the centre of m ass. A lso, if you increase the kinetic energy, you can get a switch in the spectator strip p ing lim it. This depends upon the tra n s la tio n a l energy, ro ta tio n a l energy, and the isotopes involved. J .M . PAULUS: Do you think that at present the study of hot atom reactions fo r low m ass species, for which no seeded nozzle beam s can be used, is hopeless? Y. L E E : At present, if you want to m easure angular and velocity d is tr i butions and le a rn about the dynam ics, then you do need to have a seeded beam so that you can have 1 0 10 m o le cu le s/c m 3 in the c o llisio n region. On the other hand, if you want to shoot an isotopic species into a c o llisio n cham ber, and use gas chrom atography to iso late the products, you could do experim ents at low er in tensities because the se n sitivity would be m uch greater. J.M . PAULUS: But then you have the same advantages and disadvantages of the n u cle ar re c o il experim ents. Y. L E E : Except that you can control the in itia l energy.
IAEA-PL-615/9
GAS PHASE REACTIONS OF ATOMIC TRITIUM, FLUORINE AND CHLORINE F.S. ROW LAN D Department o f Chemistry, University o f California, Irvine, Calif., United Staes o f Am erica
Abstract G A S P H A S E R E A C T I O N S O F A T O M I C T R I T I U M , F L U O R I N E A N D C H L O R IN E . E x p e rim e n ta l stu dies o f h o t a to m gas phase rea ctio n s in v o lv in g a to m ic tritiu m , flu o r in e and c h lo rin e have c o n trib u te d s ig n ific a n tly t o an in creased u n d ersta n d in g o f c h e m ic a l k in etics. T h is p a p e r con sid ers th e fo r m a tio n o f h o t a to m s in th e gas phase, th e d e te rm in a tio n o f r e a c tio n p ro d u c ts and in fe re n c e o f k in e tic m ech an ism s, and th e basic h o t rea ctio n s in m o n o v a le n t system s. A ls o discussed are s te re o c h e m is try in th e gas phase, b o n d e n e rg y e ffe c ts , th resh old s f o r a to m ic rea ctio n s, e x c ita tio n e n e rg y o f th e p ro d u c ts and su b stitu en t in flu e n c e s on p rim a ry su b stitu tion y ie ld s as d e te rm in e d th ro u gh stu dies o n tritiu m , flu o rin e o r c h lo rin e system s.
INTRODUCTION The language of c h em ical kinetics has been changing durin g the past 1 5 y ears, and has now adopted the concepts and term inology long used in n u cle ar physics, in volv ing the idea of a cross-section fo r a given process — effec tive ly , the p ro b ab ility of re m ov al fro m a p a rtic le beam by a p a rtic u la r re actio n (p a rtia l cross-section) or by the sum of a ll possible rem ov al processes (total cross-section). The usual kinetic expression of the rate constant fo r a th e rm a l ch em ical re action as a function of te m p e ra ture , the tw o-param eter A rrh e niu s term in o lo g y k = A ex p(-E /RT ), can then be d e s cribed as the integration of the a p prop riate reaction cross-sections over a M axw ellian d is trib u tio n of atom ic or m o le c u la r energies. The advantages of the cross-section term inology — and the kinds of experim ents which have led to its adoption — include a deeper understanding of the th e rm a l processes whose actual rates are effectively s u m m a riz e d by the A rrh e niu s term inology; and a method of approach tow ards the d e scrip tio n of chem ical reactions o c c u rrin g in system s not in th e rm a l e q u ilib riu m . Incre asingly, system s which are non- therm al are becom ing m o re im p o rtan t in p ra c tic a l chem ical kinetic prob le m s. One of the u ltim a te goals of hot atom c h em istry, and of non-Boltzm ann ch e m istry in gen eral, then, lie s in the descrip tio n of chem ical reactions in term s of the respective cross-sections fo r each of the various possible processes at each of the energies at which the reaction is possible. The present situation is fa r short of this distant goal, and fo r hot atom e x p eri m e nts, u su a lly now involves the averaging of cross-sections over another broad energy d is trib u tio n , different fr o m the usual M axw ell-Boltzm ann d is trib u tio n of th e rm a l sy stem s, but incapable of giving v ery m uch specific in fo rm a tio n about reaction cross-sections at p a rtic u la r energies. In the process of such investigations, however, m any new reaction modes have been discovered which were previously unknown in th e rm a l sy stem s, and have been described with both qualitative and sem i- quantitative treatm ents
139
ROWLAND
140
of sim p le m odels fo r re action cross- sections. N evertheless, m ost of the in fo rm a tio n about m icro sc o p ic d iffe re n tia l cross-sections rem ains to be discovered, and is not re ad ily accessible to the present generation of hot atom ch e m istry experim ents. The general questions of c h em ical kinetics tow ards which hot atom c h e m istry has provided som e answers include the follow ing: what are the m echanism s fo r chem ical reactions which occur with kinetic energies m uch greater than th e rm a l? W hat is the stereo ch em istry of these reactions? What is the tim e- scale? W hat are the energy ranges involved? Most of the in fo rm a tio n discovered concerning such processes du rin g the past 1 5 years has come through the study of the c h em ical reactions of radioactive atoms fo rm e d du rin g nu c le ar tra n sfo rm a tio n s. This in fo rm a tio n has thus been of im p o rtan t u tility to the chem ical k in e tic ist, while at the sam e tim e providing p ra c tic a l knowledge to the ra dio ch e m ist of the behaviour to be expected fro m these atoms when form ed in m a te ria ls under ir ra d ia tio n in nu c le ar reactors or other a cc e le rating devices. A large frac tio n of the quantitative in fo rm a tio n applicable to chem ical kinetics has been found through studies of the gaseous reactions of three m onovalent atom ic species: tr itiu m , flu o rin e ( 18F ), and chlo rine (chiefly 38C 1 , 39C 1 , and 34mCl). Several recent reviews have given v ery detailed descriptio ns about these hot atom experim ents, and can be consulted fo r the specific addition al m a te ria l not covered here [1 -3 ]. E a r lie r proceedings of In tern ation al A tom ic Energy Agency sym posia, although not up to date, fu rn is h broad coverage of m ost of the aspects of the c h e m ic a l effects of nucle ar tra nsfo rm a tio ns [4 , 5 ]. FO RM A T IO N OF HOT ATOMS IN THE GAS PHASE W hile atom s with high tra n sla tio n a l energies are form ed in m any chem ical system s, there are four p rin c ip a l ex perim ental routes to in fo r m a tion about such che m ical processes: (a) N u cle a r re actio ns, as illu stra te d with the fo rm a tio n of tr itiu m , 18F , and 38 Cl by Eqs (1 ) to (3 ). The m ost im po rtan t ch arac te ristic s n + 3He — ► XH + 3T
(1 )
n + 19F
(2 )
— *- 1SF + 2 n
n + 37 Cl — ► 38C 1 + y
( 3)
of a ll nu c le ar reactions used in hot atom experim ents are (a) in itia l high kinetic energy of the product atom , and (b) the product atom is radioactive with a convenient half- life. The radio activ ity is im p o rtan t because it p e rm its subsequent tra c e r detection of the labelled products of the chem ical reactions of these atom s. (b) P hotolysis of suitable m olecules with u ltra v io le t lig h t, as illu stra te d by the fo rm a tio n of 2 . 9 -eV H atom s in the photodissociation of H Br (bond energy 3 . 7 5 eV) with 1 8 4 9 A (6 . 7 0 eV) ligh t fro m a m e rc u ry HBr — ► H + Br
( 4)
IAEA-PL-615/9
141
resonance lam p . Other target m olecules have been successfully used, but e sse ntially a ll (including isotopic v ariants of HI, H 20 , H 2S) have been used as sources of energetic atom s of one of the isotopes of hydrogen, H, D or T. (c) C hem ical a cc e le rato rs , a class of devices intended to produce controlled beam s of energetic atoms (or m olecules) in the ex perim entally d iffic u lt region around 2 to 5 0 eV in which m any of the reactions of current ch e m ic a l in te re st are found. (d) Com puter sim ulation: A fourth im p o rtan t ex perim ental technique fo r in vestigating hot atom reactions is the com puter sim u la tio n of c la ssic a l tra je c to rie s of the collision s between energetic atom s and substrate m o le cules. Such calculations depend upon the a v a ila b ility of a ppropriate potential energy surfaces fo r the c o llis io n s , and th e ir u tility is dependent upon how closely the approxim ate potential energy surfaces used actually m ir r o r the p ro p e r ties of the re a l m o le c u la r potentials. Each of the three lab o ra to ry methods has its p a rtic u la r advantages and disadvantages, and some com parison of these methods is w orthw hile. The com puter sim ula tio n s need to be checked against re a l lab o ra to ry re su lts , and are especially valuable when close com parisons with lab orato ry e x p eri m ents can be used to check on the v a lid ity of p a rtic u la r potential energy surfaces while at the sam e tim e provoking new lab orato ry experim ents. An example of such in teraction is (a) the ex perim ental investigation of the T + CH3NC system by the n u cle ar re c o il method [6]; and (b) the com puter sim u la tio n of the system , leading to a proposed explanation (the im portance of high ro tational energy in hot atom excited product m olecules) [7 ] , and a proposed experim ent to test the explanation (T + CD3NC, fo r which no results have yet been reported). B r ie fly , the fo rm a tio n of radioactive atom s in nu cle ar reactions leads to kinetic energies m uch higher ( 1 0 2 to 1 0 6 eV) than the possible energies fo r bond-form ing reactions of these atom s. In alm ost every ex perim ental system , a reasonable frac tio n of these hot atom s fa ils to fo rm a stable c h em ical bond in collisions o ccurring while k in e tic ally hot, becom es th e rm a liz e d , and reacts to give the fin a lly observable product by som e c h a ra c te ris tic th e rm a l re a c tion pathway. In this situ atio n , the in itia l fo rm a tio n of the atom in a nucle ar re action ensures com plete sa m p lin g of the entire range of energies over which such hot and th e rm a l reactions can occur. T hus. a ll processes should be potentially observable in the nu c le ar re c o il sy stem s, which is an advantage in obtaining knowledge of the fu ll range of reaction fo r a p a rtic u la r atom with a c ertain substrate m olecule. Conversely, since these reactions can be in itiate d by atoms over a wide range of c h em ically sig nifican t energies, the p recise energy involved fo r any p a rtic u la r observed reaction or fo r the fo rm a tio n of any p a rtic u la r product is not re ad ily determ ined. The observed reaction yields in the n u c le ar re c o il system s are thus an average taken over a wide range of ato m ic kinetic energies. These yields are taken fro m a d is trib u tio n which is c le arly non-Boltzm ann, but nonetheless is an in te g ra l and not a d iffe re n tia l m e asurem ent. When the in itia l energy of an atom injected into the gaseous substrate system fa lls w ithin or just above the energies at which c h em ical reaction can actually occur — as with 2 . 9 eV H atom s fr o m photolysed H Br — a closer approach can be m ade to a d iffe re n tia l m e asurem ent of re action yields versus energy. However, unless the p o ssib ility of reaction is carefully
ROWLAND
142
lim ite d to the fir s t c o llis io n , the hot atom m ay lose energy in a series of non-reactive encounters and the observed reaction yields again become in te g ra ls over a broad d is trib u tio n of energies. Since single c o llis io n con ditions (i.e. crossed beam s of the two reactants) have not yet been applied to hot atom system s except in isolated instances, one can only m easure d iffe re n tia l yields or cross-sections by in d ire c t m ethods. P re c is io n control of the in itia l ato m ic energies in photolysis or beam experim ents p erm its the d e term ination of energy thresholds fo r p a rtic u la r reactions sim p ly by noting the appearance of specific new re action products as the in itia l kinetic energy of the atom s is ra ise d . This d e term inatio n of thresholds has been su ccessfu lly dem onstrated both with photochem ical and controlled beam ex perim ents. In gen eral, the photochem ical procedure offers greater p r e c i sion in the in itia l energy, and therefore in m easurem ent of thresholds, but has only a v ery lim ite d upper range of energies available (about 5 eV). The controlled beam s can cover the entire range of desirable reaction ener gies, but lose p re c isio n in the im p o rtan t region below 1 0 eV. In the ex p eri ments to date they have also suffered fr o m unwanted im p u ritie s in the re acting species, e.g. T+ reactions in a system designed to investigate atom ic T re actio ns. E x p erim e nta l studies can be crudely divided into (a) those in which the hot atom processes lead to the fo rm a tio n of a known product, often by a p rev iou sly unknown route; and (b) those in which som e of the products a ctu ally are prev iously unknown c h e m ic a l species whose synthesis is one of the im p o rtan t goals of the experim ent. The efforts with hot atom s of t r itiu m , chlo rine and flu o rin e in the gas phase have la rg e ly been directed tow ards the understanding of reactions leading to a known product, ra the r than tow ards the synthesis of pre v iously unknown compounds. P rog ress in the ide n tific a tio n of the m a in paths of hot chem ical reaction has been strongly dependent upon the developm ent of suitable a n a ly tic a l techniques, e sp ecially that of radio gas chrom atography. Most of the quantitative data used in fo rm u la tin g and testing v arious hypotheses has v ery larg e ly been obtained through gas p ro p o rtio nal or s c in tilla tio n counting of the separated lab elled m olecules contained in the effluent strea m of a gas chrom atograph. W ith each of these hot m onovalent species, the usual situation is that the fo rm a tio n of a new bond in a stable m ole cule occurs in a single step. Inform a tion about this in d iv id u a l step is therefore u sually e a sie r to obtain than for polyvalent species which m ust (or m ight) need se v eral steps in order to com plete a ll of the new bonds which m u st be created, e.g. fo r 11С or 31 Si.
BA SIC HOT R E A C T IO N S IN M O N O V A LEN T SYSTEMS The fund am ental hot re action processes identified fo r energetic atoms of tr itiu m , 18F , or one of the isotopes of chlorine through experim ents with m any substrate m olecules can be grouped into three general categories: (a)
A b stra ctio n of a to m s, u sually hydrogen atom s; T + RH —► HT + R
(5)
18F + RH — H 18F + R
(6 )
IAEA-PL-615/9 (b)
143
Substitution fo r another atom or ra d ic a l; T + RX —
RT* +X
18F + R X — »-R18F* + X
(7 ) (8)
(X = H , D , F , C l, B r , I, N H 2, OH, etc.) (c)
A ddition to a pi-bond system , such as olefins, acetylenes, or a ro m a tic m o le cu le s. T + ^ C = C ^ —► ^ C T — C ^
38C 1 + ^ C = C ^ — ^ C 38C 1 — C ^ *
(9 )
(1 0 )
A ll three re action types are re g u la rly found with each of the three re acting elem ents. The addition of th e rm a lize d atom s to a pi-bonded system is its e lf quite exotherm ic; when reaction is in itiate d by an atom with excess kinetic energy, the re sulting ra d ic a l is even m o re highly v ib ra tio n a lly excited, as sym bolized by the aste risk s in Eqs ( 9 ) and ( 1 0 ). The substitution products, R T * , R 18F * ( and R 38C 1 * , have been found ex p erim entally to carry high v ib ra tio n a l a n d /o r ro tational energy in m o st, if not a ll, cases, and are m arked as excited species in equations such as (7 ) and (8). W hether the abstraction products also c a rry unusual excitation energy is m o re difficu lt to dete rm in e , since energy g re a te r than the bond energy leads to im m ediate d issociatio n (and hence no evidence of ever having been fo rm e d), while energy less than the bond energy w ill in m o st cases not lead to fu rth e r detectable reaction. The m ost im p o rtan t addition al re action modes in p roviding a greater d iv e rsity of labelled m olecules in hot atom system s are the secondary is o m e riz a tio n or decom position of the highly excited products form ed in the in itia l p rim a r y substitution o r addition re actions. One or m ore of these c h a ra c te ris tic hot reactions can u sually also occur th e rm a lly in each system , and the diffe rentiatio n between hot and th e rm a l reactions m ust therefore re m a in som ewhat in d istin c t in these cases, since the d iffe re n tia l cross-section fo r re action presum ably shows no discontinuity to m a rk a separation between hot and th e rm a l energies. A m ong the c rite ria used to establish the "hot" nature of such reactions are: (a) O bservation of re action products unknown in the corresponding th e rm a l system , e.g. CH318F fr o m 18F plus CH 4; (b) In s ensitiv ity to the concentration of scavenger present in the system , the id e a l scavenger having no re ac tivity at a ll towards hot species, but being highly reactive towards th e rm a l species; (c) P ro g re ssiv e suppression of the hot reactions by the in clu sio n of in c re a s in g am ounts of a non-reactive m o derator m o le cu le (e.g. argon), which is able to rem ove the excess kinetic energy of the atom before ch e m ic a l reaction can occur; and (d) In s e n sitiv ity to the tem perature of the re acting substrate m o le cu les, since the "hot" atom furnishes m ost of the required energy fo r the reaction.
144
ROWLAND
M oderator and scavenger experim ents are standard parts of m ost gas phase studies in hot atom ch e m istry , and re lative energy losses fo r T, 18F , and 38C 1 have been m easured with a variety of m o derator m o le cules, not a ll of them noble gases. Scavengers have also been sought and found fo r m any system s, and are u sually taken fro m am ong the follow ing list: O2, N O , B r2 , I 2, H2S, C 2H 4, butadiene. Searches fo r tem p erature effects in hot atom reactions have shown that changes of 1 0 0 to 2 0 0 °C have no appreciable effects on substitution y ields, as with the replacem ent of H by T in cyclobutane or m ethane. On the other hand, the decom position of excited butyl ra d ic a ls form ed by T atom addition to 1-butene o r 2-butene is m e asu ra b ly affected by the tem perature change fro m 2 4 °C to 1 2 5 °C. Even in this case, however, the tem perature effect is esse ntially acting through the secondary reaction — the in itia l addition re a c tion of the energetic tr itiu m atom to the double bond is apparently not influenced by the extra energy of the substrate m o le cule, and the te m p e ra ture — and in te rn a l energy — of the substrate only becom e im po rtan t when the in itia l kinetic energy is added to that of the substrate in determ ining the rate of the subsequent reaction.
S T E R E O C H E M IS T R Y IN THE GAS PHASE E x p erim e ntal investigations of the stereo chem istry of the hot substitution process have been an area of p rim a r y in terest since the discovery that substitution reactions occurred with good yield in the gas phase, with the greatest interest centring on the re te n tio n /in v e rsio n problem during the replacem ent by an energetic m onovalent re c o il atom of one of the four substituents of an a sy m m e tric carbon atom . F o r m any y ears, a ll the gas phase experim ents showed only substitution with retention of configuration, and there exists s t ill no positive evidence fo r the in versio n of configuration of any m olecule durin g the substitution of T-for-H in the gas phase. Recent experim ents with energetic chlorine atom s re acting with a-chloropropionylch lo rid e , however, have disclosed a system in which the predom inant (8 0 %) gas phase m e ch anism is the in v e rsio n of configuration [8]. When a ll e x p e ri m e n ta l re sults showed strongly predom inant retention of configuration, subtle c h em ical v aria tio n s fr o m one substrate m olecule to another seemed unlikely to be found, and were not very actively sought. Now, the substitution at an a sy m m e tric carbon atom has been shown to be capable of being either p re dom inantly retention or in v e rsio n , and is thus m uch m o re sensitive to r e la tive ly s m a ll chem ical factors than had previously been thought. An u n d e r standing of the chem ical control of re te n tio n /in v e rsio n durin g gas phase hot substitution is ju st beginning, and m any addition al experim ents can both be contem plated and expected. In com puter-sim ulated tra je c to ry experim ents, in version and retention are re ad ily distinguished, even fo r m olecules such as CH4 fo r which laborato ry experim ents are not possible [9 ]. Both retention and in version of configura tion are observed in som e of these tra je c to ry c alcu lations, with different energy dependences. C om puter sim u la tio n has not yet been carrie d out with any species having fo ur distinguishable substituents — such as fo ur different m asses — on a carbon atom , as is re qu ired fo r a ll the lab orato ry e x p eri ments in which the eventual determ ination is dependent upon the chem ical separation of two is o m e ric m o le cu les. Such com puter calculations w ill
IAEA-PL-615/9
145
certainly be c arrie d out in the next two or three years, p e rm ittin g close com parison between actual lab o ra to ry system s and the com puter sim ulations fo r the sam e m olecules. The stereo ch em istry of substitution at a sy m m e tric carbon atom positions in the liq u id phase involves the additional p o ssib ilitie s of variou s condensed phase influences, such as the "cage" effects, and is also being extensively investigated at present.
EXCIT A T IO N E N E R G IE S OF PRODUCTS An extensive series of m easurem ents has been m ade of the secondary decom position of the product m olecules form ed by hot substitution reactions in itiated by T, 18F , and 38C 1 , and involving replacem ent of H, D, F , CH3 , C l, etc. The early experim ent which involved m e asurem ent of the yields versus p re ssure of £ - C 4H 7T form ed by energetic tr itiu m atom substitution in to £ - C 4Hg, and of its secondary decom position product CH2= C H T , s till re m a in s as an excellent example of such fu rth e r reactions of m olecules activated in hot atom reactions. The increased yield of c - C 4H 7T and decreased yield of C2H 3T with increased p ressure in the gas phase are assum ed to a ris e fro m successful c o llis io n a l sta b iliza tio n of highly excited _c-C4H 7T* m o le cules, and can be re ad ily converted into estim ates of the life tim e s of the excited m ole cules on the assum ption that a single c o llisio n is n o rm a lly sufficient fo r such sta b iliz a tio n . W ith th e fu rth e r assum ption of random d is trib u tio n of in te rn al energy, approxim ate excitation energies can be in fe rre d fo r these m olecules fro m R R K M (R ice-Ram sperger-K asselM arcus) theory, which relates in te rn a l excitation energy to kinetic rates of decom position. The ex perim ental behaviour found in m any system s suggests that the usual re sult is a broad dis trib u tio n of excitation energies, typically with m edian energies of 4 to 7 eV, high enough that a substantial fra c tio n of the product m olecules are capable of either secondary decom po sition or is o m e riza tio n in the gas phase. Condensed phase m easurem ents u su a lly show that some decom position p ersists even under such very rapid c o llis io n a l stab ilizatio n conditions, signifying the presence of som e extrem ely highly excited m olecules with decom position rates approaching 1012 s '1. In recent years, the assum ption of random d is trib u tio n of in te rn al energy has been reexam ined with sp e c ia l em phasis on the p ro b ab ility that substitution re actions, at least in certain system s, w ill occur at ra the r la rg e im pa c t p a ra m e te rs, and which w ill therefore necessitate high ro tational angu lar m om entum in the product m olecule [7 ]. (The conservation of angular m om entum prevents the ro ta tio n a l energy fro m being rando m ly distributed together with the v ib ra tio n a l degrees of freedo m , and prevents the effective p articip atio n of the ro tational energy in secondary decom position of the product m olecules except under unusual circum stan ces.) The o v e ra ll in te r pretation of secondary decom positions in term s of conversion of known decom position rates into in te rn a l excitation energies is now being re-studied in sev eral system s. In general, the observed rates of secondary reaction are m o re nea rly representative of the v ib ra tio n a l energy than of the sum of v ib ra tio n a l plus ro ta tio n a l energy in the activated m o le cule, and appreciable amounts of ro tatio nal energy m ay also be present which have re la tiv e ly little influence on the total rate of decom position.
146
ROWLAND
Q uestions are also being ra ise d , both in hot atom c h em istry experim ents and in other kinds of c h em ical kinetics ex perim ents, about the v a lid ity of the "stro n g c o llis io n " assum ption fo r de-activating excited m o le cules. If the loss of v ib ra tio n a l/ro ta tio n a l energy on c o llisio n by an excited m olecule is not larg e , then m ore than one co llisio n can be re qu ired to sta b iliz e , and the tim e scale fo r decom position in fe rre d fro m pressure-dependent experim ents becom es less certain. F u rth e r experim ents in this area are certainly d e s ira b le with the excited species form ed by hot atom substitution reactions.
SUBSTITUENT IN FL U E N C E S ON P R IM A R Y SUBSTITUTION Y IELD S The ea rly understanding of the abstraction and substitution processes was often based larg e ly upon the re lative yields fo r these two reactions (e.g. the H T /R T ratio) fro m a series of different m o le cules. The theory of ste ric interference with the T-for-H substitution reaction by neighbouring a lk y l groups, with an obstruction p ara m e te r of 4 5 % per alkyl group, was based on the observation of in cre as in g H T /R T yields fro m the sequence CH3-H , CH3CH2~H, (C H 3)2 CH -H , (С Н з)зС - Н and with supplem entary m easurem ents fo r other hydrocarbon substrates, each m easured in d iv id u ally . S im ila r v aria tio ns in product ratios fro m other hot atom chem istry system s have often since been explained as a ris in g fro m s im ila r ste ric interference effects of other substituent groups, or upon other re acting energetic atom s. Subsequent tests fo r possible ste ric interference have em phasized (a) separate m easurem ents of HT and RT to determ ine the v a ria tio n in each and not ju st in the H T /R T ra tio ; and (b) corrections fo r secondary decom po sitio n , which was not considered in the in itia l H T /R T m easurem ents. These m e asurem ents with hydrocarbons dem onstrated that m ost of the v aria tio n in H T /R T ratios lay with the abstraction reaction, and showed that the substitution of T-for-H showed re la tiv e ly much less v a ria tio n fro m m olecule to m o le cu le. The p rim a r y yields fo r the substitution reaction fro m sev eral alkanes and haloalkanes showed, after c orrectio n fo r v ary ing amounts of secondary decom position, an excellent c o rre latio n with the proton N M R shift in the substrate m o le cule. This c o rre latio n suggests that the p rim e effect of halogen substituents on this substitution reaction is through an influence on the electron density in the С —H bond, and not through any size or m ass effect of its e lf — in actuality, substitution is less probable with flu orine atom substituents than with chlo rine , although the size fa c to r would work in the opposite d ire c tio n. The s m a ll re m a inin g alkyl-substituent effect in hydro carbons also correlates w ell with the sam e proton N M R shift plot, and indicates that this alkyl-substituent effect (about 10% in isobutane, com pared with the facto r of 3 fro m an obstruction p a ra m e te r of 4 5 %) is p resum ably also of electron-density o rig in ra th e r than fro m a ste ric interference with the substitution process. The findin g that ste ric interference is a negligible process in the su b sti tution reactions of re c o il tr itiu m , at least fo r halogen atom s and s m a ll alkyl groups, suggests that reco nside ratio n should be given to those other systems in which s im ila r effects have been postulated without the m ore detailed e x p eri ments necessary to distin guish am ong the variou s possible kinds of s u b sti tuent effects in these m o le cules.
IAEA-PL-615/9
147
BOND ENERGY EFFECTS The yields fo r the abstraction of hydrogen fro m C-H bonds by energetic t r it iu m atom s have been shown to c o rrelate w ell with the known bond d is sociation energies of alkanes and cyclanes. D eviations fro m this c o rre latio n with m olecules such as CHF3 and CH 3CD =CD 2 have been attribu ted, respectively, to the le s s e r and greater sp a tia l alterations (relative to CH4 and other hydrocarbons) existing between the m o le c u la r distances and angles and those of the re s id u a l C F3 and CH2 —CD —CD2 ra d ic a ls left after a b s tra c tion. It has been suggested fu rth e r that the tim e scale fo r the abstraction process is too rapid (~ 2 - 5 X 1 0 ' 14) fo r complete adjustm ent of the ra d ic a ls to th e ir e q u ilib riu m configuration p r io r to rem oval of the H atom . More re cently, less num erous m easurem ents with energetic 18F atom s have shown that the abstraction of H by these ato m s, both energetic and w ell m oderated, also shows a trend p a ra lle l to the bond d issociatio n energy trend established fo r these sam e substrate m o le cu le s, while other correlations of yields with bond d issociatio n energies have been proposed fo r additional reactants and re action types. The existence of some of these bond energy c o rrelation s seem s now to be fir m ly established, although not ne c e ssarily a ll of them ; the o rig in of such effects is the subject of continuing speculation, theory, and experim ent. The lowest activation energy re action fo r t r itiu m atoms with alkanes is the a bstraction of hydrogen atom s, and reaction by this m e ch anism is also found in good yield fo r in itia lly very energetic tr itiu m atom s fro m nucle ar re c o il, and fro m photochem ically produced hot atoms in the 3 -eV range. The question has been in dispute fo r som e tim e as to whether the HT m olecules observed in re c o il tr itiu m system s are fo rm e d larg e ly by an abstraction process that is b a sic a lly s im ila r to that of the th e rm a l a bstraction process, or whether there exists in addition a m a jo r high energy contribution fro m a fundam entally different m e ch a nism , in p a rtic u la r fro m the "s trip p in g " p rocess. There seems little doubt that the m a jo r contributor in m o st system s is the basic low-energy abstraction process, although it m ay w ell be in itiated by atom s of energies as high as 5 - 1 0 eV. W hile these m ore energetic atoms a re , because of th e ir extra kinetic energy, less re stricte d in th e ir necessity to pass through the p recise m in im u m energy configuration fo r th e rm a l ab stra ctio n , the m e ch anism is fundam entally the sam e, and the portion of the ov e ra ll potential surface which is involved is the sam e as fo r the th e rm a l ab straction . T rajectory calculations have indicated that v aria tio ns in b a r r ie r height have strong effects on abstraction reactions oc c urrin g at energies well above that of the b a r r ie r its e lf [1 0 ] . The evidence fo r high-energy strip p ing processes is subject to dispute, and the question of whether there is an appreciable contribution, but not the m a jo r one, fr o m such processes is s t ill an open one.
T H RESHO LD S F O R A T O M IC REA CT IO N S The threshold energy fo r an atom ic reaction can be defined as the m in im u m kinetic energy of the atom required fo r re action with a stationary m o le cule. The cross-section fo r this re action can be expected to increase at energies above this threshold, and then at s till higher energies, to leve l off, decline, and eventually to become zero again. As the kinetic energy
ROWLAND
148
in creases through these stages, thresholds fo r additional reaction channels w ill be passed, and additional reaction products w ill be observed. As lower energy processes decrease in cross-section, they are usually supplanted by different reactions re q u irin g m ore energy; fo r exam ple, abstraction and substitution can be replaced by d issociativ e scatterin g and chem i-ionization. Such detailed m easurem ents of reactions over a broad range of energies, p re c ise ly known, do not yet exist fo r hot atom system s, but analogies can be m ade with the data fro m ionic system s fo r which the m easurem ents exist. Threshold energies fo r the abstraction process have been m easured fo r isotopic hydrogen atom s re acting with several alkanes and with m o le cu lar hydrogen. The total yield fro m such a reaction is effectively an in tegral over a very non-Boltzm ann dis trib u tio n of hydrogen atom collision s with the substrate m o le cule. As the in itia l hydrogen atom energy is slow ly v aried, this in te g ra l yield w ill also v a ry , providing the p o ssib ility of an evaluation of the d iffe re n tia l yield. The cross-section fo r the abstraction of D fro m n - C 4 D i0 by H atom s has been evaluated in this m anner, and shows a peak in cross-section in the v ic in ity of one electron volt. No m easurem ents were m ade fo r in itia l lab o ra to ry H atom energies greater than 2 . 0 5 eV, and obviously no in fo rm a tio n is therefore contained in these data about the possible existence of higher energy processes fo r abstraction. W ith higher in itia l hydrogen atom kinetic energies, substitution reactions have also been observed to have thresholds in the v ic in ity of 1 . 3 -2 . 0 eV as m easured with photochem ically excited tr itiu m atom s. W hile the abstraction reaction thresholds m entioned above can also be obtained with reasonable accuracy fro m the activation energies m easured in th e rm a l system s, these substitution processes are true hot atom re actions, and are not observed in the corresponding th e rm a l system s. B eam m easurem ents have been made over the energy range between 1 and 2 0 0 eV, and have shown the substitution into solid cyclohexane by energetic tr itiu m atoms to have a threshold of 1 . 5 ± 0 . 5 eV, with s im ila r values fo r other so lid substrates. In each of the beam experim ents published to date, however, there have s till been som e difficu ltie s in rem oving the reactions of T + fro m those attributed to energetic T, especially at the very lowest energies.
REFERENCES Ц ]
R O W L A N D , F . S . , M T P R e v ie w o f S c ie n c e , Phys. C h e m . 9 (1 9 7 2 ) 109.
[2 ]
U R C H , D . , M T P R e v ie w o f S c ie n c e , In o rg . C h e m . 5 (1 9 7 2 ) 149.
[3 ]
DU BRIN, I . , A n n . R ev. Phys. C h e m . 24 (1 9 7 3 ) 97._
[4 ]
IA E A , C h e m ic a l E ffe c ts o f N u c le a r T ra n sfo rm a tio n s — 1960. (P r o c . S y m p . P ra gu e, 1960) £ and 2^
[5 ]
IA E A , C h e m ic a l E ffe c ts o f N u c le a r T ra n sfo rm a tio n s — 1964 (P r o c . S y m p . V ie n n a , 1964) 1^ and
IA E A , V ie n n a (1 9 6 1 ).
2,
IA E A , V ie n n a (1 9 6 5 ). [6 ]
T IN G , C . T . , R O W L A N D , F .S . , J. Phys. C h e m . 74 (1 9 7 0 ) 4080.
[7 ]
B U N KER, D . L . , J. C h e m . Phys. 57 (1 9 7 2 ) 332.
[8 ]
W O L F , A . P . , P E T T U O H N , R . , unpublished.
[9 ]
V A L E N C IC H , T . ,
BUNKER, D. L . , C h e m . Phys. L e tt. 20 (1 9 7 3 ) 50.
[ 1 0 ] C H A P M A N , S . , 167th A m . C h e m . S o c . M e e t in g , Los A n g e le s , C a l i f . , A p r il,
1974, Pap er P - l l .
IAEA-PL-615/9
149
DISCUSSION G. STÔCKLIN: I think that it is w ell that you concentrated on tr itiu m , chlorine and flu o rin e , since these are the only neu tral m onovalent atoms available which do not have side problem s such as the conversion of low energy gam m a rays, etc. One m ight recom m end that if you re a lly want to look at hot atom reactions of ne u tra l monovalent species, then you should concentrate on these three species. Now, a question concerning the bond energy effect on substitutions. There is this controversy between s te ric effects and electron-density effects, as you pointed out. There are probably a num ber of things which you could also plot, and always find some kind of re la tio n. W hat about the p o ssib ility of the ease of excitation decom position as a function of the bond strength? In other w ords, that you see esse ntially a secondary effect on the deexcitation ra th e r than som e p rim a r y effect itself? F .S . ROW LAND: The reactions of tr itiu m atom s with the v arious halomethanes were carrie d out in two ways — they were done in the liq uid phase in com petition with the substitution reaction with m ethyl ch lo rid e , and in the gas phase with correctio n fo r excitation decom position. There is also some excitation decom position in the liq uid phase, but m uch less. One can then say that if the basic p rim a r y reactions are the sam e in the gas and liq uid phases that this is the observation: a c le a r trend of substitution yields versus electron density. However, it is always possible that there are addition al liq uid phase effects, such that the conclusions did not apply d ire c tly to the gas phase. However, the sam e c o rre latio n appeared in the gas phase experim ents as w ell. The corrections fo r excitation decom position are m uch la rg e r in the gas phase, but we obtained essentially the same trend in both phases. The decom position paths fo r these halom ethane m olecules are very diffe rent. The weakest bond in CH4 is the 1 0 4 k c a l/m o le C -H bond, and you don't find v ery m uch decom position of CH 3T follow ing T-for-H in m ethane. W ith CH2 C l2 and CH2F2 , the reaction is not a sim ple bond break at a ll, but the release of HC 1 or HF leaving a carbene frag m en t behind. If you want to m e asure the o rig in a l p rim a r y yield with these dihalocom pounds, then you have to m easure both the yield of the stab ilize d parent and also the yield of the decom position product. The best way to make this estim ate of decom position is to catch the decom position product, and m easure it, and add the stab ilize d and decom posed yields together. A m olecule such as CH 3F shows no weak bonds, and little decom position is observed fo r CH2TF after T-for-H substitution. W ith m ethyl chloride and m ethyl b rom id e, the p rim a r y yields observed at one-half to one atm osphere p ressure fa ll below those fr o m m ethyl flu o rid e , but when you add in the yields of the decom posi tion products, the yields of CH2T Br and CH2TC 1 are higher than those for C H 2T F. F ro m the p rim a r y yields without correctio n fo r decom position, it looks like a c o rre latio n with ste ric interference: b rom in e interferes m ore than chlo rine which interferes m ore than flu o rin e . However, when the decom position products a re included, the trend turns around and becomes a c o rre latio n in the other dire c tio n — with electron density. In these tritiu m substitution experim ents with halom ethanes, the am ount of decom position is at w orst about equal to the am ount of stab ilize d p rim a r y product, and you can do a reasonable job of m e asu rin g both and adding them together. However, in som e of the reactions of flu orine and chlo rine ato m s, fo r which the excita-
150
ROWLAND
tion energy of the re sid u a l substitution product is gre a te r, and the percentage decom position is therefore also gre a te r, experim ents at one atm osphere gas p ressure often give you very little useful in fo rm a tio n unless you have also trapped and m easured the decom position products as w ell, and are m e a su rin g a sum of the p rim a ry yield plus that of the decom position p ro ducts. If you m easure only the p rim a r y yield what you observe is m ore a re flectio n of the excitation decom position rate — and this is not a very in te re stin g re su lt. If you want to lea rn about the rates of decom position of the various m o le cu les, then there are other experim ents which provide the in fo rm a tio n m o re d ire c tly . However, if you want to understand the o rig in a l p rim a r y hot yields before decom position, then you have to add in the contribution fro m the decom position products as w ell. G. STOCKLIN: I understand your point about the decom position in the gas phase. My question is: how does this re lation look in the liq uid phaee? You say that it is e sse ntially the sam e in both phases? F .S. ROW LAN D: A fter correctio n fo r decom position, the two c o rr e la tions — liq uid phase and gas phase — are in good agreem ent. The c o rre c tions are m uch la r g e r fo r the gas phase, and the re su ltin g e r ro r lim its are som ewhat la r g e r , but the gas phase results agree with the liq u id phase re su lts . D .J . M A LC O LM E-LA W E S: Could you te ll us exactly what you intended to im p ly when you said that the m e ch anism fo r the abstraction reaction could be the sam e fro m half an electron volt to ten electron volts? F .S . ROW LAN D: The question becomes one of what kinds of tra je c to rie s can be traced on a p a rtic u la r potential energy surface going through a p a rtic u la r potential m in im u m . The potential m in im u m fo r abstraction in a ll of these hydrocarbon system s corresponds to the lin e a r T-H-R system , with the lowest activation energy of the order of 1 0 k c a l/m o le . If the tritiu m atom has 3 0 k c a l/m o le kinetic energy it can s t ill go through the sam e part of the potential energy surface. It doesn't have as m uch re s tric tio n on it, since it has considerable extra energy and needn't go through the exact potential m in im u m . The tra je c to ry calculations which Sally Chapm an and Don B unker have done a re interpreted by them as being p r im a r ily affected by the b a rr ie r height. When they leave everything else the sam e, and the b a r r ie r height is changed, there is a large effect on the abstraction yield. The b a r r ie r height has an influence on the tra je c to ry even at energies which are very m uch higher than the energy of the b a r r ie r its e lf. S m all v aria tio n s in a 1 0 - k cal/m ole b a r r ie r height have effects on the yields of 6 0 - 7 0 k ca l/m o le t r itiu m a to m s, or even 1 5 0 k c a l/m o le atom s. D .J. M A LC O LM E -LA W E S: Y ou 're not saying that you can't come in at an angle that is not 1 8 0 °, are you? F .S. ROW LAN D: No. D .J . M A LC O LM E -LA W E S: Then you can come in at an angle of 1 5 0 °, or perhaps 9 0 °? F .S. ROW LAN D: W ith m ethane, there is an abstraction p o ssib ility centred around 1 8 0 ° and a substitution reaction elsew here. At 1 5 0 °, a b s tra c tion is c ertainly possible. At 9 0 °, it m ight be hard to have abstraction because the potential energy surface there m igh t favour substitution. If you did have a b straction , it m ight involve a different path over the potential energy surface — a greatly different path — than the one being followed over the b a r r ie r with the m in im u m at 1 8 0 ° and 10 k ca l/m o le .
IAEA-PL-615/9
151
I think that you can get a reasonable explanation of the abstraction yields by re s tric tin g yourself to the potential energy m in im u m used fo r low-energy a b stra ctio n , and u sin g a cone roughly fro m 1 3 5 to 1 8 0 ° fo r the abstraction process. In other w ords, m ost of the a bstraction processes we actually observe have gone through this m in im u m , although not n e c e ss arily at the m in im u m energy. D .J . M A LC O LM E - LA W E S : Why do you put a re s tric tio n in it? Is n 't the only re s tric tio n a d y n am ic al one determ ined by the available energy? F .S . ROW LAND: I don't m ean that one puts a re s tric tio n on the c a lc u la tions, but ra th e r that when the calculation is done, y o u 'll find a yield of a bstraction fro m that cone around 1 8 0 ° which is high enough to account fo r the amount of HT observed in the experim ents, and that the other m e ch a n ism s, w hile p o ssible, are not that im p o rtan t in d e term inin g the ov erall yield. D .J . M A LC O LM E -LA W E S: A re you saying that 1 0 -eV tr itiu m atoms w ill contribute m o re by the c o llin e a r approach than they would by approach at 9 0 °? F .S. ROW LA N D: I'm saying that abstraction by 1 0 -eV t r itiu m atom s is probably not a v ery large fra c tio n a l contribution anyway to the observed abstraction y ie lds, so that whether it is being form ed by the c o llin e a r or som e other mode is not so im p o rtan t in its contribution to the total yield. The to ta l yield is dom inated by contributions at low er energies. In these low er energy re actions, the yield becom es m o re and m o re prevalently this c o llin e a r re action m e chanism . D .J . M A LC O LM E-LA W E S: How would you co rre late D ubrin's e x p eri m e ntal results with Chapm an's calculations? F .S. ROW LAND: D ub rin d id n 't do an experim ent that changed the b a r r ie r height. D .J . M A LC O LM E -LA W E S: No, I'm asking a separate question — I m ean his observation that the abstraction yield peaked at one electron volt. C hapm an's calculations showed a m a x im u m at m uch higher energies. It's true that her calculations were fo r a different isotopic m a s s , and so on. Do you feel that this difference is sig nifican t? F .S . ROW LAND: P robably what it suggests is that in the re a l e x p e ri m ent there a re a lot of c o llisio n s being m ade in the energy range around one electron v o lt, and that this cross-section is weighted ra th e r heavily in the yield d e term inatio n in the lab o ra to ry because there have been m any chances fo r re action. A t 1 0 eV you don't get as m any trie s as in the v ic in ity of 1 eV. K la ra B E R E I: I would like to re tu rn to the discussion of chem ical effects. There have been v ery few experim ents done in the gas phase with aro m atic system s. There are some re sults fr o m M achulla and Stocklin, and we have heard about Lee's experim ents with beam s. N evertheless, there a re n 't very m any experim ents in this field. O bviously, only a few a ro m atic system s have enough v o la tility fo r gas phase m e asurem ents. I think, however, that it is im p o rtan t to choose som e of these system s because these are suitable fo r the study of chem ical effects, fo r d istin guish ing am ong in te r tia l, s te ric , and c h e m ic a l influences on hot re actions. W ith different benzene d e riv ativ es, one can cause v aria tio n s in im p o rtan t ch em ical param eters such as electron density, dipole m o m ent, p o la riz a b ility without having appreciable changes in the in e r tia l and ste ric p ro p e rtie s. A nother point is the well-known stab ility of a ro m atic sy stem s, which can help in m in im iz in g excitation decom position.
152
ROWLAND
F.S. ROW LAN D: We have made a ra th e r detailed study of 18F reactions with a ro m atic flu o rid e s , m ono, d i, and t r i, with th e rm a l atoms created in itia lly as hot atoms and then th e rm a lize d in m u ltip le collision s with S F 6. We have found it very d ifficu lt to get in form atio n about the in itia l location of attack in a clean way because we haven't been able to locate a ll the radioactive products. The problem can be illu stra te d in this way — we have fo ur possible products fro m 1SF reaction with fluorobenzene: 18F - for- F, and the three 18F-for-H substitution products giving the ortho, m eta, and para compounds, p lu s, of course, the p o ssib ility of abstraction to fo rm H 18F . In the gas phase, we find very little yield of fluorobenzene-1SF , and the total yield of the ortho, m e ta, and para compounds is about 9 %. N in e ty p e r cent of the 18F is not located in those experim ents. It's presum ably on the w all, but the ch e m ic a l fo rm is not known. However, when we put O2 into the system , the yields of the para and meta difluorobenzene compounds go way up, and the yield of the ortho compound actually decreases. The total yield is now 6 5 %. Now, in this system we have found m o st of the ra d io activ ity , but we have d ifficu lty saying anything about ortho, m eta, para dire c tio n a l influences. We are s t ill m is s in g 3 0 %, and we don't know whether the fa ilu re to find m ore ortho-difluorobenzene is the re su lt of the interm ediate ra d ic a l not being fo rm e d , or because of its fa ilu re to react with O2 in the same way as the m eta and para d ifluo ro p re c u rs o r ra d ic a ls did. Because we do not easily identify a ll of the interm ediate ra d ic a ls , we can't in te rp re t the overall experim ent in te rm s of in itia l dire c tio n a l effects of the addition. W ith the added O2 we can show that these d ira d ic a ls are fo rm e d , and that we need to w orry about them . In b rie f, we have tried to do s im ila r experim ents with the a ro m a tic system s and we haven't been able to get very conclusive data. G. STOCKLIN: Extensive studies have been m ade with a ro m atic system s on the selectivity and re ac tivity of chlorine atom s. It's not too different fo r fluorine and chlo rine , and L in dner has s im ila r results as w ell. I wouldn't w o rry about the m is s in g activity in a ro m atic system s. It is not worse than in alip h a tic system s. Substitution products always have s m a ll yields. In a ll a ro m atic system s, depending on the phase and the density, you find fro m 1 0 -3 0 % of the ra d io ac tiv ity m issin g . But you have a certain am ount of addition product which w ill react with other aro m atic m olecules fo rm in g p olym eric products. In m y paper (IA E A - PL - 6 1 5 / 1 0 ), in F ig .8, I have out lined a reaction scheme which explains why you can't expect to find a ll these y ie ld s , and why you are m is s in g quite a frac tio n . It is not d istu rbin g at a ll. F.S. ROW LAN D: It disturbs m e in this system . One of the things which your experim ents certainly show is that there is not very m uch selectivity — t h a tis , reaction occurs at ortho, m eta, and para positio ns, and a ll the re a c tion yields are w ithin a facto r of two of one another. If you were to take the m is s in g activity and a r b itr a r ily assign it to any one of the positio ns, then it changes the whole pictu re. If you assign it to m eta, then m eta becomes the m a jo r reaction p osition. I don't think you can re a lly say anything about the details of selectivity in the a ro m atic system s u n til you have shown that the m is s in g a ctiv ity did not react at any of the positions being com pared. G. STÜCKLIN: No, this is som ething com pletely different. You are just interested in the substitution channel, and you have essentially the sam e p ro b le m when you look fo r decay ions. F .S . ROW LAN D: The experim ents I have just described with flu o r o benzene a ll involve addition to the. rin g , and it is th e rm a l 18F , and not hot.
IAEA-PL-615/9
153
G. STOCKLIN: It may make quite a difference that you have th e rm a l a to m s, and not hot ones. A .P . W O L F : I ' l l play the de v il's advocate here, esp ecially since you have given a ra the r ancient h istory perspective to hot atom c h em istry . It seem s to me that Rowland is saying, "W hat is the nature of the p rim a r y event? " and saying that u n til one can account fo r everything that one cannot say m u ch about the p rim a r y event. But Stocklin is saying, "T his is the product a n a ly s is ". But the product analy sis doesn't re a lly te ll you anything about the p r im a r y event. The point that Rowland is m aking is that the question you should be asking is — of the nu m be r of tim es that the meta position is je op ardized, what frac tio n shows up in the product an a ly s is. That you can't say anything about the p rim a r y je o p ardization of o rth o /m e ta /p a ra positions without com plete a n a ly s is, and I think that is a v alid point. This re a lly gets into what hot atom c h em istry is a ll about. W e've a ll played this game of c o rre latio n with nu cle ar m agn etic resonance sh ielding, with stru c tu ra l effects, etc., but the latest game one plays is to use the H am m ett sigm a-rho function and sig m a- star and pi- star and sigm a double s ta r , and look fo r c o rre latio n s. Take the nu cle ar m agnetic resonance effect — fir s t of a ll, what is the effect? It has to do with the sh ielding of the atoms which are involved. Is the effect on the in co m ing atom ; or is the effect on the ease with which the bond breaks because the attack is affected as the atom comes in ? F o r exam ple, with carbon — which I think is analogous although it is not m onovalent — one can get a beautiful straigh t line c o rr e la tion between the yield of acetylene, fo r exam ple, and the N M R shielding of a p a rtic u la r bond. The less it is shielded, the higher the observed yield of labelled acetylene. If one takes the sam e m o le cu le s, and plots the num ber of v ib ra tio n a l degrees of freedo m fo r those m o le cu le s, corrected fo r the p e r turbations in the system , one also gets a stra ig h t lin e . W hat is it that con tro ls the yield? Is it the p rim a r y event of the in co m ing atom , or is it the excitation-decom position? As fa r as I'm concerned, it's s t ill a wide-open fie ld . But what the hot atom chem ist can try to answ er is — what is the nature of the p rim a r y event? The fir s t attack on the m o le cule. How can we distin guish the relative num ber of attacks on the different positions in the m o le cu le? We a ll try to do this by product a n a ly s is, but I think we've come to the stage where we have to ask whether product analysis is that le g itim ate a reflectio n of the p rim a r y event. F .S . ROW LAND: In com m ent on th is, let m e say that I aid exhibit data fo r two photochem ical reactions in which thresholds fo r substititio n were shown — substitution into CD4 and into CH3F . Both experim ents have also been done with the oppositely labelled isotopic m o le cu le s, C H 4 and C D 3F. In these photochem ical ex perim ents, the threshold fo r T-for-D in CD4 is h igher in energy than T-for-F in CD3 F and low er than T-for-D in CD 3F . It looks to me as though what we are seeing is that the substitution of a flu o rin e atom fo r deuterium in the target m olecule has caused an increase in the threshold fo r the T-for-D replacem ent. The effect of the substituent flu o rin e atom appears to be on the b a r r ie r fo r substitution, and this fa c to r is probably the one which causes low er yields in higher energy re c o il tr itiu m experim ents — low er yields that is fo r T-for-H in CH 3F as com pared with T-for-H in C H 4. The observed yield is low er per bond in CH3F , and it is probably because the b a r r ie r fo r substitution has been raise d. D .J . M A LC O LM E -LA W E S: In your photochem ical m e asurem ents of thresholds do you plot absolute y ie ld s , or the ratio of a b stra ctio n to substitution?
154
ROWLAND
F .S . ROW LAN D: We have never plotted absolute yields in these system s. It is the ratio of substitution to a bstraction in a m ixed system so that we know that we are com paring reactions c arrie d out in the sam e tr itiu m atom flux. D .J . M A LC O LM E-LA W E S: Then in fact the plots could be shifting because the abstraction yield is changing? F .S. ROW LAN D: No. That's what I m ean by a mixed system . If you do CD4 , C H 3F , you can observe substitution into both, and these substitution yields are then m easured re la tiv e to one another, with any v a ria tio n in the a bstra ctio n yields cancelled out in this d ire c t com parison. K la ra B E R E I: A re you re a lly as sc e p tica l, D r. W olf, as you say about the c o rre latio n between product analysis and the o rig in a l p rim a r y event? If we com pare v ery s im ila r compounds — even replacem ent reactions in the sam e m olecule with di-substituted derivatives — and if we m ake these com pariso ns in different phases, perhaps we can get an answ er about the re al reactions which happen in this system . This was m y point — there are too few experim ents which have been m ade in gaseous a ro m a tic system s. There is a lac k of gas phase experim ents fo r com parisons with the liq uid phase system s in which m o re studies have been made. A .P . W O LF: I'm not sceptical about the use of product analysis — it's the only tool we re a lly have. The point was raise d whether one need to be concerned about the frac tio n of ra dio activ ity that one cannot account fo r. The answ er that Stocklin gave was that the m is s in g activity was not re a lly that sig n ific an t. And Rowland said that what he's interested in is the p rim a ry attack, and that you can't use d ire c t product analysis fo r that purpose. No one has devised an experim ent that distinguishes the p rim a ry event fro m the fin a l product in this p a rtic u la r type of study. Take the example of flu o ro benzene. Suppose you expose fluorobenzene to hot flu orine atom s, and you have convinced yourself that you have scavenged the th e rm a ls , so that you are ju st looking at the hot atoms. If you observe a p a r tic u la r dis trib u tio n of o r th o /m e ta /p a ra , but fu rth e r know that you haven't accounted fo r 6 0 -7 0 % of the to ta l flu o rin e ra d io ac tiv ity , and that it had m any re action channels open to it fo r re action. Suppose the m is s in g flu o rin e activ ity had a ll attacked in the ortho position — then the observed d is trib u tio n would not reflect this. One of the d istin g uish in g features of hot atom c h em istry as we p ractice it is that we approach the activated complex — fo r want of a better te rm — fro m the high energy end of the re ac tivity range — and you're in a position that the com plex can get into any one of m any potential valleys. Getting into the box is a one-jeopardization event, but getting out of the box there are m u ltip le pathways involved. Suppose that 9 0 % of these valleys do not involve ortho-difluorobenzene, but suppose that in the m eta case only 20% of the valleys do not involve m eta-difluorobenzene. Then you can't re a lly correlate these two because you don't know what the other products were that m ay have been created by attack on the ortho position. You can develop a ll sorts of ra tio n a liz a tio n s, and that is what we do to get around this difficulty; nevertheless, the d ifficu lty s t ill exists. This is true not only of m onovalent hot atom s — it is also true fo r polyvalent hot atom s. Here the inorganic chem ist has an advantage over the organic chem ist because they have available tools such as the M ôssbauer effect, perturbed angu lar c o rre latio n , etc., to try to get a m e asu rem ent clo ser to the p r im a r y event. Perhaps the theoretician can give us a hold e a r lie r in the p ro cess, too, or help us to devise an experim ent
IAEA-PL-615/9
155
that w ill give the necessary results to answ er questions about the nature of the p r im a r y event. Y. L E E : These are re a lly quite com plicated system s. If you re a lly want to know what happens in the p rim a r y event fo r flu o rin e plus flu o ro benzene, you can put these species into the crossed beam s. The beam method always looks at the p rim a r y event, and you can d istin g uish the o rth o /m e ta /p a ra m olecules through the ir frag m en tatio n patterns in the m ass sp ectrom e ter detectors. It could be done in the beam s, but we haven't done it because we are concerned about m o re sophisticated problem s ra th e r than such sim p le che m ical p ro b le m s. But this could be done. It would take som e tim e to exam ine the frag m entatio n patterns in order to distin guish the is o m e ric m o le cules. If one com pares flu o rin e atoms going to o rth o /m e ta /p a ra positions of fluorobenzene, then you have different sta b ilitie s ; you excite different m odes of the m olecules; the life tim e s a re different. When you do the product a n a ly s is, it is not n e c e ss arily going to te ll you m uch about the p rim a r y event, which is a two-body event. There are m any relaxation processes o c c u rrin g in the other experim ents — s ta b iliz a tio n of the ra d ic a ls , p o ly m e riza tio n , etc. You have to do the experim ents in the crossed beam s. M . NEW TON: The burden seem s slow ly to be shifting to the th e oretician. I would lik e to get a consensus of your opinions about what factors are i m p ortant beyond the sim p le reactive cross-section. F o r instance, the actual c alc u la tio n of a product yield re qu ires the total in e la s tic cross- section, as w e ll as an assessm ent of the fra c tio n a l decom position of the product. One aspect of this which M alcolm e- Law es, am ong others, has m entioned is that even the fo rm a tio n of HT by a b stra ctio n m ay be affected by decom position depending upon the in te rn a l and tra n s la tio n a l energies of the product m o le cule. How im p o rtan t do you think elastic effects are? F o r instance, I think — as opposed to W olfgang's o rig in a l fe eling that reactive p ro b ab ilitie s are close to unity at high energies — that the feeling is now that reaction p ro b a b ilitie s a re a facto r of 1 0 le ss. This then m akes in e la stic collision s an im po rtan t fa c to r, since m o st c o llisio n s are in fact in e la stic energy loss collision s and not reactive co llisio n s. A ll this in fo rm a tio n is pertinent to the ability of the the oretician to give in fo rm a tio n about the expected products of p rim a r y in te ra c tio n s. N. G E T O FF: I agree with D r. W olf. I would lik e to add that v arious iso m e rs of the a ro m a tic in term ediates could be fo rm e d . Each is o m e r could lead to different products fro m a stru cture in e q u ilib riu m . G. STÜCKLIN: P erhaps we a re a little too p e s s im is tic concerning the a b ility of nucleogenic atom s fo r c la rify in g re action m e ch a nism s. If you want to know the p r im a r y reaction event, fo r exam ple, in an a ro m atic m o le cu le, what is the site of the o rig in a l event? W here does it end up? There is no technique available to te ll you th is. We a re probably o v e r e stim a tin g the beam technique, which does not te ll you about the detailed c h e m istry . It can't te ll you anything about the o rth o /m e ta /p a ra d ire c tio n a l effects n o r about stereo ch em istry . So fa r the studies have only been carrie d out with the exotherm ic substitution of flu o rin e fo r hydrogen in the beam s. If you com pare the results obtained by re c o il ch lo rination with photolytic cyanation, the product dis trib u tio n s are v ery s im ila r , a lm o st id e ntical. F .S . ROW LA N D: P a r t of what we are talk ing about is what we are going to do in the next ten y ears, and not ju st what was done in the past ten years. As Yuan Lee has said, in p rin c ip le one can find out a ll these details by
156
ROWLAND
atom ic and m o le c u la r beam experim ents. In fact, however, alm ost a ll our in fo rm a tio n was not found by the beam method, but by some other technique. The question fo r us to consider is — in the next ten y ears, w ill this situation continue to re m a in true? This becom es chiefly a question of how fa r along the ato m ic and m o le c u la r beams have com e, and at the sam e tim e , how sophisticated can the hot atom chem ists become with th e ir tra d itio n a l experim ents ? G. H A R B O T T LE : It is n 't true that you don't know the d irectio n of in itia l attack. You know it perfectly w ell because it is sim p ly a Monte C arlo situ ation . The in itia l d irec tio n is every d irec tio n of attack. You know this because it is a system with com plete random ness, with no order at a ll. W hat you do is to sta rt out on the the oretical tra ck and run a series of tra je c to ry calculations in which the target m olecule and the in co m ing chlorine atom come together in random d ire c tio n s. Then you observe what frac tio n lead to in e la stic c o llis io n s , how m uch energy goes into ro ta tio n a l m o tio n, etc. M. NEWTON: You also need to see how m uch money the A to m ic Energy C o m m ission is w illin g to put into this experim ent. There is n 't enough money available to do in e la s tic scatterin g. P ut in specific term s: if large im pact p a ra m e te r c o llision s are im p o rtan t, then it takes a lot m o re money and com puter tim e to span the p aram eters of in terest. The reactive collisions tend to involve a s m a lle r am ount of phase space and therefore of d o lla rs. G. H A R B O T T LE : I think that it is im p o rtan t not to ask what is the attack on the ortho position or the m eta position. You are attacking the m olecule fro m every conceivable directio n because the in itia l conditions are random . F .S. ROW LAN D: P roduct analysis has the great advantage that it selects only those tra je c to rie s which are successful. If there are large num bers of in e la stic sc a tte rin g events, then you can be w asting your tim e looking at a ll these events in which no reactions occur. W ith o r th o /m e ta /para ex peri m ents, we want to find out what frac tio n of these events lead to the fo rm a tio n of som e reasonably stable interm ediate com plex, sufficiently stable that it leads to a distinguishab le product. G. H A R B O T T LE : Y our definitio n of success is a pretty narrow c irc u la r definitio n. Success is that which gives us a successful outcome — som ething we can m e asu re . But how m any cases were there in which the thing held together fo r a w hile, or went through a series of v ib ra tio n s, and then flew a p art. There are m any kinds of possible success, and you are looking only at a v ery re stricte d group. F .S . ROW LAN D: This is the tra d itio n a l orientation of the reaction k in e tic ist. He looks fo r the product in the th e rm a l system in which 1 0 14 c o llisio n s occur, and only one is successful. A .P . W O LF: I would like to re tu rn to som ething w hich Lee, Newton and Rowland have a ll touched upon — the purpose of this m eeting. Yuan Lee said that the re action of flu o rin e with fluorobenzene is a very easy question to answer — just put them in the crossed beam s. Rowland puts the quotation m ark s "in p rin c ip le " after that statem ent. Now, we've heard statem ents like this fo r a long tim e . W e 're chem ists and w e're interested in certain p a r tic u la r questions and the answ ers to them . Yuan Lee has also said that there were m o re sophisticated things to look at, and we hope to find out about them la te r when he gives his talk. Can the theoreticians show us a d irec tio n in which we as ex p erim entalists can look fo r answers to m o re sophisticated questions? Things we haven't thought about. Or is it s im p ly
IAEA-PL-615/9
157
that Lee and I are interested in different questions because he is a beam m a n and I'm an organic chem ist? I would like now to re tu rn to the question of the point of attack in the m o le cu le. I don't think it is quite as sim p le as H arbottle has indicated. We did som e experim ents once which were designed to answ er this kind of question in a different system . W ith a carbon atom attacking an a ro m a tic m o le cu le, you can set up three m odels. One of them involves attack on a pi bond; one involves attack at a sigm a bond; the third involves attack in a random fashion on the whole pi system . We were able to distin guish among these possible products by using a double-label system , and com pletely elim inated the p o ssib ility of any attack on the pi-system as a whole. It can only be by attack on the sigm a bond or by attack on the isolated pi-bond â&#x20AC;&#x201D; and the la tte r is contrary to the d e scrip tio n of pi-bonding in such m olecules â&#x20AC;&#x201D; they a re n 't supposed to be iso lated. The point in this case is that, although the in itia l approach is rando m , as the atom gets closer to the target m olecule other forces begin to act and the atom can be deflected fro m its o rig in a l course. In other words you don't re a lly know anything about these interm o le c u la r forces in a hot atom system , and what these forces w ill do to the tra je c to rie s when the atom s approach w ithin bonding distance. You can m ake a ll these sim p le s ta tis tic a l assum ptions, but when you look at the c h e m istry of the system s, it doesn't seem quite that sim p le . Can the theoreticians help to lead the hot atom chem ists in new directio ns in which they can do som ething creative as chem ists? D .J . M A LC O LM E -LA W E S: I wonder if the ex p erim entalists pay m uch attention to what the theoreticians do? Let me give you an example fro m m y own experience, and please forgive me fo r that. As you probably know, there has been a certain am ount of argum ent about the fo rm a tio n of tr a n s la tio n a lly excited HT fro m re c o il tr itiu m re actions. Now, I notice that Rowland did not m ention this in his ta lk , either fo r or against. Now, this s u rp ris e s m e. Is this because this is not of in te re st to hot atom chem ists? This is a s m a ll exam ple, but it is som ething which comes fro m a theory, however crude the theory doesn't m a tte r, and which no ex p erim entalist appears to be interested in . M . NEWTON: D u b rin has m entioned it in his review a rtic le in A nnual Reviews of P h y sic al C hem istry. He d id n 't sp e c ific ally give a reference to your w ork, but he did m ention it as an im p o rtan t fact. F .S. ROW LAND: In te rm s of the specific example of tra n sla tio n a lly excited HT m o le cu le s, if there is n 't any high energy strip p in g reaction, there a re n 't any high energy HT m o le cules. D .J . M A LC O LM E -LA W E S: I would certainly accept that, but I don't accept that there is n 't any high energy stripping . F .S . ROW LAND: F ro m m y point of view , this is not n e c e ss arily an in te re stin g aspect of re co il tr itiu m c h em istry u n til it is dem onstrated to m y satisfactio n that such high energy tr itiu m abstraction processes exist in substantial yield. The experim ental evidence fo r th e ir possible existence is based upon the re sults of experim ents c a rrie d out with dilute solutions of the substrates in ra re gases, fo r which the observed ratios of abstraction to substitution are different with different ra re gases. Now, as I have indicated to you p riv a te ly , we have had great d ifficu lty in dup licating these re sults in our own lab o ra to rie s â&#x20AC;&#x201D; that is , in m e asu ring the HT to RT ratios at high dilution in ra re gases. When we see difficu lty in reproducing the e x p erim ental data fr o m such experim ents, I am re lu ctant to seek detailed
158
ROWLAND
explanations fo r som ething which actually m ay ju st be an experim ental a rtifa c t. D .J . M A LC O LM E - LA W E S : You m ust be aware that there is other ex perim ental evidence fo r it: scavenger effects and so on. And your own experim ents of adding deuterium gas to m ethane. F .S . ROW LAN D: I don't think that it is as easy to do an appropriate experim ent in re c o il tr itiu m c h em istry as one m igh t guess fro m reading the lite ra tu re . I have been interested in the prop o sal of tra n s la tio n a lly excited HT fr o m abstraction re actions. I've asked Max W olfsberg at our u n iv ersity , who has done som e calculations on the reverse experim ent — shooting rare gases into hydrogen. One ought to get the sam e re su lt fr o m both sides if you take both experim ents into the centre-of-mass fo r com parison. W o lfsberg has done calculations in which he changes the m ass of the rare gas being shot into hydrogen, and he doesn't see the m ass dependence which should be there according to the re c o il tr itiu m experim ents. My in clin a tio n is to think that these results in the re co il tritiu m experim ents are somehow in c o rre c t — ju st artifa c ts of the experim ents. The im pingem ent of excited argon atoms on m o le c u la r hydrogen w ill c ertainly cause dissociatio n when the energies a re in the range of 3 0 eV. I ju st don't think that there are enough m o le c u la r hydrogen HT m olecules with such high energies to be an appreciable contribution to the yields observed in the ty pical n u cle ar re co il t r it iu m experim ent. N. G E T O FF: I have a question fo r the th e oreticians. By taking into account the inductive effect, one can calculate the m o st favourable position of attack. That is , it doesn't m a tte r where the p rim a r y attack occurs. The m o st im p o rtan t facto r is the sta b iliza tio n of certain is o m e ric structures afte r the p r im a r y attack — the structure w ith the longest life tim e w ill control the fin a l products. Can this be handled by the oretical m ethods, and give in fo rm a tio n about the hot atom re actions? As W olf m entioned, the attack could be on the sig m a position or the pi p ositio n, but it doesn't m a tte r. A fte r attack, there are m any p o ssib ilitie s fo r sta b iliza tio n . The p rim a r y attack leads to c ertain species, but a picosecond la te r, everything starts e q u ilib ra tin g among the possible is o m e ric structures. M. NEWTON: Do you m ean u n im o le c u la r decay of the hot atom product? A .P . W O L F : He m eans in tra m o le c u la r re arrangem ent, N. G E T O F F : Yes, in tra m o le c u la r re arrangem ent. F .S . ROW LA N D: The assum ption that you have just stated is that no m a tte r where the o rig in a l attack takes place, it w ill always move around to the m o st stable position am ong the is o m e rs . I don't think that this assum ption is c o rrect. G. STOCKLIN: B a s ic a lly , this is an in te re stin g question and organic chem ists have la rg e ly m issed this point. They have looked at the iso m e ric d is trib u tio n and have m isse d the p o s s ib ility that som ething m ight have happened between the p rim a r y attack and the fin a l product — fo r instance, re arra n g e m e n t. In hot atom reactions of ne u tra l species, fo r exam ple, flu o rin e atom s with dihalogen compounds as substrates, one looks after substitution to determ ine whether the sam e is o m e r is fo rm ed. B e re i and I, fo r instance, have found that the substitution of halogen fo r halogen occurs exactly at the point of o rig in a l substitution — that is , that there is no re arra n g e m e n t. On the other hand, if you go to decay io ns, in the gas phase, the is o m e r d is trib u tio n depends on the p ressure and the environm ent, because after the fir s t fo rm a tio n of the a ro n ium ion, there are two processes
IAEA-PL-615/9
159
competing: hydrogen tra n sfe r and re arra ng em ent. The com petition is controlled by the environm ent, and one can see both therm odynam ic and kinetic control under different conditions. Eventually you reach the th e rm o dynam ic position in which the m eta position product is m o st abundant. Thus, it depends upon the sy stem , and on the substituting atom , and the p o ssib ility of re arrangem ent is som ething which the organic chem ists have larg e ly overlooked. This is why the organic chem ists are becom ing so interested in this kind of experim ent. I don't know how fundam ental this type of e x p e ri m ent re a lly is , but I can only say that it is being recognized now by the organic ch em ists, and that gives me some satisfaction. F .S. ROW LAND: Many years ago when we were investigating the re a c tions of re c o il t r itiu m atom s with substituted benzoic a cid s, we found that there was som e substitution other than in the position of the displaced atom . F o r ex am ple, w ith para- chlorobenzoic acid, we observed m o stly p a ra - tritia ted benzoic a cid, but there was som e replacem ent of chlorine with the fo rm a tio n of m e ta - tritiate d benzoic acid. This substitution in the different location represented a re la tiv e ly s m a ll fra c tio n of the total y ie ld , with m ost of the substitution appearing as a d ire c t replacem ent of chlo rine by tr itiu m in the sam e m o le c u la r position. K la ra B E R E I: We have recently m ade experim ents with di-substitut.ed benzene derivatives such as chloronitro- and chlorofluorobenzene. In these system s, we have also not found any is o m e riz a tio n or rearrangem ent. Y. L E E : I think the p ro b le m of m ig ra tio n should use the chem ical re action tim e as a com parison — to determ ine whether re action is fa ste r or slow er than m ig ra tio n . We did investigate one sp ecial system , looking fo r flu o rin e m ig ra tio n in dichloroethylene. By taking C H 2= CC 1 2 and CHC 1 = C H C 1 and re acting them with fluorine ato m s, if the flu o rin e m ig ra tio n is fa s te r than the chem ical reaction then the substitution reaction re p lacing ch lo rine or hydrogen w ill be dom inated by the exoergicity. On the other hand, if the m ig ra tio n is slow , then the hydrogen-substitution re action w ill be propo rtionate ly high er in yie ld. In the C H 2= CCl2 case, H -substitution and Cl-substitution would become a lm o st equal. If this is found, it m eans that m ig ra tio n is not im po rtan t. In the CHC 1 = C H C 1 case, we did see about 2 5 0 to 1 , favouring replacem ent of Cl ra th e r than H. In CH 2 = CC 12, we did see su b stan tia l hydrogen lo ss, which m eans that m ig ra tio n is very slow. J .P . A D L O F F : At the beginning of this d iscu ssio n , Stocklin m entioned that there are three ne u tra l m onovalent species which can be used fo r hot atom studies. There are at least two m ore: p o sitro niu m and m u onium . Of course, these are re a lly isotopes of hydrogen. Perhaps la te r we can have som e discu ssio n of these hot species. M. NEWTON: I w ish to ask Rowland about the fin a l status of the bond energy effect upon abstraction . You were the fir s t to m ention this ten years o r so ago, and now the tra je c to ry calculations find that it is re a lly a b a r r ie r effect? Is this in te re stin g , or is it obvious? Is there always a monotonie re la tio n sh ip between the b a r r ie r and the bond energy? If there is , then it's not very in teresting . If there is n 't, then you should find a m olecule that doesn't fit on to this m onotonie pattern, and you would get a v io la tio n of your bond energy dependence. B ut of course, you'd need an ab in itio c a l culation to te ll you what the b a r r ie r actually was. F .S . ROW LAN D: Your fir s t question was, "Is the re la tio n sh ip between bond energy and b a r r ie r height fa ir ly w ell e s ta b lis h e d ? " and is it always the sam e? The answ er to this question is that fo r a homologous series
160
ROWLAND
there are what are called the Evans-P olanyi c o rrelation s fro m the 1 9 3 0 s which relate the activatio n energy â&#x20AC;&#x201D; which is a m easure of the b a r r ie r height to the bond energy. If we make a c o rre latio n of yields with bond energy fo r hydrocarbons, then im p lic itly we also m ake a correlation with b a r r ie r height. M. NEWTON: But you can extend your bond energy re lationsh ip beyond sim p le homologous series. F.S. ROW LAN D: The Evans-P olanyi plots are different fo r different homologous s e rie s. The coefficient re la tin g activation energy to bond energies is not constant fro m one homologous series to a different one. One could probably m ake checks of the kind you suggest.
IAEA-PL-615/10
HOT ATOM CHEMISTRY OF MONOVALENT ATOMS IN ORGANIC CONDENSED PHASES G. STO CKLIN Institut f端r Nuklearchemie, K F A J端lich GmbH, J端lich, Federal Republic o f Germany
Abstract H O T A T O M C H E M IS T R Y O F M O N O V A L E N T A T O M S IN O R G A N IC C O N D E N S E D PH A S E S . T h e advantages and disadvantages o f h o t a to m studies in con d en sed o rga n ic phases are con sid ered , and r e c e n t advances in c o n d en sed phase o rg a n ic h o t a to m ch e m is try o f r e c o il tritiu m and h a lo gen atom s are discussed. D eta ils are p resen ted o f th e p resen t status and u n d ersta n d in g o f liq u id phase h o t a tom ch e m is try an d.also th at o f o rg a n ic solids. T h e con seq u en ces o f th e A u g e r e ffe c t in con d en sed organ ic system s are also co n sid ered .
INTRODUCTION W hile it is already d ifficu lt and alm ost im po ssible to establish a general theory for hot tr itiu m and halogen atom reactions in the gas phase, it seems not even worth while try in g for condensed organic system s. The liq u id and solid phases are often considered as "m e ssy " and, indeed, m any of the num erous studies w hich have been c a rrie d out since the ea rly 'th irtie s co n firm this p e s s im is tic view since they do not provide any pertinent in fo r足 m ation except product yields and highly speculative explanations. The question, however, a rise s why should we study liq u id or even solid organic system s, esp ecially when even gas phase studies do not provide enough detailed in fo rm a tio n and when the present trend is shifting tow ards beam techniques with single c o llisio n conditions? To evaluate this question we have to analyse the advantages and disadvantages of hot atom studies in condensed organic phases as com pared with the gas phase. The advantages can be su m m a rize d as follow s: (1 ) (2 ) (3 ) (4 )
G reater v arie ty in choice of systems H igher yields G reater relevance to applied aspects P o s s ib ility for in-situ detection of unstable interm ediates
Gas phase studies are n e c e ss arily lim ite d to s m a ll m olecules; thus, a num ber of in teresting organic system s and biom olecules cannot be studied. In gaseous system s the apparent yields, p a rtic u la r ly those of the m ost in te re stin g substitution products, are in general considerably s m a lle r than in the condensed phases and lie in the range of 0 . 1 to 1 0 %. W ith respect to applied aspects, such as la b e llin g and transm u tatio n effects in b io lo gical system s, the condensed phase is of great im portance. F in a lly , the solid organic phase in some cases lends its e lf to the in-situ detection of specific
161
162
STÔCKLIN
secondary ra d ic a ls by p h y sical m ethods, in p a rtic u la r the ESR-techniques, as w e ll as to the investigation of post- recoil annealing reactions. The disadvantages, however, are severe. V ery generally they can be s u m m a rize d as follow s: (1 ) (2 ) (3 ) (4 )
G reater m u ltip lic ity of overlapping in teractions T h erm al processes obscure hot re action channels L ittle in fo rm a tio n on energy and charge states E x istin g theories are not applicable
In the condensed phases a v a rie ty of p rocesses, such as energy loss, charge tra n s fe r, ra d ic a l fo rm a tio n and u ltim a te bond fo rm ation occur in a c om paratively s m a ll volum e with an effective diam eter of a few to some hundred  . Thus, there is a considerable overlap of com peting processes. B esides, the greater uncertainty about the charge and energy state of the re acting species at the tim e of bond fo rm a tio n , com bination and re com bination w ith secondary or parent ra d ic a ls , lack of bond rupture etc. , m ake it alm ost im po ssible to distinguish unam biguously between contributions fro m hot and th e rm a l reactions to the product fo rm ation, m a in ly owing to a lack of ex peri m e ntal techniques such as m o dera to r and to a c ertain extent scavenger m ethods, w hich can be successfully applied in the gas phase. F u rth e rm o re , the oretical approaches with respect to energy loss, charge exchange, k inetics and m e ch anism of in teraction are extrem ely d ifficu lt or a lm o st im po ssible to apply in condensed phase organic hot atom c h em istry . Even though the kinetic theory of hot atom reactions was tentatively applied to the liq u id phase [1] by u sing a num ber of d rastic a ssum p tions, its a p p lic a b ility to the condensed phases seem s highly question able. It should be pointed out, however, that some of these difficu ltie s are also v alid to a large extent for m any of the gas phase hot atom studies, if nu cle ar re c o il m ethods are used. If one wants to study hot atom reactions under single c o llis io n conditions, the only answer is a beam experim ent. Thus, gas phase hot atom c h e m istry of m onovalent species by m eans of n u cle ar m ethods m ay have already reached its lim its and only beam ex peri m ents m ay provide fu rth er in fo rm atio n. Hot atom c h e m istry in the condensed organic phases, on the other hand, re lie s alm ost exclusively on nuclear m ethods (and to a le sse r extent on im plantatio n of accelerated ions). In the follow ing we outline some recent advances in condensed phase organic hot atom c h e m istry of re c o il tr itiu m and halogen, p redom inantly in the liq u id phase, and discuss some aspects of decay-induced reactions and consequences of the A uger effect in condensed phases. The follow ing questions sh all be raise d: How im p o rtan t is caging? Is the im pa c t m odel applicable? Does W alden in versio n occur? Is com plex fo rm a tio n possible? A re conform ation al effects a c rite rio n fo r direct processes? How significant are re ac tiv ity and selectivity effects? Is the A uger explosion m odel applicable? How im po rtan t is A uger ra d io ly sis?
IAEA-PL-615/10
163
LIQ U ID PHASE HOT A T O M CH EM IST RY O F R E C O IL T RIT IU M AND H A L O G EN In this section we discuss some aspects of liq u id phase hot atom ch e m istry of m onovalent re c o il atom s re sulting from induced nuclear re actions. We w ill consider only those cases where c o m p licatin g factors such as the charge state of the re acting re c o il atom or the A uger effect and its consequences can be avoided, i. e. we w ill re s tric t ourselves to re c o il tr itiu m , fluorine and chlorine. A lip h atic systems Cage effects In saturated gaseous system s p rim a r y products are p redom inantly form ed by abstraction and replacem ent o c c u rrin g by d ire c t m e ch anism s, i. e . single-step processes not involving p ersiste nt in term ediates. E xcitatio n decom position of such products often occurs but is generally identifiable (for review s cf. R efs [2 - 7 J). W hile hot hydrogen reactions are frequently re la tiv e ly phase independent [8 , 9 , 2 3 , 2 4 , 3 2 ], yield spectra of hot halogen processes are at least quantitatively d ra s tic a lly different when going to condensed m edia (cf. e. g. , R efs [2 , 3 , 7 , 3 2 ]). Despite m uch discussion on these differences th e ir o rig in re m a in s speculative. This is la rg e ly because scavenger techniques are unable to distinguish between direct hot reactions and th e rm a l processes o c c u rrin g in the solvent cage. A cen tra l concept in condensed phase hot atom ch e m istry is FranckR abinow itch caging [1 0 ], which had already been postulated in the e a rly days of hot atom c h em istry [2 , 1 1 , 1 2 ]. There are two types of im m ediate cage effects which contribute to substitution products fro m m onovalent hot atom s: ( 1 ) C om bination after displacem ent of a substrate atom X by a hot atom [Y]*
[ Y - ]* + R X - { R â&#x20AC;&#x2DC; + X ' + Y ' }
cage
^
RY
(2 ) R eco m bination after excitation decom position of a fir s t form ed hot substitution product
[Y-]*+ R X ^
[RY ]
-desame.-Âť {r - + y ' }
exc.
cage
-RY
Caging can be influenced by a ll phenom ena which affect out-of-cage diffusion, nam ely m a ss of the ra d ic a ls , density, v isc o sity , tem perature and, of course, the re a c tiv ity of the solvent m olecules tow ards the ra d ic a ls involved. A straigh tforw ard approach to the investigation of cage effects is to study the influence of a continuous increase in density fro m the gas phase up to clo sely packed liq u id or solid on the yields of m a jo r products. In a pioneering experim ent of this type an apparently lin e a r rise in yields was
164
STÓCKLIN In te rm o le c u la r 3¿.6
5.15
3.25
2.33
U3
D is t a n c e , Â 1.31
100
0.7¿
0.52
0.35
020
0.07
D ensity [g cm-3] FIGД. Absolute yields of substitution products as a function of density in the C H 3F-I2 system: O: C H 31SF; A : C H 2F18F. Temperature at 55°Cunless otherwise specified. Richardson and Wolfgang [14].
observed by R ice and W illa r d [1 3 ]. More recently R ich ard son and W olfgang [1 4 ] p e rform ed a very inform ative experim ent on the gas to liq u id phase tra n s itio n in the 18F /C H 3F system . The re su lts are shown in F ig . 1 . Absolute yields of 18F-for-H and 18F-for-F substitution in CH3F were m e asu red as a function of densities from 0 . 0 0 1 4 g c m "3 (gas at 1 . 1 atm ) to 1 . 1 g cm "3 . Both the C H 318F and the C H 2F 18F yields show a sharp rise fro m the lowest gas densities, le v e llin g off at about 0 . 2 g c m '3 (5 0 - 1 0 0 atm ), a behaviour c h a ra c te ristic for c o llis io n a l de-excitation of highly excited p r im a r y products. Above the c r itic a l density a second ris e , even greater in m agnitude, was observed and ascribed to cage effects. It was also indicated on the basis of F ig . 1 that caging becom es im po rtan t only when
IAEA-PL-615/10
P r e s s u r e [torr)
F IG .2 .
165
P r e s s u r e Itorrl
C o m p a riso n o f pressure and phase d e p e n d e n c e o f h ot T - f o r - H and 38C l - f o r - C l substitution y ie ld s
in r a c e m ic and m e s o (CHC1FJ¡¡.
M a c h u lla and S to c k lin [ 3 2 ] .
m ean in te rm o le c u la r distances shrink to about h a lf the diam eter of the fluorine atom . It is in te re stin g to note that the F /C H 3F is highly se lf scavenging (hydrogen abstraction by F-atom s has an activation energy close to zero); this m eans that the "cage w all" is highly reactive. F u rth e r m o re , no tem perature effect could be observed. Is the large second rise of the product yields re a lly only due to ra d ic a l- ra d ic a l cage (re)com bination? We w ill discuss this question now in the context of stereo-chem ical aspects. E xcitatio n decom position The high c o llisio n density in the liq u id phase obviously allow s an effec tive energy tra n sfe r. It is therefore not su rp ris in g that the phase change also affects the extent of excitation decom position of a fir s t form ed sub stitution product. This generally contributes sig nifican tly to the increase in apparent substitution product yields and a concom itant decrease in the yields of the decom position products. W hile reactions of re c o il tr itiu m with sim ple alkanes give quantitatively the same product spectrum in both
166
STÓCKLIN
6 -
гас.
сн3он
CH30 H
.c ' ^ 1 2 n -C g H ^
o
O
0.2
OA
0.6
0.8
1.0
M ole Fraction o f Solvents FIG.3. Solvent effect on the stereochemical course (ratio retention/inversion) o f 38C l-for-C l substitution in rac- and meso-2,3-DCB. Key: ®:
rac-DCB-Br2;
□ : rac-DCB-CH3OH;
Д: C:
rac-DCB-n-C5H i 2 ; meso-DCB-Вгг;
O: rac-DCB-c-C6Hi2 ; ■ : meso-DCB-СНзОН;
A:
meso-DCB-n-C5Hi 2 ;
• : meso-DCB-c-C6Hi2
Vasaros, Machulla and Stocklin [3 1 ].
gaseous such as ficantly fro m an
and liq u id phase [8, 9 , 1 5 ], the rm o d y nam ically less stable system s cyclobutane [1 6 ], butenes [1 7 ] and alky l halides [1 8 -2 0 ] are sig n i affected with respect to the yields of those products which arise interm ediate excited substitution or addition product.
Stereochem istry The stereo ch em ical course of a hot re action can provide im po rtant in fo rm a tio n on the basic m e ch anism of the substitution process. E nergetic substitution re actions by hot nucleogenic tr itiu m and halogen atom s at a sy m m e tric carbon atom s occur p redom inantly w ith retention of configura tion [2 1 -3 3 ]. This has been dem onstrated for both the gaseous and the condensed phases, even though the stereo sp ecificity in the condensed phase is generally somewhat s m a lle r. The alm ost com plete retention of con figu ratio n in the gaseous phase has been generally explained on the basis of the W olfgang Im pact M odel [3 4 ], assum ing a direct replacem ent process on a tim e scale com parable to m o le c u la r tra n s it tim e s for the hot atom .
167
1АЕА-РЬ615/10
Retention _ Qrt IR T I + QrgÍR G K opg ÍRG ] Inversion ' Ort IRT] + OrgIRG]
[ Retention 1 . 0 m tIM T 1 *0 ^ t[M G ] [Inversion] " Omt IM TI + OmgIMG]
7.0- rae. DCB 1
m eso DCB | 60Q. X q>
6.0
5.0
4 *7
i5 £ H
с 4.0-
с 4.0о с Чл _0) а> 3 0 "ф > С О
СС с
О
V )
fc 3 0 >
с-Нехап □ n-Rentan • Bromine 0 Methanol х pure meso ОСВ а
2 .0 -
1.0-
д с-Нехап о n-Pentan • Bromine 0 Methanol х pure meso DCB
20
1.0-1
/
Ф
1.0
2.0 3.0 4.0
0.0
0.0
0.0 1.0 20 3.0 4.0 5.0 6.0 ( Retention \ _ 0.13 mt > ¿.so mg I Inversion L ,J 1 . x mt ♦ 0 26 mg
0.0
( Reten tion ^
Cl
C l y 'K y H
с Н з ч Н 'ч - н
С Н з^Р ^Н
C H j- ^ Ç ^ H
5.0 6.0
g25RT«2.06RG*1.7¿RG' 1.00RT. 0.15RG
I In v ersio n /calc.
ci
ci
Н ^ И ^ -С Н з
F IG .4 .
c =
Cl c t y + Y CH3 CH3^ Ç ^ - H
Cl H y '+ 'y a CH 3^Ç>H
Cl
СНз
Cl
H
CH3
MT
MG
RT
RG
RG’
C o m p a riso n o f c o m p u te d and m easu red r a tio (r e te n tio n / in v e r s io n ) in r a c e m ic and m e s o 2 ,3 -D C B in
d iffe r e n t so lv e n ts .
M a c h u lla and S to c k lin (u n p u b lish ed results;
s ee a lso R e f . [ 3 1 ] ) .
estim ated to be about 1 0 "14 - 1 0 "13 s. Thus, on the ba sis of the im pact m o del, W alden in versio n should not be possible in hot atom reactions. There are, however, recent e x p erim ental re s u lts , which indicate that W alden in v e rsio n can occur in halogen hot atom substitution reactions [3 1 -3 3 ]. The im pact m o del was o rig in a lly established for re c o il tritiu m in the gas phase, and it re m a in s questionable whether it can be sim p ly applied to h eavier atom s o r even to the liq u id phase. The som ewhat sm a lle r stereo specificity in the condensed phases was generally ascribe d to a con trib u tio n from ra d ic a l- ra d ic a l cage com bination re actions, allow ing racem ization of the interm ediate ra d ic a l. A gain, there are indication s (see below) that this is not n e c e ss a rily the case and W alden in version m igh t occur to a la rg e r extent in the liq u id phase [3 2 ]. How does a continuous increase in density affect the stereo ch em ical course? T his is shown in F ig . 2 for T-for-H and C l-for-C l substitution in m eso and ra e . 1 , 2 - d ic h lo ro - l, 2 -difluoroethane 1 3 2 ]. It can be seen that the density dependence in T-for-H substitution is either v ery s m a ll [meso (CHC 1F ) 2] or negligible [rac. (CHC 1F ) 2 ] when com pared with C l-for-C l substitution. In the latte r case the effect is alm ost id e ntica l in both d ia ste re o m e ric substrates. The inverted product yield re m a in s n eg lig ibly s m a ll over the entire p re ssure range and the ju m p occurs only
168
STOCKLIN
when going to the liq u id phase, w hile the retained product shows the fa m ilia r increase already in the gas phase, as expected on the basis of enhanced c o llis io n a l stab ilizatio n of the firs t form ed excited product. The density dependence of the 38Cl-for-Cl substitution yields resem bles the trend observed by R ich a rd so n and W olfgang in the 18F / C H 3F system (see below), and it is tem pting to attribute the gas-to-liquid phase effect and the con com itant decrease in stereo specificity to ra d ic a l caging. This would affect the chlorine but not so m uch the s m a lle r tr itiu m atom s. Even though there is little or no in fo rm a tio n on the therm odynam ic and kinetic p ara m e te rs of ra c e m iza tio n versus (re )com bination and on the stereo chem ical course of the • CHF - CHC 1F + 38C 1 - (re)com bination (is the ra d ic a l p lan ar? ), the a lm ost id e n tica l phase effect in both dia ste re o m e ric substrates, together w ith the different density behaviour of the two dia ste re o m e ric products, are difficu lt to explain on the basis of ra d ic a l caging. How im po rtant is ra d ic a l caging in a self-scavenging system such as 18F / R X or 38C 1 /R X , where H -abstraction by th e rm a lize d halogen atom s is extrem ely fast? There are a num ber of ex perim ental re su lts which contradict a m a jo r c ontribution fro m caging in these system s: (1 ) (2 ) (3 ) (4 )
Lack of m ass effects [3 1 ] Lack of tem perature effects [1 4 , 3 1 ] Lack of v isc o sity effects [3 1 ] Ide ntical phase effect on stereochem ical course in m eso and rac. (C H FC 1 )2 [3 2 ] (5 ) C onfo rm ational effect on stereochem ical course [3 1 ].
The c onform ation al effect deserves a separate discussion.
C o nform ational effect It has recently been dem onstrated by V asaros, M achu lla and St'ocklin [3 1 ] that the stereo chem ical course of Cl-for-Cl substitution in ra c . and meso 1 , 2 -dichlorobutane (DCB) can be d ra s tic a lly influenced by solvents with different d ie le ctric constants (cf. F i g . 3 ). The stereo chem ical course of the substitution can be v arie d over a wide range (from about 3 5 to 8 5 % retention) by changing the type and the concentration of the solvent. The magnitude of the effect is quite different for meso and rac. 2 , 3 -DCB. The different behaviour of the two d iastereo m ers cannot be explained by secondary effects, such as different contribution from th e rm a l ra d ic a l reactions or solvation. In fact the lack of s im ila r ity seems to indicate the absence of such effects. The m ost lik e ly explanation of the solvent effect is a changing stereochem ical influence caused by changing the concentration of the s te ric a lly different ro ta tio n a l is o m e rs . A fter c arry in g out a conform ational analysis the authors c o rrelated the stereo ch em ical course (rete ntion/inversion) to the concen tra tio n of the conform ers assum ing different reaction cross-sections for substitution with retention (a) and substitution with in version (a1) for each of the conform ers. A tria l- an d - e rro r treatm ent showed a cle ar dependence of the stereo ch em ical course on the substrate conform ation. The relative cross-sections have now been recalculated by the authors with a com puter p ro g ra m for the determ ination of v aria b le s .in a non-linear function. The fitting together with the kinetic equation and the calculated cross-sections
169
IAEA-PL-615/10 -x-
RY
[Y R X ]* ►Products
( T ) caged complex'
•Y * + RX hot atom 1 -100 eV
YRX 10 -10'lJsec
■Y+R+X ( ! ) cag e combination 10‘’0-10'9
RY
( 3)
RY
direct
------- ► [ R Y ] * -x-
excited product 1-10 eV
F IG .5 .
+M 10‘'3-10'8sec
decomposition Products elimination
P o ssib le path w ays for h a lo g e n - fo r - h a lo g e n substitu tion in a lip h a tic system s.
are shown in F ig . 4 . W ithout going into de ta il on the p h y sic a l m eaning of the c ross-sections, it can be stated that the fittin g is reasonably good, thus p roviding fu rth er evidence that the observed solvent effect is indeed a con fo rm a tio n a l effect. If so, the conform ation al effect m igh t provide evidence for a one-step substitution m e ch anism . A change of conform ation during the im pa c t and before bond fo rm a tio n also seem s u n lik e ly since the transien t tim e du rin g which bond fo rm a tio n takes place (about 10"13 s) is several o rders of m agnitude s m a lle r than the tim e re qu ire d for ro ta tio n a l changes of conform ation (1 0 “9 - 1 0 ' 8 s). Even though the observed effect does not unam biguously dem onstrate the absence of caged ra d ic a l reactions (conform a tio n a l effects m ight also affect interm ediate ra d ic a ls provided they are non-planar) it deserves further attention as a possible new tool in hot atom c h em istry . Studies on conform ational effects in the gas phase are desirable but d iffic u lt, since the changes have to be induced by tem perature changes. In p rin c ip le , tem p erature effects can be expected but th e ir detection turns out to be v ery d ifficu lt because of the low yields in the gas phase, p a rtic u la r ly for the inverted substitution products. The e q u ilib riu m concentration of the conform ers in the gas phase is com parable to that in liq uid system s at high d ilu tion with a non-polar solvent, but the stereo chem ical course is com pletely different. In p rin c ip le , conform ation al effects m ay overshadow the p rim a r y substitution event in stereo chem ical investigations when u sing diastereo m ers for ex perim ental sim p lic ity . F ir s t experim ents on the stereo ch em ical course
170
STOCKLIN
101
102
103
104
" quid
P r e s s u r e [torr] -------- â&#x2013;ş F IG .6.
C o m p a riso n o f pressure and phase e f f e c t on 38C l - f o r - H and 38C l - f o r - F su bstitu tion in th e flu o ro b e n z e n e
system .
M a c h u lla and S to c k lin [ 4 0 ] .
of hot C l-for-C l substitution with enantiom eric m olecules (d- and 1 - 2 chlo ropro pionylchlo ride) have been c a rrie d out by W o lf and co-workers [3 3 ]. In tere stingly enough, the authors observed 8 1 % in version even in the gas phase. Conclusion The m e ch anism leading to T-for-H substitution seem s to be ide ntical in the gaseous and liq u id phases. Caging is u nim p o rtant and substitution pred om in an tly occurs with retention of configuration even in the condensed phases. As in the gas phase T-for-H substitution in saturated liquid system s proceeds v ia a direct process, possibly according to the im pact m odel. H alogen-for-halogen substitution in the liq u id phase cannot be s a tis  fa c to rily explained by the im pa c t m odel nor by ra d ic a l caging. There is in cre as in g evidence that ra d ic a l caging in self-scavenging system s plays a m in o r ro le and that W alden in version m ight be favoured in the liq uid phase. It m ig h t be possible that the high densities favour a caged complex [3 2 ], i. e. an e le c tro n ic a lly unstable m olecule which is held together by
1АЕА-РЬ615/10 Recoil Tritonotion [38]
Recoil Chlorination [45]
171 Electrophilic Substitution [46| (for comparison)
CH3
CH, 178Г
C6H5CH3
CH3
-,178
2 0 .0 [< ¿ ^ 2 0 .0
30.5i^ ¿ N 3 0 . 5
2 1 .6 ls ^ 2 1 .6
1 4 .5 l^ ~ ¿ J l4 .5
r o i y j r o
21.2
31.0
=1
37
0.25 (nucleus only)
60
Radiolytic Chlorination 1441
Photo Cyanation [47]
k c 6He
Recoil Chlorination [411
20.5 r y - C 4! 20.5
16.8r<-^|16.8
18.5|^>j18.5
le.sLV—^J 18.5
10.9
1 9 8 lw J l9 8
220
10.9 44.4
1.0
23.6
0.59
k c«H6
F IG .7 .
S e le c t iv it y and r e a c t iv it y in liq u id p hase h o m o ly t ic su bstitu tion o f to lu e n e and flu o ro b e n z e n e
(e le c t r o p h ilic c h lo r in a tio n o f to lu e n e in s o lu tio n show n fo r c o m p a ris o n ).
the surro un ding m olecules for a tim e sufficient for configu rational changes. In th is case three pathways m ay lead to the substitution products (Fig. 5 ): (1 ) A d ire c t process (2 ) Caged ra d ic a l com bination,and (3 ) A caged complex F u rth e r ex p erim ental and th e o re tic a l work is needed to understand hot hom olytic substitution in the liq u id phase. A ro m a tic system s The in-situ production of nucleogenic species in nu cle ar tra n sfo rm a tio n s can be conveniently used to study hot hom oly tic a ro m atic substitution (SHH 2 ). The technique p rovides a v arie ty of com plem entary features when com pared w ith c la s s ic a l m ethods, such as solution c h e m istry , p hotolysis, ra d io ly sis or high-tem perature c h em istry . The inhe rently s m a ll concentration of the nucleogenic species does not lead to double substitution nor to chain re actions. The disadvantage is that a re la tiv e ly large frac tio n of the hot atom s (about 10 to 20%) appears as unidentifiable higher b o ilin g o r "p o ly m e ric " products, probably re su ltin g fro m addition of the hot or th e rm a liz e d atom at
17 2
STOCKLIN
the a ro m atic rin g in the firs t step. G arland and Row land [3 5 , 3 6 ] suggested the fo rm a tio n of an interm ediate T- cyclohexadienyl ra d ic a l, which in the gas phase m ay decompose to yield benzene-t. F u rth e rm o re , the inorganic yield is v ery high (6 0 - 8 0 %) and the relevant substitution product yields are only in the range of 1-20% in scavenged liq u id system s. The in tra m o le c u la r selectivity of re c o il tr itiu m in m ono-substituted benzenes is ex trem ely low and close to a s ta tis tic a l d is trib u tio n both in the gaseous [3 7 ] and liq u id phase [3 8 ], while the halogens exhibit sm a ll selectivity and re ac tiv ity effects [3 9 , 4 1 , 4 4 , 4 5 ].
The gas to liq u id phase tra n sitio n The density effect on the substitution yield in a ro m atic system s, as recently determ ined by M a c hu lla and Stocklin [4 0 ] is considerably less d ram atic than in alip hatic system s. F ig u re 6 shows the p ressure and phase effect on Cl-for-H and Cl-for-F substitution in the 38C l/ C gH5F system . The gas to liq u id phase effect is either not apparent at a ll (C l-for-F substitution) or s m a ll (Cl-for-H substitution) com pared with 38C 1 in an a lip hatic system (see below and F ig . 2 ). The iso m e r d istrib utio n in Cl-for-H substitution re m a in s e sse ntially unchanged when going fro m the gaseous to the liq u id phase (also indicated in F ig . 6). The s m a ll phase effect, together with the alm ost id e ntica l is o m e r dis trib u tio n , seem s to indicate that ra d ic a l caging cannot be v ery im po rtan t. This is in teresting since the self-scavenging a b ility of the a ro m atic system (o ccurring v ia addition and H -abstraction) is s im ila r or less effective than that in the aliphatic system m entioned below. Hot hom olytic substitution in a ro m atic system s m o st lik e ly proceeds v ia complex fo rm a tio n [4 1 , 4 2 ], at least in the case of T-for-H re placem ent [4 1 ], even in the gase phase. It has been dem ons trated by m eans of m o le c u la r beam experim ents that even under single c o llisio n condition F-for-H substitution in the F / C SH 6F system proceeds v ia a long-lived interm ediate complex [4 3 ]. The different phase effect in a ro m atic and a lip hatic system s m igh t therefore be attributed to the different a b ility of com plex or non-complex fo rm ation ra th e r than to ra d ic a l caging.
Selectivity and re ac tiv ity Even in th e rm a l ra d ic a lic hom olytic a ro m atic substitution selectivity and re a c tiv ity effects are generally s m a ll, and it is therefore not s u rp risin g that in hot hom oly tic substitution these effects are even s m a lle r. System atic studies on the selectivity and re ac tiv ity of re c o il halogen atom s in sim ple a ro m atic system s have been c a rrie d out by Stocklin and co-workers [4 0 , 4 1 , 4 4 , 4 5 ]. In F ig . 7 some re su lts obtained in the liq u id phase with re c o il tr itiu m and chlo rine are com pared with other liq u id phase substitution procedures. It can be seen that re c o il tritĂł n a tio n [3 8 ] proceeds alm ost s ta tis tic a lly while s m a ll selectivity effects are observed for re c o il chlorine [4 4 , 4 5 ]. Com pared with c la s s ic a l e le c tro p h ilic substitution via positive chlorine in solution [4 6 ], both selectivity and re activity effects are ex trem ely sm a ll. On the other hand, this is also true for ra d io ly tic c h lo rination [4 4 ] and photolytic cyanation [4 7 ]. The iso m e r d is trib u tio n therefore gives only lim ite d in fo rm a tio n with respect to the
IAEA-PL-615/10
FIG . 8.
173
P ossib le re a c tio n p ath w ays in th e r e c o il c h lo r in e - h a lo b e n z e n e system .
re lative contribution from th e rm a l ra d ic a l re actions v ersu s that fro m hot substitution processes. As outlined above, hydrogen substitution by re c o il t r itiu m and halogen is lik e ly to proceed v ia ir, Ń Ń&#x201A;-complex fo rm a tio n . Halogen replacem ent is in p rin c ip a l also possible v ia the same in term ediate as a com peting process; it m ig h t, however, also occur v ia a d ire c t replacem ent [4 1 ]. Bond energy effects which have been dem onstrated for T-for-halogen [4 8 , 4 9 ] as w e ll as for halogen-for-halogen substitution [4 1 , 5 0 ] neither prove nor disprove the one or the other m e ch a n ism . If we tentatively assum e that in the p r im a r y re action event between re c o il chlorine and a halobenzene C 6H 5X , only H -abstraction and halogen abstraction compete w ith w- and a-complex fo rm a  tion, a v arie ty of possible re action pathways are in p rin c ip a l possible as outlined in F ig . 8. The fin a l products are H C 1 , X C 1 , p o ly m e rs, C 6H 5C 1 and C6H4XCI. If a ll the re action cross-sections and p a rtia l re action crosssections would be id e ntical, the in d iv id u al substitution products would only be form ed in yields of a few per cent, which is the o rd e r of m agnitude which one indeed finds ex perim entally.
174
STÓCKLIN
Conclusion
The nu cle ar re c o il m ethod is an in teresting supplem entary technique for the study of hom oly tic a ro m atic substitution. Hot hom olytic substitution ( s h h 2) provides some noteworthy features, such as the replacem ent of sub stituent group, the low but apparent selectivity and the sm all-phase effects. It seem s lik e ly that a ll substitution processes occur v ia complex fo rm ation. The detailed m e ch a nism , however, e. g. the fin a l hydrogen- or X - transfe r (cf. F ig . 8), is s till obscure and deserves fu rth er attention.
O RG A N IC SOLIDS P o ly c ry sta llin e and glassy phases In the 'fiftie s and' sixties a num ber of studies were c a rrie d out on compounds with m e ltin g points below room tem p erature. The inform atio n w as, how ever, conflicting and even such t r iv ia l questions concerning whether the to ta l organic yields and yields of specific products d iffe r, when the target compound is in the liq u id or in the solid (at low tem perature) state, could not be c la rifie d . A yres and R ack [5 1 ] found that the correspond ing yields for p o ly c ry stalline alkyl iodides at 7 7 K are ide ntical to the liq uid state values; however, the y ields fo r glassy n-propyl and n-butyl iodides are 8 to 1 2 % higher. (It m ay be appropriate to note here that the m easured yields were found to be influenced by the isotopic exchange du rin g the ex traction, and respective c o rrectio ns were obtained fro m a plot of organic yield v ersu s tim e of extraction. S u rp ris in g ly enough, in m any older studies this was m isse d , despite the fact that the phenomenon of separation-induced isotopic exchange had long been known in ra d io c h e m istry . ) Studies of c h e m ic a l effects of nu cle ar tra n sfo rm a tio n s in glassy and p o ly c ry stallin e states, in itiated in W illa r d 's lab orato ry, offer some essen tia l advantages, e. g. they can be com pared w ith the ra d ia tio n c h em istry data fo r the sam e system s and under id e ntica l conditions. A representative study along this line is that by Hahne and W illa r d [5 2 ]. A c r itic a l review er m u st, however, adm it that, as fa r as hot atom ch e m istry is concerned, these studies are d ra m a tic a lly less in form ativ e than the com parative works in glasses and p o ly c ry stalline solids irra d ia te d with io n izin g ra d ia tio n or light. The m o st im p o rtan t feature of low -tem perature ra d ia tio n and photo ch e m ic a l studies [5 3 ] is the application of ESR techniques for elucidation of the nature of electron tra p s and the nature and c h a ra c te ristic s of decay of trapped ra d ic a ls . U nfortunately, this could not be established in hot atom c h e m istry studies in a s im ila r system , because the low concentration of reactive in term ediates did not allow a successful application of the ESR spectrom etry. A nnealing in organic cry stals C ry sta llin e organic halogen derivatives of sufficiently high m e ltin g points are suitable objects for hot atom c h em istry, since they allow annealing experim ents to be c a rrie d out. Solid tribrom obenzenes provided p a rtic u la r ly in te re sting re su lts . S iek ierska and H alpern [5 4 ] reported that sym- and
1АЕА-РЬ615/10
F IG .9.
25
24
127I l n .y ) 128I
125Xe(EC ),25I
liquid [411
liquid [68]
17 5
ï 66f 125Xe(EC),25I g a s [67]
I-for-H[%]
6.1
7(1)
0.6
I-for-F[%]
2.1
3(0,3)
0.6
Is o m e r d istrib u tio n and su bstitu tion y ie ld s resu ltin g fro m th e Ш Х е ( E C ) 1251 d e c a y in liq u id [ 6 8 ]
and gaseou s [ 6 7 ] flu o r o b e n z e n e (v a lu e s in b rack ets a re fo r 0.5 X 1 0 " 2 m o le > I 2 c o n ta in in g s y s te m ) and fro m th e 127 l(n , y ) 1281 process in liq u id flu o r o b e n z e n e [ 5 0 ] .
asym -tribrom obenzenes are is o th e rm a lly annealed at different tem peratures to the sam e plateau value, only the tim e re qu ire d to reach the plateau is tem perature-dependent. This m eans that the tem p erature accelerates the course of the annealing re action, without influencing its extent. This apparently sim p le kinetic pattern diffe rs esse n tially fro m that described in num erous papers concerning inorganic cry stals and re sem bles ra th e r the solution kinetics. The authors [5 5 ] found that the annealing reaction involves pred om in an tly brom ine atom s w hich recom bine with the parent compound or one of its iso m e rs . The data fit fa ir ly w ell a first- o rd e r kinetics. B rom ine atom s appeared to also have a m a jo r contribution to the annealing process in p-dibrom obenzene [5 6 , 5 7 ], but s u rp ris in g ly enough the annealing pattern in th is compound is m o re com plicated. A n in te re stin g contribution to the organic solid state hot atom ch e m istry has been m ade by C o llin s and H arbottle [5 8 ]. The authors studied the in fluence of radiation-produced defects on the subsequent th e rm a l annealing of 82B r in c ry stalline hexabrom oethane and found that both " in tr in s ic " (i.e . those which occur in the absence of defects) and defect-sensitive reactions contribute to the annealing. The studies published so fa r, however, are not m any and the field deserves future attention, p a rtic u la r ly in the lig h t of the ra p id p rog ress in organic solid state ch em istry.
CONSEQUENCES O F THE A U G E R E F F E C T IN CONDENSED O RG A N IC SYSTEMS So fa r we have dealt with hot atom s which do not im pose serious p rob le m s concerning the charge state. In m any nucle ar p rocesses, however, we are faced with the consequences of the A uger process w hich, to a greater or le sse r extent, determ ines the u ltim a te product fo rm ation. This is true to some extent for (n, 7)-processes on heavier atom s owing to the in te rn al conversion of low-energy 7-transitions; it is p a rtic u la r ly true for high ly converted is o m e ric tra n sitio n s and ex trem ely im p o rtan t for EC-decay processes. In the gas phase the "A uge r explosion" seem s to occur because of Coulom b re pu lsio n follow ing charge generation and dis trib u tio n . This m odel was developed by C arlson and W hite [5 9 ] on the basis of charge spectrom e tric
STOCKLIN
176
Dose Effect on Radical Formation in Solid lUdR
______ Absorbed energy " [ergxIO5] F IG .10.
Lack of Annealing at 75 °C
-------- *-Tim elh)
C o m p a riso n o f dose e f f e c t in 5 -io d o d e o x y u r id in e (IU d R ) resu ltin g fro m Е С - d e c a y o f 125I in IU dR
l a b e lle d w ith 1251 and fro m ir r a d ia tio n w ith m o n o e n e r g e tic 6 .4 - k e V X -r a y s . d e te r m in e d b y ESR ( c f . R e f.[7 0 ] ) p lo tte d versus absorbed dose ( l e f t s id e ).
R a d ic a l c o n c e n tra tio n as
L a ck o f a n n ea lin g at 7 5 °С
(rig h t s id e ).
re su lts and aim ed at explaining the observation of m u ltip ly charged fragm ent ions as a consequence of A uger charging at a constituent atom of a m olecule. It seems now to be w idely applicable for the gas phase under low-pressure conditions. K azanijan and Libby [6 0 ] assum ed that the charge created during the A uger process causes the m olecule to explode even in condensed phases, but the n e u tra lizatio n of the ions is ra p id in contrast to the gas phase, thus giving rise to n e u tra l re c o ilin g atom s. Solids, however, m ight be able to tra n sfe r charge v ery ra p id ly over m any m o le c u la r diam eters and it m ight therefore be possible that n e u tra lizatio n occurs before an explosion can be accom plished. C ruset [6 1 ] has recently pointed out, on the ba sis of a M ossbauer ex perim ent, that at least a p art of 57Fe from the E C of 57 Co2+ in the F e C l3 lattice is neu tralized already during the A uger cascade and stab ilize s without the fo rm a tio n of transien t highly charged species. It should be pointed out now that a u n iv e rsa l a p p lic a b ility of the "explosion m o del" was questioned even in the gas phase when Shiokawa et al. [6 2 ] found hydrogen-deficient species such as CH2B r +, C H B r+ and C B r+ in the charge spectrum after X - ra y ir ra d ia tio n or iso m e ric tra n s itio n at the B r atom of C H 3B r. Hot atom chem ists som etim es explained the product fo rm a tio n in condensed system s as re su ltin g from se lf- ra dio ly tica l action of conversion and A uger electrons. This hypothesis has never succeeded in becom ing m o re than a useful speculation. It was proposed by G e issle r and W illa rd [6 3 ] and then applied by several investigators (cf. , e . g . , R ef. [6 4 ]) to explain the s im ila r ity of the observed product dis trib u tio n fro m different nuclear processes (n, 7, IT) and fro m 60Co--y-radiolysis in alky l h a lides. A ccording to this hypothesis the reactions leading to sta b iliza tio n of re c o il atom s are
IAEA-PL-615/10
177
Z) с ^ о 8. С 0 — £
2* 2 ~ J3 о
с — 01
F IG .1 1 .
R eson an ce e f f e c t on r a d ic a l p ro d u ctio n in 5 - b r o m o d e o x y u r id in e (th y m id in e fo r c o m p a riso n ).
R a d ic a l c o n c e n tra tio n as d e te rm in e d b y ESR p lo tte d as a fu n ctio n o f X - r a y e n e rg y .
H a lp e m and S to c k lin [ 693.
c h a ra c te ris tic of ra d io ly sis ra th e r than hot atom re actions. However, the d is trib u tio n of products fro m ra d io ly s is and nu cle ar activation in some system s, e. g. brom oethane-brom ine m ix tu res [6 5 ], diffe rs m ark edly and thus does not support the general v a lid ity of the autoradioly sis hypothesis. The s im ila r ity in the product dis trib u tio n can a ltern ativ ely be explained by the above-mentioned hypothesis of K azanijan and Libby, which accom m odates argum ents that contradict the G e issler and W illa r d view. It is in teresting to note that arom atic substitution v ia halogen decay ions, which in the gas phase pred om inantly occurs by positive halogen ions [66, 6 7 ], seems to proceed v ia n e u tra l atom s in the condensed phase. This is dem onstrated in F ig . 9 . The iso m e r dis trib u tio n and the yields obtained fro m the 125X e (E C )125I decay in liq u id fluorobenzene [68] are alm ost s ta tis ti cal and p ra c tic a lly id e ntica l with those obtained fro m 127I(n, -y)128I in liq uid fluorobenzene. These re su lts are in d ram atic contrast to the re la tiv e ly high selectivity observed, when 125 Xe is allow ed to decay at low p ressures in the same substrate [6 7 ]. The low selectivity in the liq u id phase can only be explained by hom olytic processes. O bviously the positive iodine species are so ra p id ly ne u tra lize d in the condensed phase that they undergo chem ical reactions only as ne u tra l atom s. W h ile the significance of the A uger effect in hot atom ch e m istry is obvious, the actual m odes of post-Auger processes in condensed phases are s till obscure. P a r tic u la r ly in solid phase studies it is open to question
178
STÔCKLIN ■о a>
-О
¿3 F IG .12.
Fraction of Electron Energy ------------------ ►
R a d ic a l c o n c e n tr a tio n in b ro m o d e o x y u rid in e per u n it e n e r g y absorbed versus fra c tio n o f to ta l
e le c t r o n e n e r g y c a rrie d b y p h o to - or A u g e r -e le c tr o n s .
H a lp ern and S to c k lin [ 6 9 ].
whether the A uger effect m anifests its e lf owing to the increased electron ra d io ly s is o r to the p r im a r y charging. A straigh tforw ard approach to the p rob le m was suggested recently by H alp ern and Stocklin [6 9 ] in a study in which conditions s im ila r to those existing after c ertain nucle ar processes were sim ulated by inducing the A uger tra n sitio n v ia a photoeffect. To study the role of the A uger effect in organic solids it was necessary to use m ono energetic X - rays fro m an adjustable source to create selectively the inner shell vacancy in a p re c ise ly defined region of a m o le cule. F o r this purpose the biom olecule 5 -brom odeoxyuridine (BUdR) o r 5 -iododeoxyuridine (IUdR) was used. H alogenated nucleosides are extrem ely suitable objects for relevant studies, not only because of the im po rtant role they play in radiobiology and nu c le ar m e dicine, but also because of the strik in g stab ility of c ertain ra d ic a ls produced in them upon ir ra d ia tio n with io nizin g radiation. The authors found [7 0 ] that E C decay of 125I in iododeoxyuridine also yields v ery long-lived ra d ic a ls . Thus, in contrast to m ost previous attem pts in hot atom c h em istry , ESR spectroscopy was applicable. F o r exam ple, the ra d ic a l fo rm a tio n in IU dR was studied by using (a) Е С -decay of 125I in lab elled IU dR , and (b) ir ra d ia tio n w ith m onoenergetic X - rays of 6 . 4 keV, w hich is the prope r energy to ionize the L - shell of iodine. It can be seen in F ig . 1 0 that both Е С -decay and irra d ia tio n with the resonant energy for the L- sh ell leads to very s im ila r yields. A fter a short in itia l lin e a r ris e a pseudo plateau is reached at re la tiv e ly low doses corresponding to about 1 0 3 rad. This saturation behaviour does not re sult fro m the eventual e q u ilib riu m between the fo rm a tio n and disappearance of ra d ic a ls . These ra d ic a ls are known to be ex trem ely stable at room te m p e ra ture . F u rth e rm o re , attem pts to anneal at 7 5 °C (right-hand side of the figure) failed . The sig nal re m a in s unchanged.
IAEA-PL-615/10
17 9
If the A uger effect is p a rtic u la r ly effective in producing ra d ia tio n dam age, one should observe a resonance effect at the К-edge of the absorbing heavy atom . This resonance effect is shown in F ig . 1 1 . Here the ra d ic a l concen tra tio n s in BUdR, n o rm a liz e d per unit dose absorbed, are plotted versus the energy of incident ra d iation. An abrupt change occurs in the ra d ic a l concentration per unit energy absorbed at the К-edge of B r, showing a threefold in crease. The changes in the ra d ia tio n effect are not due to a g reater am ount of energy absorbed — they are expressed per u nit dose — but apparently due to the fact that the efficiency of producing ra d ic a ls diffe rs w ith photon energy. We deal here with a true ra d ia tio n resonance effect. In contrast, no resonance behaviour was observed in thym idine (TdR) irra d ia te d under the same conditions, obviously because of lack of the B r atom in th is m olecule. This resonance behaviour bears evidence that when ever the A uger tra n s itio n occurs it a m p lifie s the re su ltin g m o le c u la r conse quences. R ecently a s im ila r a m p lifie r effect was observed [7 1 ] for the in activation of the m etalloenzym e carbonic anhydrase irra d ia te d with monoenergetic X - rays of energies in the v ic in ity of Z n К-edge. This effect was accom panied by a concom itant release of Zn. To obtain in fo rm a tio n on the re lative im portance of A uger ra d io ly sis v ersu s charge buildup, the re su lts fro m BUdR (cf. F ig . 1 1 ) have been fu rth er evaluated. In F ig . 1 2 the ra d ic a l yie ld, as determ ined fro m ESR m e asu rem ents, is plotted v ersu s the calculated energy c a rrie d off by A uger electrons and by photoelectrons. It can c le a rly be seen that, while the ra d ic a l concentration increases lin e a rly with the energy fraction c a rrie d by A uger electrons, it concom itantly decreases with the fraction of energy of photoelectrons. This m ay appear strange at firs t sight since the energy deposited by photoelectrons is alm ost the same or even higher than by A uger electrons. Both photoelectrons and A uger electrons have re la tiv e ly short ranges in solids and should produce ra d io ly tic decom position in the v ic in ity of the o rig in a l m o le cu les. Thus, the ra d io ly tic a l damage produced by photoelectrons is expected to be of the same extent or even high er than that fro m A uger electrons. F ig ure 1 2 , however, dem onstrates that this is not the case. We therefore have to conclude that another m e cha n is m , closely related with the em issio n of A uger electrons, is operative. This leaves the preceding charging process as a m a jo r source of ra d ia tio n damage.
Conclusion In the liq uid phase positiv ely charged halogen ions re su ltin g from the A uger process are probably so ra p id ly ne u tra lize d that they undergo ch e m ic a l reactions as neu tra l atom s. In solids it m ay not even come to a Coulom b explosion. The m u tlip ly charged p rim a r y ion, however, m ay supply sufficient energy for further m o le c u la r decom position. P re su m a b ly , this does not occur v ia Coulom b re pu lsio n , as in the gas phase, but ra the r v ia a fast n e u tra lizatio n , w hich m o st lik e ly leads to excitation decom position. The im portance of the A uger effect with respect to a selective m ic r o su rge ry on large biom olecules and its possible the rape utical use w ill be discussed in a paper by L . F . Feinendegen.
STÓCKLIN
180
REFERENCES М., R a d io c h im . A c ta 2 (1 9 6 4 ) 180.
[1 ]
M IL M A N ,
[2 ]
W IL L A R D , J.E.,
[3 ]
C A M P B E LL, J.G .,
An n . R ev. Phys. C h e m . 6 (1 9 5 5 ) 141. A d v a n c e s in In o rg a n ic C h e m is try and R a d io c h e m is try V , A c a d e m ic Press (1 9 6 3 )
1 3 5 -2 1 4 .
—
[4 ]
W O L F , A .P .,
A d v a n c e s in P h y s ic a l O rg a n ic C h e m is try II, A c a d e m ic Press
[5 ]
W O L F G A N G , R.,
[ 6]
S T O C K L IN , G .,
[7 ]
U R C H , D .S .,
[8 ]
M E N Z IN G E R ,
(1 9 6 4 ) 2 0 1 -2 7 7 .
Progress in R e a c tio n K in e tic s I I I , P e rg a m o n Press (1 9 6 5 ) 9 7 -1 0 9 . C h e m ie h eisser A t o m e , V e r la g C h e m ie (1 9 6 9 ).
M T P In te rn a tio n a l R e v ie w S c ie n c e , In o rg a n ic C h e m is try , Ser. 1, 8 R a d io ch em istry,
B u tterw orth (1 9 7 2 ) 1 4 9 -2 1 2 .
[9 ]
М.,
W O L F G A N G , R.,
J. Phys. C h e m . 72 (1 9 6 8 ) 1789.
LEE, E .K .C ., R O W L A N D , F .S ., J. C h e m . Phys. 36 (1 9 6 2 ) 554.
[1 0 ]
F R A N C K , J., R A B IN O W IT C H , E., T ran s. F a r a d - T o c . 3 £
[1 1 ]
LIBBY, W .F .,
[1 2 ]
LU , С .,
[1 3 ]
RICE, W .E .,
J. A m . C h e m . S oc.
S N Y D E N , S.,
(1 9 3 4 )
120.
62 (1 9 4 0 ) 1930.
J. C h e m . S o c. (1 9 3 9 ) 1273.
W IL L A R D , J.E.,
J. A m . C h e m . S oc. 75
(1 9 5 3 ) 6156.
[1 4 ]
R IC H A R D S O N , A .E .,
W O L F G A N G , R., J. A m . C h e m . S o c. 92 (1 9 7 0 )
[1 5 ]
E L -S A Y E D , M . A . ,
[1 6 ]
LEE, E .K .C ., R O W L A N D , F .S .,
J. A m . C h e m . S oc. 8 £
[1 7 ]
LEE. E .K .C ., R O W L A N D . F .S .,
J. Phys. C h e m . 74 (1 9 7 0 ) 439.
ESTRU P, P ..
W O L F G A N G , R.,
(1 9 6 3 ) 8 9 7 ~
[1 8 ]
T A N G , Y .- N .,
LEE, E .K .C .,
[1 9 ]
T A N G , Y .-N .,
R O W L A N D , F.S.,
J. A m . C h e m . S oc. £7^ (1 9 6 5 ) 3304.
[2 0 ]
C IP O L L IN I, R.,
S T Ó C K L IN , G .,
R a d io c h im . A c t a 9 (1 9 6 8 ) 105.
[2 1 ]
KELLER, H .,
R O W L A N D , F .S ., 7 7 A m . C h e m . S o c. 86 (1 9 6 4 ) 1280.
R O W L A N D , F .S .,
[2 2 ]
K A Y , J.G .,
[2 3 ]
T A N G , Y .-N .,
M A L S E N , R.P.,
[2 4 ]
P A L IN O . G .F ., HENCHM AN,
[2 6 ]
W A I, C .M .,
[2 7 ]
R O W L A N D , F .S .,
J. Phys. C h e m . 62 (1 9 5 8 ) 1373.
R O W L A N D , F .S .,
T IN G , C . T . ,
[2 5 ]
3480.
I. Phys. C h e m . 62 (1 9 5 8 ) 1356.
J. A m . C h e m . S oc. И
R O W L A N D , F .S .,
(1 9 5 9 ) 5050.
J. Phys. C h e m . 74 (1 9 7 0 ) 675.
R O W L A N D , F .S ., J. Phys. C h e m . 75 (1 9 7 1 ) 1 2 9 § Г
М., W O L F G A N G , R., J. A m . C h e m T s o c . 83 (1 9 6 1 ) 2991. T IN G , C .T .,
R O W L A N D , F .S .,
W A I, C .M .,
J. A m . C h e m . S o c. 86 (1 9 6 4 ) 2525.
T IN G , С . T . ,
M IT T E R , G .,
C h e m ic a l E ffe c ts o f N u c le a r T ra n sfo rm a tio n s -
1964, (P ro c . S y m p . V ie n n a , 1 9 6 4 ) 1, IA E A , V ie n n a (1 9 6 5 ) 333. [2 8 ]
W A I, C .M .,
R O W L A N D . F .S .,
J. Phys. C h e m . 71
(1 9 6 7 ) 2652.
[2 9 ]
W A I, C .M .,
R O W L A N D , F.S.,
J. Phys. C h e m . 74
(1 9 7 0 ) 434.
[3 0 ]
P A L IN O , G .F .,
R O W L A N D , F .S .,
R a d io c h im . A c t a 1 ¿ (1 9 7 1 ) 57.
[3 1 ]
V A S A R O S . L .,
M A C H U L L A , H .J .,
[3 2 ]
M A C H U L L A , H .J.,
S T O C K L IN , G .,
[3 3 ]
P E T T U O H N , R .R.,
R A C K , E.P.,
S T O C K L IN , G .,
J. Phys. C h e m . 76 (1 9 7 2 ) 501.
J. Phys. C h e m . 78^ (1 9 7 4 ) 658.
W O LF, A .P .,
A bstracts 7th Int. H ot A t o m C h e m is try S y m p ., jü lic h ,
FRG (1 9 7 3 ). [3 4 ]
H E N C H M A N , М .,
[3 5 ]
G A R L A N D , J .K .,
R O W L A N D , F.S.,
U R C H , D ..
R a d io c h im . A c t a 4 (1 9 6 5 ) 115.
[3 6 ]
G A R L A N D , J .K .,
R O W L A N D , F .S .,
J. Phys. C h e m . 70 (1 9 6 6 ) 735.
[3 7 ]
A C H E , H .J.,
HERR, W .,
S ym p . P ra gu e, 1 9 6 0 ) [3 8 ]
C IR A N N I, E.,
[3 9 ]
W IL L A R D , J.E.,
W O L F G A N G , R.,
T H IE M A N N , A . ,
Can. J. C h e m . 38 (1 9 6 0 ) 1722, c f. also R e f . [ 5 ] .
C h e m ic a l E ffects o f N u c le a r T ra n s fo rm a tio n s -1 9 6 0 (P ro c.
2, IA E A , V ie n n a (1 9 6 1 ) 111.
C IR A N N I, G „
G U A R IN O , A .,
G a z . C h im . It a l. 95 (1 9 6 5 ) 52.
C h e m ic a l E ffe c ts o f N u c le a r T ra n s fo rm a tio n s -1 9 6 0 (P ro c . S ym p . P ragu e, 19 6 0 ) _I.
IA E A , V ie n n a (1 9 6 1 ) 215. [4 0 ]
M A C H U L L A , H .J.,
S T O C K L IN , G .,
K F A Ju lich , u npublished results, c f. also abstracts o f th e 7th
In t. H o t A t o m C h e m is try S y m p ., J u lich , FRG (1 9 7 3 ). [4 1 ]
BEREI, K .,
[4 2 ]
K O N T IS , S .S.,
S T K c K L IN , G .,
R a d io c h im . A c ta 1 £ (1 9 7 1 ) 39.
[4 3 ]
S H O B A T A K A , K .,
L E E .Y .T .,
S H O B A T A K A , K .,
PA R S O N , J .M .,
U R C H , D .S .,
R a d io c h im . A c t a 1 £ (1 9 7 1 ) 21. RICE, S t.A .,
J. C h e m . Phys. _59 (1 9 7 3 ) 1435;
L E E .Y .T .,
RICE, S t .A .,
[4 4 ]
S T O C K L IN , G ., T O R N A U , W .,
R a d io c h im . A c ta 6_ (1 9 6 6 ) 86.
[4 5 ]
S T O C K L IN , G ., T O R N A U , W .,
R a d io c h im . A c t a 9 (1 9 6 8 ) 95.
[4 6 ]
BRO W N , L .O .,
[4 7 ]
S P A G N O L O , P ., T E S T A F E R R I, L .,
[4 8 ]
P O Z D E V , V .V ., N E S M E Y A N O V , A . N . ,
SOPER, G .F ..
and
ib id . 59 (1 9 7 3 ) 1427.
J. C h e m . S o c. (1 9 5 3 ) 3576. T IE C C O ,
М., J. C h e m . S oc.
D Z A N T IE V , G .B .,
(В)(1 9 7 1 )
R a d io c h im ija
2006. 5 (1 9 6 3 ) 395.
181
1АЕА-РЬ615/10
[4 9 ]
W H IT E , R .M .,
[5 0 ]
BEREI, K „
R O W L A N D , F .S .,
V A S A R O S , L .,
J. A m . C h e m . S oc. 82 (1 9 6 0 ) 4713.
N A G Y , G .A .,
K A R D O S , Z s.,
abstract o f th e 7th Int. H o t A t o m C h e m is try
S ym p o siu m , J ü lich , FRG (1 9 7 3 ). [5 1 ]
A Y R E S , R .L .,
[5 2 ]
H A H N E , R .M .A .,
R A C K , E .P.,
[5 3 ]
WILLARD,
J.E.,
R a d io c h e m . R a d io a n a ly t. L e tt. 3 (1 9 7 0 ) 213.
W IL L A R D , J.E.,
J. Phys. C h e m . 62 (1 9 6 4 ) 2582.
S c ie n c e 180 (1 9 7 3 ) 553 (a r e v ie w ).
[5 4 ]
S1EKIERSKA, K .E .,
H A LPE R N , A .,
[5 5 ]
SIE K IE R S K A , K .E .,
H ALPE R N , A .,
[5 6 ]
N A R B U T , J.,
[5 7 ]
D A N C E W IC Z , D .,
[5 8 ]
C O L L IN S , K .E .,
[5 9 ]
CARLSO N, T . A .,
D A N C E W IC Z , D .,
R a d io c h im . A c t a 5 (1 9 6 6 ) 51. M A D D O C K , A .G ., J. C h e m . S o c. ( A ) (1 9 6 8 ) 1645.
H ALPE R N , A .,
H ALPE R N , A .,
H A R B O T T L E , G .. W H IT E , R .M .,
R a d io c h im . A c t a 7_(1 9 6 7 ) 55.
R a d io c h im . A c ta 12 (1 9 6 9 ) 52. R a d io c h im . A c t a 3 (1 9 6 4 ) 21;
3 (1 9 6 4 ) 29.
C h e m ic a l E ffects o f N u c le a r T r a n s fo r m a tio n s -1964 (P ro c . S ym p.
V ie n n a , 19 6 4 ) 1, IA E A , V ie n n a (1 9 6 5 ) 23. [6 0 ]
K A Z A N IJ A N ,
aTr.,
[6 1 ]
C R U S E T, A .,
R a d io c h e m . R a d io a n a ly t. L e tt. 16 (1 9 7 4 ) 309.
[6 2 ]
LIB B Y, W .F .,
T A K I D A , Y . , H IR A G A ,
J. C h e m . Phys. 42 (1 9 6 5 ) 2778.
М., Y O S H IH A R A , M ~ S H IO K A W A , T . ,
R a d io c h e m . R a d io a n a ly t. L e tt. 7
(1 9 7 1 )3 1 3 . [6 3 ]
GEISSLER, P .A .,
[6 4 ]
M E R R IG A N , S .A .,
W IL L A R D , S.E .,
[6 5 ]
D A F O N S E C A , A .J .R .,
J. Phys. C h e m . 67 (1 9 6 3 ) 1675.
ELLGREN, W .K ., FULLER, K .,
R A C K , E.P.,
J. C h e m . Phys. 4 4 (1 9 6 6 ) 174.
L A T H A N , A .,
S H A W , P .F .D .,
R a d io c h e m . R a d io a n a ly t. L e tt.
2
(1 9 6 9 ) 69. [6 6 ]
C A C A C E , F „ S T O C K L IN , G .,
[6 7 ]
K N U S T , E.J., H ALPE R N , A .,
J. A m . C h e m . S o c. 94 (1 9 7 2 ) 2518.
[6 8 ]
VE R N O IS , J.,
[6 9 ]
H A LP E R N , A .,
S T O C K L IN , G .,
[ 70]
H A LP E R N , A .,
S T O C K L IN , G .,
S T O C K L IN , G .,
ABDEL G H A N I, A .H .,
J. A m . C h e m . S o c .,
M U X A R T , R.,
in
R a d io c h e m . R a d io a n a ly t. L e tt.
press. 12 (1 9 7 2 ) 7.
R ad iat. Res. 58 (1 9 7 4 ) 329. K F A jü lic h , unpublished results, c f. a lso abstracts o f th e 7th In t. H ot
A to m C h e m is try S y m p ., jü lic h , FRG (1 9 7 3 ). [7 1 ]
D IE H N , B.,
H A LPE R N , A .,
S T O C K L IN , G .,
Proc. 5th Int. C o n gr. R a d ia tio n Research, S e a ttle ,
W a sh in gto n , U S A (1 9 7 4 ).
DISCUSSION F . S . R O W LA N D : I want to com m ent on one aspect of the halogen experim ents that is v alid for both gas and liq uid phase — and that is the assum ption that some of them are self-scavenging. The concept of s e lf scavenging is dependent upon the activation energy for the scavenging process. F o r com parison, consider the c o llisio n efficiencies of two reactions as m easured in the gas phase. The re action of F atom s with Cl has a rate constant at room tem perature that is about 1 0 ' 10 cm 3 m o le cu le "1 s '1 — alm ost every co llisio n . The abstraction of H fro m Cffi^ by F atom s has a rate constant at room tem perature of about 2 X 1 0 ' 13 , so that already at 3 0 0 K about 5 0 0 tim e s m ore c o lli sions are re qu ire d for reaction with CHF3 than with C l 2. Stocklin was talk ing about CH3F and not CHF3 , but the rate constant as a function of tem p erature has only been m easured with CHF3 , and both represent abstraction fro m a fluorinated m ethane. A fluorine atom m ight be scavenged fa ir ly ra p id ly at 3 0 0 K in CHF3 , re q u irin g co llisio n num bers of the order of hundreds, which is n 't too highly efficient in the scavenging process. The activation energy for abstraction of H from CHF3 has been m easured to be 2 . 4 k c a l/m o le , which is not very high, but is c ertainly not zero — and the difference is im po rtant. L e t's look at the available plots of the yields of CH318F fro m CH3F versus density. The points up through the fir s t plateau in yield were
182
STOCKLIN
a ll m easured at 2 4 °C. At higher densities, the tem perature was lowered sim ultaneously, going down to -1 9 6 °C eventually. Suppose one had the same rate constant for abstraction from CH3F as is observed for CHF3 . Then the rate constants would go from 2 X 1 0 ' 13 at 2 4 °C to 1 X 1 0 "i3 at - 1 0 °C to 5 X 1 СГ14 at -5 0 °C; 4 X 1 СГ15 at - 1 2 0 °C; and fin a lly , 2 X 1 СГ18 at - 1 9 6 °C. By the tim e you have reached a rate constant of 4 X 1 0 ' 15 , you are already re q u irin g an average of 105 c o llisio n s for scavenging, which is a long way fro m being a self-scavenged system . The situation is even worse with 38C 1 for which the activation energies are higher. The self-scavenging of 1SF and 38C 1 is only true with a few v ery reactive system s if one m eans re m o v al on the firs t few c o llisio n s. M olecu lar CI2 would be self-scavenged. G. STO CKLIN : I am quite aware of th is, and I did n't make a very strong point about self-scavenging, because it is an open question. F ir s t, and of course this doesn't prove the point, these system s do not show any scavenger effect. A t high den sities, of course, we have im m ediate caging, and some re action then occurs very efficaciously although perhaps not with the scavenger — in some sense it is self-scavenging. Secondly, there m ay be a difference when you go from CHF3 to CH3F — you have m o re hydrogen atom s available, and there is also an electronegativity effect. We could, of course, m easure the tem perature effects at different densities, and you y o u rse lf observed a s m a ll change with tem perature in the stereo ch em ical course in your experim ents with W a i, but didn't observe any difference in the yields. This is pu zzlin g. It says that the change in tem perature doesn't bring in any very im po rtan t new p a ram eter beyond that contained in the density effect. F . S. R O W LA N D : My only com m ent is that they m ay re a lly be se lf scavenged — that is , that reaction w ithin a few hundred c o llisio n s m ay be good enough at room tem p erature. That is n 't v ery efficient scavenging, but it m ay be good enough. However, as you go down in te m p erature, the scavenging keeps getting worse a ll along, and if you find some other process beginning to compete. G. ST O CKLIN : That is b a s ic a lly true , and I agree. F . S. R O W LA N D : You want to be very careful that it is not a te m p e ra ture effect instead of a density effect. J . P. A D L O F F : I don't agree with the statem ent in your paper that the self- radiolysis m odel is an hypothesis which has never been m ore than a useful speculation. There is plenty of evidence from M Sssbauer spectro scopy done in our lab o ra to ry at Strasbourg and by Danon in R io that indicates that this self- radioly sis m odel re a lly applies, and is the dom inant process for the m o le cu les we have investigated — inorganic compounds and m e tal chelates. G. ST fjC K LIN : In inorganic compounds, it m ight be quite different because you have m o re electronic processes available. How can you distinguish between different environm ents with respect to charge state? You cannot distinguish between som ething form ed after ne u tra lizatio n with excitation decom position and that which is form ed by A uger electron ra d io ly sis. They m ay eventually lead to the same re sult. J . P . A D L O F F : We m easure the net effect of the beta decay. F o r instance, if you take a M ossbauer e m issio n spectrum of a compound, and then irra d ia te it with electrons o r g am m as, you get an absorption spectrum which com pletely m atches the e m issio n spectrum . This m atch in g indicates
IAEA-PL-615/10
183
that we have the same re su lt whether we have in te rn a l ra d io ly sis fro m the A uger effect, or if we have external ra d io ly sis of the same m olecule. G. STÜCK LIN : It s till does not give you an unam biguous answer. J . P . A D L O F F : W e ll, but it is m ore than a useful speculation. G. STRICKLIN: I agree com pletely. G. H A R B O T T L E : I have v e ry serious doubts whether any experim ent such as the M ossbauer effect can unam biguously dem onstrate the ra d io ly tic self- radiolysis as postulated by W illa rd and G e issle r. A s I m entioned in m y paper, I would like someone to make calculations on the actual am ount of ra d io ly sis involved in this process. A fter a ll, we know the spectrum of A uger ra d io ly s is , and we ought to be able to m ake order-ofm agnitude calculations on the ra d io ly tic production of ra d ic a ls in the v ic in ity of the M ossbauer atom , o r of the organic m olecule in which this decay took place. It shouldn't be too d ifficu lt to do th is, but what I think you are going to find is that the range of even soft electrons — and c ertainly of X - ra d ia tio n — is so great that the damage is going to be deposited in the c ry stal fa r, fa r away fro m the site of where anything in teresting is happening. This is only a feeling, and not an assertion. I want to come back to the data D r. Stocklin presented on the brom ine compound irra d ia te d with absorption edge X - ray s. I think this is extrem ely in te re stin g , and I can cite one additional piece of data which supports the idea that m o st of the observed effect is caused by lo c a l electronic d is turbances ra th e r than by coulom bic explosion in the sense of m ass m o ve m ent of heavy atom s along the Carlson-W hite m odel. That piece of evidence is connected with the old question of the V arley m e ch anism in a lk a li halides. The supposition was that io nization of an inner shell, producing an inner sh ell vacancy, in brom ine in K B r, that the brom ine would now be positively charged, and consequently be in the wrong kind of potential w ell — in the opposite potential w e ll from what it needed for binding. In that case, V arley guessed that the atom m igh t hop out. If you exam ine th is process y o u 'll see that it is v ery m uch like coulom b explosion. However, in the attem pt to v e rify the V arley m e ch a nism , people have irra d ia te d a lk a li brom ide w ith absorption edge brom ine ra d ia tio n , and have found no change at a ll in the production of in te r s titia l p a irs as you pass through the brom ine absorption edge. I think that this re su lt is v e ry m uch p art and p a rc e l of what you are seeing with the bro m o u rid ine experim ents. A . G. M ADDOCK: Two or three com m ents. F ir s t, the calculations that D r. H arbottle re fe rs to have been m ade. This paper showed that although some A uger electrons go some distance, quite a lot tra v e l only one or two lattice units. The second point concerns these X - ra y experim ents which re m ind me • of the Mayaguez experim ents in 1 9 6 8 when G om berg was doing some very s im ila r experim ents on some s im ila r substrate — but I have never seen the re su lts of those experim ents published. G. H A R B O T T L E : I believe there is a p u blication. At least the thesis was recorded in N uclear Science A bstra cts. A. G. M A DD O CK: The la s t point is that, b earing in m ind the experience w ith in organic solid s, it does seem desirable at some fa ir ly e a rly stage to check these system s for n o rm a lly unexpected th e rm a l exchange reactions. A fter a ll, the so-called "tra n s fe r annealing" re actions were ra th e r unexpected, and at the m om ent one could not say that such things do not occur in organic system s. F a ir ly e a rly , there is a need for doping-type ex perim ents.
184
STÔCKLIN
G. ST O CKLIN : D r. H arbottle' s com m ent about the V ar ley experim ents is v ery in te re stin g , but I think it depends on the type of solid you use. In the inorganic solid , it is probably ra p id ly annealed, and you do not see the effect. It is fortunate that in this type of bio-m olecule used in our e x p eri m ents these ra d ic a ls are v ery stable. It m ight be interesting to look in other inorganic system s for these effects. A s fa r as other К-edge resonance experim ents are concerned, there are two so fa r. The G om berg experim ent has not been presented in the open lite ra tu re , but there is a P uerto R ico re po rt on it. He has esse ntially m easured the bio lo g ic a l effect and observed some kind of resonance. The A uger effect was not p ro p e rly treated, in m y opinion. A second experim ent was c a rrie d out by Diehn. He didn't monoch rom atize h is X - rays enough, but he s till observed some positive effects. I think that our experim ent proves the effect unam biguously under better ex perim ental conditions. A s fa r as annealing effects in organic solids is concerned, there is a lot that can be done. Some in teresting experim ents by H arbottle which he can te ll us about. A nother by H alpern who did n't observe the tem perature plateaus. They observed only one plateau even though they m easured at different tem p eratures. O f course, an organic solid in a way is re a lly difficu lt and m essy. But in te rm s of solid-state organic ch em istry, it c ertainly deserves further attention. The field in solid-state organic physics and c h e m istry is ra p id ly expanding, and it m ay be worthwhile having a new look at annealing in organic solids. L . F E IN E N D E G E N : I happen to have w ith m e the paper that gives a ll of the calculations on the range of the low energy electrons, and I ' l l give you the data. There are three 0 . 5 -keV A uger electrons per decay on the average, with a range of 2 5 0 Â in w ater. This is the lowest range of these electrons, and the highest range for the M -conversion electrons w ith an energy of 3 4 . 2 keV had a range in water of 2 0 . 5 p m . These c a lc u la tions agree with the e a rlie r com m ent that nearby effects are u n lik e ly to have been the re su lt of ra d ia tio n absorption fro m these A uger electrons. I w ill talk this afternoon about some effects of 125 I decay in DNA when the 125I is bound to the p y rim id in e rin g . More recent work in Z im m e r 's lab o ra to ry in K a rlsruh e and by K risch at Argonne indicates that there is a p ro b ab ility of 0 . 5 to 1 . 0 of double-strand breaks per 125I decay in that position. This is the m ost toxic event that has so far been observed from a decaying isotope in DNA. Even heavy atom s accelerated and shot into the DNA do not produce double-strand breaks w ith such an efficiency. 1 2 5 1 produces double-strand breaks — a sim ultaneous break at the site where the isotope decayed and opposite it on the adjacent strand — with an e ffi ciency of 0 . 5 to 1 . 0 per decay. This is extrem ely efficient when com pared w ith what is observed with electrons, for which the re la tio n sh ip between single-strand and double-strand breaks is of the ord e r of 20 to 1. G. H A R B O T T L E : I am indebted to D r. Feinendegen for having come up so quickly with supporting calculations. If you take a ra d iu s of 2 5 0 Â and put that into the fo rm u la 47rr3 , y o u 'll find a v ery large num ber of m o le cules in that sphere over which you are going to divide the io nization energy. C om ing back to the question of organic annealing, it is of course ex trem ely easy to observe organic annealing in organic halides and of the halide atom itse lf. W hat I would like is for someone to look for annealing
IAEA-PL-615/10
185
in a pure and sim ple 11 С system . Let me propose som ething like benzoic acid, or oxalic acid, for which you can ju st do a su blim ation without re q u irin g anything fancy such as vapour phase chrom atography. L e t's see if one can get annealing in some sim ple organic system of the carbon r e c o il atom. A . P. W O L F : It's been done and it 's in the lite ra tu re . The Canadians did it — oxalic acid; sodium carbonate. G. H A R B O T T L E : Oh you m ean the M c C a llu m work. F . S . R O W LA N D : There was a Swedish thesis on n C (Stenstrom ) that looked into the M c C a llu m re s u lts , and explained them as being caused by ra d ia tio n dam age. A. P. W O L F : The Swedish work was in aqueous system s, and the M c C a llu m work was in solids. S tenstrom 's w ork was a ll in aqueous system s, and what he looked at was the ra d ic a l reco m bin ation reactions. In fact, m o st of what Stenstrom saw in h is n C work was ra d ic a l re co m bination re action in aqueous solution and had nothing whatever to do with hot atom ch em istry. W e've trie d annealing for organic solid solutions and in m ixed c ry stals. There is some annealing, but nothing spectacular. J . DANON: I would like to in s is t again that the M ossbauer experim ents with tra n s itio n m e ta l chelates re a lly do show the difference in intensity between ex ternally irra d ia te d sam ples and the re c o il ex perim ents, but q u a li tativ ely there was no difference at a ll in sym m etry surroundings or any of the other p a ra m e te rs we could m e asu re. J . P. A D L O F F : Let m e say that the figures of D r. Feinendegen re fer to the range of electrons in the liq u id phase in w ater, while in M ossbauer work we ty p ic a lly are involved with the solid phase for w hich the range of the conversion electrons is m uch s m a lle r, and should not be la rg e r than a few A ngstro m s. K la ra B E R E I: I have a com m ent on the question of replacem ent in liq u id phase aro m atic compounds. There are some experim ents in p rogress in our lab o ra to ry with di-substituted benzene derivatives. O u r aim is to investigate the role of ch e m ic a l behaviour as it is changed by different substituents in the m olecule — the effect on the hot replacem ent reaction by halogen atom s using different stubstituents on the target m o le cule, and de te rm inin g the re la tive yields in replacem ent re actions in in tra m o le c u la r com petition. This kind of procedure has some advantages because the secondary effects of the m e dium are the same for both com peting reactions in an in tra m o le c u la r system . We have had some difficu ltie s in the in te r p retatio n of our re s u lts , as you can im ag ine , but two or three m a in con clusions can be draw n fro m the re su lts so fa r w ith 38C 1 reactions with the different is o m e rs of chloronitrobenzene, chlorofluorobenzene, and chlorophenol. F ir s t , there is a bond energy effect — the substituent with low er bond energy is always replaced at a higher rate. Second, the replacem ent of each heavy atom substituent — we haven't studied hydrogen — is always noticeably dependent on the nature and the p o sitio n of the other substituent in the m olecule. The th ird observation is that we did not find any d rastic cage effects in these system s. We trie d to observe cage effects by adding 5 0 m ole per cent of m ethanol, a very good scavenger fo r 38Cl atom s, and did not find any effect on yields to indicate a scavenger effect. The firs t two points — the m in o r details of the
186
STÓCKLIN
ch em ical p rop e rties of the m olecule — as connected in a ro m atic system s w ith inductive and m e so m e ric effects — and the existence of bond energy effects are consistent with D r. S tocklin's statem ents about complex form ation in these re actio ns. It is very in teresting that even inductive effects seem to p lay a ro le . I would like to defend once m ore the investigation of arom atic system s, even in spite of the ex perim ental and the o re tic a l com plications. One of the im p o rtan t aspects of the arom atic system s is ju st the significance of d ire c t la b e llin g techniques. I believe that the evaluation of new techniques for la b e llin g of bio-m olecules or ra d io p h arm a ce u tic als, which often contain one or m o re a ro m atic nu cle i in the m o le cule, can be p a rtly based on the re su lts of such fundam ental studies of aro m atic system s. Y . L E E : I think that m aking a p a ra lle l c om p arison of the hot atom method and the m o le c u la r beam m ethod is ra th e r unfortunate, and also m isle a d in g because hot atom c h e m ic a l phenom ena involve m any, m any atom ic and m o le c u la r processes — relax atio n, etc. If you use hot atom c h e m istry to try to derive the dy n am ical c h em ical re la tio n s in a single c o llis io n — then I would say if you can do it in a beam , then you w ill get m ore in fo rm a tio n . However, if you go to the condensed phase, and examine the re action of iodine atom s plus a ro m atic hydrocarbon — here , you are not m ak ing a d ire c t c om p arison because in the liq u id phase you have a p rim a r y step and afterw ards there is in tra m o le c u la r energy tra n s fe r, and ra d ic a l re ac tio n s, and we can w rite perhaps 1 0 to 1 5 steps in the m e chanistic p rocesses. In the m o le c u la r beam you can only look at one step. This doesn't m ean that the m o le c u la r beam method w on't be able to help in u nderstanding these processes because the m o le c u la r beam technique can also te ll you about in te rm o le c u la r energy tra nsfe r processes. A ctually, the re aso n that we don't understand the m e ssy liq u id phase or solid state phenom ena is that in m any cases the argum ents we have put in — such as cage effect, and stereo sp ecificity — are not very sound. It m igh t be this, it m igh t be that, without a fir m b asis. If we have a strong basic understanding of how energy is tra n s fe rre d , then w e 'll be able to understand a ll of the m e ssy p rocesses. The com p arison is not re a lly a p a ra lle l com parison. F o r exam ple, in the gas phase we have been talk ing about the structure of the m o le cu le, but we have never talked about the im pact p a ra m e te r, or the angular m o m entum , although with a benzene rin g , and a fluorine atom substituted on to it, the different carbon sites w ill have different im pact p a ra m e te rs, and the angular m om entum im pa rted at different sites of attack w ill be v ery different. In the hydrogen case, it is not v ery im po rtant, but with chlorine it is im po rtan t. A ll these dynam ic things are u su a lly not considered because we don't understand very m uch. I don't think the com p arison between the m o le c u la r beam and hot atom methods should be a p a r a lle l com parison. We should be a little m o re specific about what can be done with the beam method. G. ST O CK LIN : I perfectly agree with you. I am ju st asking that you try to help u s, on the basis of your experim ents with fluorine atom s on benzene in the beam , explain some m ore of the re su lts which are actually observed in the liq u id phase. O rganic chem ists and biochem ists and life scien tists cannot w ait for ten o r twenty years u n til a ll these data are a v a il able. We have to live with it now, and we have to have some postulates to guide our ex perim ents. Y. L E E : We are s till talk ing about different things. We cannot wait, but on the other hand, what can you do? You have a m e ssy system , and
1АЕА-РЬ615/10
187
it w ill re m a in m essy unless you go in and look at the in tra m o le c u la r energy tra n s fe r, or the ra d ic a l re ac tio n s, and a ll the steps w hich are involved. If you accum ulate enough in fo rm a tio n , then you can begin to understand. K. R O S S L E R : W e've already had a question concerning whether there are large differences in yields and substitution behaviour between the liq u id phase and the solid phase. Have you studied the liq u id and solid phase in the same system ? You reporte d a big jum p in yields when you go fro m gas to liq u id system s. W hat happens when you go fro m liq u id to solid phases? G. ST O CKLIN : The answer depends upon the system . F o r m onovalent atom s, there is u su a lly a fu rth e r continuous increase in yield as we have seen in the W o lfgang-R ichardson experim ent — no a ddition al ju m p or sharp break. I am m uch m ore concerned now with L e e 's com m ents, because he has ra is e d a basic question. We ju s t cannot give up the condensed state and w ait for fundam ental re su lts fro m each of these in d iv id u al steps. P robably with single c o llis io n conditions you w ill answer a lot about the p rim a r y events, but in m y opinion you w ill not answer the prob le m s of the condensed phase. Y. L E E : I think we s till have some m isunderstanding. I'm not talk ing about the beam method alone — it's not going to solve a ll the m a croscop ic phenom ena. If you try to understand the m ac ro sc o p ic phenom ena, you do have to know a ll the dynam ic processes in the in d iv id u al steps, and this is what the m o le c u la r beam experim ents are try in g to do. If you have some v ery im p o rtan t and in te re sting hot atom phenom ena, and these processes involve one hundred steps, then you w ill have to do one hundred different experim ents to investigate the hundred different processes. You can't solve it a ll in a single beam experim ent. W hat we want to find out is the nature of the energy tra n sfe r in a ll its d e ta il, so that you can get some idea of what is taking place in the liq u id . I think we have to be patient, and not be m is le d into saying that if you do the flu o rin e atom plus benzene re action in the gas phase that it w ill te ll you everything. The p rim a r y step is not the im po rtan t one — the im p o rtan t steps are the re lax atio n steps on which we have no in fo rm a tio n as yet. As the tim e goes on, the beam e x p eri m ents w ill be able to te ll you how efficient the energy tra n sfe r is in twobody c o llisio n s between large m o le cu les, etc. G. STO CKLIN : I'm with you one hundred p e rc e n t. T hat's why we started beam work two years ago. F . S . R O W LA N D : I have a com m ent about the p roblem of conform ation in the 2 , 3 -dichlorobutane system . If you re m e m b e r S tocklin's re s u lts , he dem onstrated conclusively that as you start dilu ting this compound w ith a series of solvents — b ro m in e , m ethy l alcohol, n-pentane, etc. — that the re te n tio n /in v e rsio n c h a ra c te ris tic s of the 2, 3 -dichlorobutane products changed. Stocklin attributes this v a ria tio n to differences in the fractions of c onform ers present in the different solutions. However, it can be shown fro m in fra - re d analyses that there, are differences in the populations of these c onform ers. If you take these solutions down in tem p erature to - 7 8 °C, then the am ount of each conform er w ill change because these differences c o r respond to differences of about a k ilo c a lo rie per m ole in conform er energy. We have taken these system s down in te m p e ra ture , without finding any appreciable change in the retention of configuration in m ost of the system s we have trie d . We argue fro m this re su lt that the effect of solvent on per cent retention which Stocklin has observed is not an effect on the ground-
188
S T O CK LIN
state co n fo rm a tio n s . In tr y in g fo r a d iffe r e n t approach and explan ation , w e have p lotted the p e rc e n ta g e reten tio n s again st the d ie le c t r ic constant o f the m ed iu m , and get a good continuous cu rve in th is w ay. W hen the e x p e rim e n ts a re done w ith so lven t m ix tu re s whose d ie le c t r ic constant changes w ith te m p e ra tu re , then w e can get te m p e ra tu re v a ria tio n s in p e rc e n ta g e re te n tio n as we go down to -78°C . O u r fe e lin g is that these re a c tio n s a re m o r e dependent on the c o n fo rm a tio n o f the r a d ic a l which is re c o m b in in g w ith the c h lo rin e atom , and that the r a d ic a l c o n fo rm a tio n is b e in g c o n tro lle d by the e ffe c t o f the d ie le c t r ic constant on the stru ctu re o f the r a d ic a l, and not by the co n fo rm a tio n s o f the ground state o f the m o le c u le . G. S T Q c K L IN : W e cannot a b so lu tely exclu d e th is explan ation because we don't know w h eth er the re c o m b in in g r a d ic a l is p la n a r. If the r a d ic a l is p la n a r, then you a re w ron g. A s fa r as the te m p e ra tu re is con cern ed , if you change the te m p e ra tu re you change the c o n fo r m e r con cen tration . W e have studied the te m p e ra tu re ra n ge fr o m about 0 to 100°C with the pure dich lorob u tan e. You can ca lcu la te in the pure s y s te m s that th ese changes a re s m a ll, and a 100°C change in te m p e ra tu re g iv e s r is e to on ly a 10% change in the c o n fo r m e r co n cen tra tion , so that the e ffe c t is s m a ll and you w o n 't p ro b a b ly see it. N ow , you argu e that i f you go fu rth e r down, you can see it. F . S . R O W L A N D : N o, we don't see any change in reten tio n in your s y s te m s as we go down in te m p e ra tu re . G. S T O C K L IN : W h ich s y stem did you u se? F .S . R O W L A N D : The sam e one you did — the d ich lorobu tan e is o m e r s . G. S T O C K L IN : In solu tion , o r w ith the pure system ? F .S . R O W L A N D : W e take them fr o m about 100°C down to -78°C with both the pu re liq u id s and also d is s o lv e d in you r so lven ts. G. S T O C K L IN : I'm s u rp ris e d that you see any change. F .S . R O W L A N D : W e see a lm o s t no change in re te n tio n / in v e rs io n ra tio in m o s t o f th ese s y s te m s o v e r a te m p e ra tu re ra n g e o f a lm o s t 200°C. G. S T O C K L IN : Y ou w ou ldn 't ex p ect to. F .S . R O W L A N D : N o, you w ould ex p ect enough change to be able to m e a s u re it. A . P . W O L F : I'd lik e to ask a sim p le question. D id you do the in fr a re d a n a ly s is o f the c o n fo r m e r s at 0 and at -78°C , becau se I think Stocklin is rig h t — in m any s y s te m s , you w o n 't see any m a jo r c o n fo rm a tio n a l change o v e r such a sh ort te m p e ra tu re ra n g e. A s a consequ en ce, you w ouldn't ex p e c t a m a c ro change in you r re s u lts . The question r e a lly is , ra th e r than argu e about it, did you a c tu a lly m ea su re the co n fo rm a tio n a l change at th ese tw o te m p e ra tu re s ? F .S . R O W L A N D : N o, what we did. . . . A . P . W O L F : Then y o u 'r e specu latin g. F .S . R O W L A N D : W hat w e 'r e sp ecu latin g fr o m is the data o f the p eop le who have m ade the in fr a - r e d m e a s u re m e n ts ..., A . P . W O L F : On 2, 3-d ich lorob u ta n e fr o m 0 to -78°C ? F .S . R O W L A N D : On 2,3 rd ich lorob u tan e o v e r a ran ge o f te m p e ra tu re s s u ffic ie n t to ca lcu la te the d iffe r e n c e s in c o n fo rm a tio n a l e n e r g ie s . O b vio u sly, th ere m igh t be s m a ll v a r ia tio n s but you can c e r ta in ly get the o r d e r o f m agnitude o f change which should be exp ected . A . P . W O L F : I f you go to E l i e l 's book on c o n fo rm a tio n a l a n a ly sis and lo o k at the b roa d sp ectru m — th is has been studied in thousands o f m o le c u le s — y o u 'll find v a r ia tio n s a ll o v e r the m ap, som e enorm ou s
IA E A -P L-6 1 5/10
189
changes. The changes depend on the h eigh ts o f the b a r r ie r s and d ie le c t r ic e ffe c ts w ithin the m o le c u le s th e m s e lv e s , and not ju st the h eigh ts o f the b a r r ie r s . U n til you have con vin ced y o u r s e lf that th e re is indeed a d if fe r e n c e in population b etw een 0 and -7 8 °C , then I think you a re ju st specu latin g. Do the e x p e rim e n t — it 's an e a s y m ea su rem en t. A . G. M A D D O C K : Is n 't th e re another p o s s ib le approach? C an 't one s o m e tim e s fr e e z e out a p a r tic u la r c o n fo r m e r by u sin g a d sorp tion — o b s e r v e the beh aviou r with an a d sorb ed s p e c ie s ? G. S T Ô C K L IN : H ow does th is h elp you? You can use in fr a - r e d tech n iqu es to m ea su re the stab le c o n fo r m e r con cen tra tion . T h is has been done, and we have done it, too. A . G. M A D D O C K : You want to distu rb it, don 't you? G. S T C )C K L IN : T h e re a re m any p o s s ib ilit ie s fo r d istu rb in g it — even by the p o la r ity o f the so lven t, o r by te m p e ra tu re . The e x p e rim e n t w hich r e a lly should be done is to show what re a c tio n o c c u rs w ith a p a rtic u la r c o n fo r m e r in the gas phase. T h e r e it is d iffic u lt, but one m igh t c o n c e iv e o f an e x p e rim e n t in which one has an e x te r n a l fie ld to change the c o n fo rm e r con cen tration . In the gas phase, you can apply the s c a v e n g e r technique and you can am b igu ou sly show that you have no r a d ic a l re a c tio n s . If you do o b s e r v e a c o n fo rm a tio n a l e ffe c t in the gas phase, then w e have an unam biguous p ro o f. Do you a g re e to th is? F .S . R O W L A N D : I f you o b s e r v e the co n fo rm a tio n a l e ffe c t in the gas phase, then th e re is no p ro b le m . N o , I ' l l take that back — th e re can be p r o b le m s . W hat you have is an in te rp re ta tio n w ith fiv e p a r a m e te r s . It 's not hard to fit data w ith fiv e p a r a m e te r s , and the p ro b le m is in d em o n stra tin g that th is r e a lly is a re a c tio n o f the in d ivid u a l c o n fo r m e r . W hat is c e r ta in ly tru e is that you have o b s e rv e d a so lv e n t e ffe c t on the re te n tio n / in v e rs io n c h a r a c te r is tic s which w e want to t r y to understand. D .J . M A L C O L M E - L A W E S : I w on d er i f D r. W o lf can t e l l us w h eth er he has studied the in v e rs io n re a c tio n m en tion ed the oth er day in the liqu id phase as w e ll as in the gas phase? The c h lo rin e atom re a c tio n s with c h lo ro p ro p io n y l c h lo rid e ? A . P . W O L F : Y e s , as a m a tte r o f fa c t, w e have studied it in the gas p hase, in the liq u id phase, and in the s o lid phase both as a g la s s and as a c r y s ta l. T h e re d o esn 't seem to be any a p p re c ia b le d iffe r e n c e in the p e r cen tage re te n tio n b etw een the g la s s and the c r y s ta l. The tro u b le w ith th ese s te r e o c h e m ic a l e x p e rim e n ts is that when the re s u lt g iv e s you a r a c e m ic 50-50 prod u ct, you have no handle on b e in g able to t e l l w h eth er what you have o b s e r v e d is 50% in v e rs io n in the p r im a r y re a c tio n , o r 100% r a c e m iz a tion a fte r the re a c tio n . But in the gas phase, ou r re s u lts show that it is c le a r ly , u n e q u iv o c a lly in v e rs io n . The liqu id phase s eem s to in d ica te e ith e r to ta l r a c e m iz a tio n o r 50% in v e rs io n — w e have no w a y o f te llin g which. T h is is a lso tru e fo r our re s u lts in the s o lid phase. M . N E W T O N : I take it that the g e o m e tr y o f this r a d ic a l is im portan t. I f it is r e a lly that im p orta n t, it should be a v a ila b le fr o m ESR stu dies on the a p p ro p ria te m o le c u le s , o r e ls e it could be ca lcu la ted . G. S T Ó C K L IN : W e would be v e r y in te re s te d i f you could ca lcu la te the r a d ic a l stru c tu re s fo r d ic h lo ro d iflu o ro e th a n e m inus one c h lo rin e atom , o r fo r d ich lorobu tan e m inus one c h lo rin e. M . N E W T O N : Th at should be a r e la t iv e ly rou tin e calcu lation . J. D A N O N : A question on the ESR e x p e rim e n ts on the b ro m o - and io d o -o r g a n ic m o le c u le s . W e r e the r a d ic a ls a lso id en tified ?
190
S T Ô C K LIN
G. S T O C K L IN : Y e s , th is is a v e r y w ell-k n o w n s y stem . The ra d ia tio n ch e m is ts have known the d eso x yu rid in e s y stem fo r a lon g tim e , and sin gle c r y s t a l e x p e rim e n ts have been m ade by H ütterm ann and M u lle r. Q u a lita tive ESR m e a s u re m e n ts have been done on this r a d ic a l. J. D A N O N : So in both c a s e s — the e le c tr o n c o n v e rs io n and the edge e x p e rim e n ts — you had the sam e type o f r a d ic a ls ? G. S T O C K L IN : I cannot sa y th is unam biguously, becau se we have not done q u a lita tiv e ESR m e a s u re m e n ts , and we have not used sin gle c r y s ta ls . It lo o k s as i f it is the sa m e, but I cou ld n 't an sw er this question d e fin itiv e ly . M . N E W T O N : P e rh a p s a le s s o n co m e s out o f the t r a je c t o r y c a lc u la tion s fo r tritiu m plus m ethane in the gas phase. D istin gu ish in g the v a rio u s a p p roach es which le a d to in v e r s io n is not as obviou s as you m igh t ex p ect — in fa c t, th ey a re h igh ly e n e r g y dependent. F o r in stan ce, you can get the standard tr ig o n a l b ip y ra m id w hich m a y lead to re te n tio n in stead o f in v e rs io n . I w on der i f you r c o m p e llin g re a s o n fo r postu latin g som e s o r t o f e ffe c tiv e c o m p lex w as that you w e r e o b s e r v in g in v e rs io n ? Is that c o r r e c t? G. S T O C K L IN : T h a t's one o f the re a s o n s , y e s . M . N E W T O N : D e ta ile d a n a ly s is o f the t r a je c t o r ie s shows that e n e r g ie s w e ll above th e rm a l b lu r the situation. It is n 't ob viou s — that is , the m a jo r so u rce o f in v e rte d prod u cts m a y not be the standard tr ig o n a l b ip y ra m id e approach that we u su a lly think about at th e rm a l e n e r g ie s . D e ta ils o f these t r a je c t o r y ca lcu la tio n s w ill be published soon, and they m a y be o f use in s te r e o c h e m ic a l p ro b le m s .
IA E A -P L-6 1 5/11
HOT ATOM CHEMISTRY OF INORGANIC LIQUID SYSTEMS N. SAITO, T. T O M I N A G A Department of Chemistry, Faculty of Science, The University of Tokyo, Japan
Abstract H O T A T O M C H E M IS T R Y O F IN O R G A N IC L IQ U ID S YS T E M S . T h e h o t a to m ch e m is try o f th e liq u id system tris (a c e ty la c e to n a to )c o b a lt in a b en zen e s o lu tio n has been stu d ied and th e results p resen ted . T h e use o f m e ta l salts as scavengers t o suppress th erm a l rea ctio n s in irrad iated system s is discussed. S everal d iffe r e n t p aram eters w e re stu d ied t o d e te rm in e th e ir e ffe c ts on th e rea ctio n s u n d er s tu d y and th e resu lts are described.
W h ile much d e ta ile d w o rk has been done on the r e c o il and subsequent re a c tio n s o f in o rg a n ic c o m p le x e s in s o lid sta te, on ly a fe w stu dies have been r e p o rte d f o r such r e a c tio n s in in o rg a n ic liq u id s y s te m s . T h e e a r lie r stu dies w e r e m o s t ly co n cern ed w ith the hot atom c h e m is tr y o f in o rg a n ic c o m p le x e s in aqueous solu tion s. T h e r e c o i l S0B r and 60C o re a c tio n s w e r e studied in aqueous solu tion s o f cob a lta m m in e c o m p le x e s such as [C o (N H 3 ) 6] 3 [С о (е п )з ]3+ and [ C o (N H 3)5B r ] 2+, [ 1 - 4 ] , and the s p ectru m o f the p rod u cts w as obtained fo r the r e c o i l 51C r r e a c tio n s in aqueous solu tion s o f a c h ro m iu m th iocyan ate co m p le x [5 ] . M o r e o v e r , s e v e r a l w o rk s have been r e p o r te d on the solu tion s o f o r g a n o m e ta llic com pounds such as m e ta l c a rb o n y ls . T h e e ffe c ts o f the co n cen tra tion , te m p e ra tu re and p r o p e r tie s o f s o lven ts on the b eh a vio u r o f 56M n r e c o il atom s w e r e studied in dilu te solu tion s o f c y clo p en ta d ien y l m anganese tr ic a r b o n y l and dim an gan ese d e c a c a rb o n y l [ 6 ] . T h e m ech a n ism o f re c o m b in a tio n re a c tio n s w as studied in a liq u id o rg a n o m e ta llic com pound, m eth y lc y c lo p e n ta d ie n y l m anganese, as w e ll as in the o rg a n ic solu tion s [ 7 ]. H o w e v e r, the study o f hot atom r e a c tio n s in the ir r a d ia te d o rg a n ic solu tion s o f m e ta l c o m p le x e s had not been re p o rte d b e fo r e our w o rk on ben zen e solu tion s o f tr is (a c e ty la c e to n a to )cob alt [ 8 ] . In 1970 w e in itia ted a s y s te m a tic study to ir r a d ia te the o rg a n ic solu tion s o f in o rg a n ic c o m p le x e s w ith th e rm a l neu trons so as to elu cid a te the m ech a n ism o f hot atom re a c tio n s in in o rg a n ic liq u id s y s te m s [ 8 ] . D u rin g ou r in v e s tig a tio n o f the re a c tio n s o f r e c o il s p e c ie s in o rg a n ic solu tion s o f m e ta l ch ela te c o m p le x e s , w e found that som e m e ta l s a lts can w o rk as e ffe c t iv e s c a v e n g e rs to su p p ress th e rm a l re a c tio n s taking p la ce both in the ir r a d ia te d solu tion s and in solu tion s ,of the ir r a d ia te d s o lid s w h e re a s unstable m e ta l c h e la te s as a d d itiv e s enhance such re a c tio n s [ 8-13] T o exp lain the beh aviou r o f the r e c o il s p e c ie s in b en zen e solu tion s o f t r is (a c e ty la c e to n a to )c o b a lt (II I ) o r ch rom iu m (III), we p ro p o sed a p robable m ech a n ism inclu ding ( 1 ) re c o m b in a tio n o f the lig a n d -d e fic ie n t r e c o i l s p e c ie s
191
192
S AIT O and T O M 1NA G A
F IG .l.
S c a v e n g e r e f f e c t o f va rio u s m e t a l l i c salts and m in e r a l acids on 60C o re te n tio n in th e irra d ia te d 0 .2 M
solu tions o f С о (а с а с )3 in b e n z e n e c o n ta in in g 10% o f e th a n o l. F e C l36H20 ; - Э - C o C l2-6 H 20 ;
- O - F e C l,;
~ 0 - A1C13; ~ Ш ~ C u C l2-2 H 20 ;
С оС 1г ; - Д - M g C l2' 6H20 ¡
- ♦ " H C l;
C u (C H 3C 0 2) 2- H 20 ;
N i C l 2- S H 20 ;
~ 0 ~ H N 0 3.
w ith fr e e a c e ty la c e to n e , ( 2 ) sca ven gin g and re a c tio n s o f m e ta l s a lts to cap tu re fr e e a c e ty la c e to n e , and (3) liga n d tr a n s fe r fr o m unstable m e ta l a c e ty la c e to n a te s to the lig a n d -d e fic ie n t r e c o il s p e c ie s . A lth ou gh the o v e r a ll phenom enon m ay be g e n e r a lly accounted f o r on the b a s is o f th is m ech a n ism , the oxid ation states o f cob alt in the lig a n d d e fic ie n t 60C o r e c o il s p e c ie s w as not c la r ifie d e x p e r im e n ta lly . W ith a v ie w to gain in g som e clu e to this question, w e have r e c e n tly in v e s tig a te d the e ffe c ts o f a tm o s p h e ric o x y g en on the fo rm a tio n o f C o (a c a c )3 in C o (a c a c ) 2 b en zen e solu tion s and the b eh a vio u r o f the 60C o r e c o il s p e c ie s in benzene solu tion s o f ir r a d ia te d C o (a c a c )3 [ 14] . A s shown in F ig . 1, the th e rm a l re a c tio n s in the ir r a d ia te d benzene solu tion s o f tr is (a c e ty la c e to n a to )c o b a lt can be sca ven ged e ffe c t iv e ly by the use o f c e r ta in m e ta l s a lts as a d d itiv e s . W e can c o m p a re the scaven gin g p o w e r in te r m s o f the red u ction in the apparent 60Co re te n tio n . T h e apparent 60C o reten tio n in the u nscavenged solu tions is as high as 8 % (w hich is h igh er than in s o lid p h a se), but it d e c r e a s e s s h a rp ly to n e a r ly 0 % in the p re s e n c e o f s c a v e n g e rs such as ir o n o r alum inium s a lts . H o w e v e r, m agn esiu m salt is not an e ffe c t iv e s c a v e n g e r . It is evid en t that w e can e s tim a te the p r im a r y re te n tio n in the w e ll-s c a v e n g e d solu tion s. F ig u r e 2 r e v e a ls the c o r r e la tio n o f scaven gin g p o w e r o f m e ta l sa lts w ith the th erm o d yn a m ic s ta b ility constants o f th e ir a c e ty la c e to n a te s . The sca ven gin g p o w e r is e x p re s s e d in te rm s o f the p e rc e n ta g e o f the th e rm a l re a c tio n s sca ven ged by an equ al m o la r con cen tra tio n o f the m e ta l s a lts in the ir r a d ia te d tris (a c e ty la c e to n a to )c o b a lt ben zen e solu tions. In F i g . 2
193
IA E A -P L-61 5/11
F IG .2.
C o r r e la tio n o f s c a v e n g in g pow ers o f va riou s m e ta l salts (2 x 1 0 " 3 M ) w ith th e th e r m o d y n a m ic s ta b ility
constants o f th e ir a c e ty la c e to n a te s . —O — :
S ep a ra ted w ith in a fe w hours a fte r irra d ia tio n ;
—
:
S ep a ra ted a fte r 7 d.
0.2M Co(acax.)} No additive Bz.soln. ~<У 3xid3MFe(acac)} -O - 3 x io3M Hi(acac)jZHzo SO -ù- 2 xw’mМц(асас)2 -0-3 x tS’M acetylacetone -e-ZxicrJM FeClj iHzO
~~A
■
Storage time (days) F I G .3 .
g
C o r e te n tio n in th e irra d ia te d 0 .2 M solu tions o f C o (a c a c )3 in b e n z e n e c o n ta in in g 10% e th a n o l
and va riou s a d d itiv e s .
the sca ven gin g p o w e r o f the m e ta l s a lts ap p ea rs to c o r r e la t e w e ll w ith the s ta b ility constants o f th e ir a c e ty la c e to n a te s , n a m ely , s a lts o f the m e ta ls , w h ich can fo r m m o r e sta b le a cetyla ceto n a to c o m p le x e s (iro n o r alum inium , f o r e x a m p le ), w o rk as m o re e ffe c t iv e s c a v e n g e r s . B y co n tra st, the sca ven gin g p o w e r o f m agn esiu m sa lt, w hich fo r m s on ly unstable a c e t y l aceton ate, is a p p a ren tly s m a ll and it d e c r e a s e s fu rth e r on standing fo r s e v e r a l d a ys. T h is im p lie s the e x is te n c e o f s lo w re a c tio n s w hich m ay con trib u te to red u ction o f apparent scaven gin g e ffe c t in the c a s e o f m agn esiu m s a lt.
194
S AIT O and T O M IN A G A
Solid Со(асас)з ---- -Ô-— Ô—
4 a /г i6 Time after dissolution(days) F IG .4.
MC o r e te n tio n in 0 . 2 M solu tion s o f th e irra d ia te d s o lid C o (a c a c )3 in b e n z e n e c o n ta in in g 10% eth a n o l
and va riou s a d d itiv e s (s y m b o ls a re th e sa m e as th ose used in F i g . 3 ) .
Time after dissolution (days> :1G. 5.
$°C o r e te n tio n in 0 . 2 M solu tion s o f th e irra d ia te d s o lid C o (a c a c )3 in b e n z e n e c o n ta in in g 3 x 10" 3 M
v4 g(a ca c)2 at va rio u s tem p e ra tu re s .
T o c la r if y such p ro b le m s , that is , the nature o f the slo w re a c tio n s , and w hy the sca ven gin g p o w e r is c o r r e la te d w ith the th erm od yn a m ic s ta b ility r a th e r than the k in e tic fa c to r , w e have fu rth e r studied the e ffe c t on r e c o il re a c tio n s o f the addition o f m e ta l a cety la c e to n a te s in stead o f m e ta l s a lts . F ig u r e 3 in d ic a te s the change o f apparent 60C o re te n tio n w ith tim e a fte r ir r a d ia tio n o f the tr is (a c e ty la c e to n a to )c o b a lt b en zen e solu tion s containing v a r io u s a d d itiv e s . T h e apparent 60Co re te n tio n re m a in s unchanged even a fte r standing fo r fiv e days in the p re s e n c e o f f e r r i c c h lo rid e as an a d d itive, w h ile it in c r e a s e s a p p re c ia b ly on standing in the p re s e n c e o f m agn esiu m c h lo r id e . An in te re s tin g o b s e rv a tio n in this s y s te m is the e ffe c t o f m e ta l a c e ty la c e to n a te s as a d d itiv e s . Both n ic k e l and m agn esiu m a c e ty la ceto n a tes as a d d itiv e s enhance re m a r k a b ly the in c re m e n t o f the 60Co re te n tio n w ith tim e as c o m p a re d w ith the re te n tio n c u rv e in the absen ce o f an a d d itiv e , w h e re a s iro n a c e ty la c e to n a te has p r a c t ic a lly no e ffe c t. P r o b a b ly the tr a n s fe r
IA E A -P L-61 5/11
195
Stability constants u f met i l acetylacetonates F IG . 6.
In c r e m e n t o f th e ap p aren t 60C o r e te n tio n ( A R ) by th e a d d itio n o f m e t a l a c e ty la c e to n a te s versus th e
th e r m o d y n a m ic s ta b ility o f th e m e t a l a c e ty la c e to n a te s .
o f the a c e ty la c e to n a te ion fr o m a m e ta l a c e ty la c e to n a te o f lo w e r s ta b ility (m agn esiu m o r n ic k e l a c e ty la c e to n a te ) to the lig a n d -d e fic ie n t 60Co r e c o il s p e c ie s m ay m a in ly con trib u te to the in c re m e n t o f the apparen t 60C o re te n tio n w ith tim e in the p re s e n c e o f such m e ta l a cety la c e to n a te s as a d d itiv e s . It is lik e ly that the liga n d tr a n s fe r m ay p ro c e e d v ia d is s o c ia tio n o f the a c e t y l aceton ate c o m p le x e s o f lo w e r s ta b ility . It is w o rth w h ile m en tion in g that the e ffe c t o f the addition o f a c e ty la c e ton ates on the beh aviou r o f r e c o il s p e c ie s is e s s e n tia lly s im ila r in the irr a d ia te d solu tions ( F ig . 3 ) and in the solu tion s o f the ir r a d ia te d s o lid c o m p le x (F ig . 4 ). Since the th e rm a l re a c tio n s taking p la c e in the solu tion s m a y be thus quite s im ila r both in the ir r a d ia te d solu tion s and in the solu tion s o f the ir r a d ia te d s o lid , w e have fu rth e r in v e s tig a te d the b eh a viou r o f r e c o il s p e c ie s in the solu tion s o f the ir r a d ia te d s o lid c o m p le x e s in the p re s e n c e o f v a r io u s a d d itives and at d iffe r e n t te m p e ra tu re s . In F ig . 5 is shown the change o f apparent 60C o re te n tio n w ith tim e a fte r d isso lu tio n o f the ir r a d ia te d so lid tris (a c e ty la c e to n a to )c o b a lt in the p re s e n c e o f m agn esiu m a c ety la ceto n a te as an a d d itiv e . T h e s e c u rv e s in d ica te the change o f the 60Co re te n tio n w ith tim e o f standing in solu tion s at v a r io u s te m p e ra tu re s . Although th ese c u rv e s a re quite s im ila r in shape to the annealing c u rv e s in so lid phase, the actual re a c tio n s taking p la c e in the solu tion s m a y not n e c e s s a r ily be s im ila r to the s o lid -p h a s e annealing re a c tio n s . M o re d e ta ile d k in etic a n a lyses o f such re te n tio n c u rv e s in solu tion phase at d iffe r e n t te m p e ra tu re s , and in the p re s e n c e o f v a r io u s a d d itiv e s , a re n e c e s s a r y fo r m o r e co m p le te u n d er standing o f what is taking p la c e in the solu tion s. C o n s id e rin g th ese data on the e ffe c ts o f a d d itiv e s , w e m ay now p ro p o s e a p ro b a b le m ech a n ism to ex p la in the o v e r a ll phenom enon including the apparent s c a v e n g e r e ffe c t o r the e ffe c t o f m e ta l sa lts and m e ta l a c e ty la c e tonates as a d d itiv e s in tr is (a c e ty la c e to n a to )c o b a lt — ben zen e s y s te m s .
196
F IG . 1 .
SAIT O and T O M IN A G A
T h e c o r r e la tio n b e tw e e n th e a pparen t s c a v e n g in g p o w e r o f m e t a l salts-in b e z e n e solu tions o f th e
ir r a d ia te d s o lid C o (a c a c )3 and th e th e rm o d y n a m ic s ta b ility constants o f th e ir a c e ty la c e to n a te s .
B e s id e s the p r im a r y re te n tio n o f the p aren t c o m p lex , v a rio u s lig a n d d e fic ie n t r e c o il s p e c ie s m a y be p rodu ced as the r e s u lt o f r e c o il and fra g m e n ta tio n . T h e s e lig a n d -d e fic ie n t 6cCo r e c o il s p e c ie s re c o m b in e with a c e ty la c e to n a te ion s s u c c e s s iv e ly to r e fo r m the p a ren t c o m p lex , thus con trib u tin g to the apparent 60C o re te n tio n . The nature o f th e rm a l re a c tio n s in the solu tion s can be studied in te r m s o f the change o f apparent 60Co re ten tio n . B a sed on our re s u lts on the e ffe c ts o f a d d itiv e s , at le a s t th ree typ es o f re a c tio n s m ay be postu lated to ex p la in the m ech a n ism o f the o v e r a ll phenom enon, o r the th e rm a l re a c tio n s taking p la c e in the solutions: (1)
R eco m b in a tio n .
(2)
S caven gin g re a c tio n .
[ 60 C o (a c a c )3 _xl + a c a c ' -Âť 60 C o (a c a c )3
(3)
L ig a n d tr a n s fe r .
M X n + acac" -* M (a c a c )n
[ 60C o (a c a c )3_x] + M (a c a c )n -Âť 60C o (a c a c )3 + M (a c a c ) (n > m ) m
In the s y s te m s w ithout an a d d itive, w e m a y s im p ly c o n s id e r reco m b in a tio n r e a c tio n s (1) alon e. In the sy s te m s containing a d d itiv e s , h o w e v e r, w e should p ostu late com b in ation s o f d iffe r e n t typ es o f re a c tio n s : n am ely, re a c tio n s (1), (2) and (3) fo r s y s te m s containing m e ta l s a lts as s c a v e n g e rs , and re a c tio n s (1) and (3) fo r sy s te m s containing m e ta l a c e ty la c e to n a te s as a d d itiv e s . A s w e have in d icated ab ove, unstable m e ta l a ce ty la c e to n a te s as ad dtives enhance the th e rm a l re a c tio n s in the solutions o f the ir r a d ia te d t r is (a c e ty la ceto n a to )co b a lt. In the p re s e n c e o f unstable m eta l a cety la c e to n a te s , w e should p re s u m e that re a c tio n s (1) and (3) a r e both taking p la c e , and the apparent 60C o re te n tio n is in c re a s e d as com p a red w ith the s y s te m s without an a d d itiv e , co rre s p o n d in g to the con trib u tion fr o m ligan d tr a n s fe r re a c tio n s (3). In F ig . 6 , such in crem en t o f the apparent 60Co re te n tio n at e q u ilib riu m is p lo tted a gain st the th erm od yn a m ic s ta b ility constant o f the m e ta l a c e ty la c e to n a te used as an a d d itive, and the exten t o f the ligan d tr a n s fe r re a c tio n s (3) c o r r e la t e s w e ll w ith the s ta b ility o f the m e ta l a c e ty la c e to n a te used as an a d d itiv e . I f the s y s te m s contain m e ta l s a lts as s c a v e n g e rs , h o w e v e r, w e m ay exp la in the o v e r a ll phenom enon, o r the c o r r e la tio n o f the sca ven gin g p o w er w ith s ta b ility , in te r m s o f re a c tio n s (1), (2) and (3). Im m e d ia te ly a fte r ir r a d ia tio n o f the solu tion , o r d isso lu tio n o f the ir r a d ia te d so lid , the co m p e titio n b etw een re a c tio n s (1) and (2) is im p ortan t, w h ile re a c tio n s (3)
197
IAEA-PL-615/11
100
í
Storage time (days) F I G . 8.
C o n v e rs io n o f MC o (a c a c )2 in to MC o (a c a c )3 in 0 .0 1 5 M b e n z e n e solu tions w ith variou s a d d itiv e s :
—O — no a d d itiv e “ A-
8 X 1 0 -3 M N i (a c a c )2‘ 2 H 20
-•-O —
8 X 1 0 ” 3 M M g (a c a c )2
1 . 5 x 10 ~2 M a c e t y la c e t o n e
no a d d itiv e ~Ш — 1 .5 x 1 0 ‘ 2 M a c e t y la c e t o n e
m a y s t ill be r e la t iv e ly unim portant. Such c o m p e titio n p red o m in a n tly fa v o u rs scaven gin g re a c tio n s ( 2 ), and the c o r r e la tio n c u rv e o f the sca ven gin g p o w e r w ith s ta b ility constant m ay approach c lo s e ly to the brokwn h o rizo n ta l lin e (lin e 1 + 2) in F ig . 7 on ly i f the ir r a d ia tio n and s to ra g e p e rio d s in solu tion s b eco m e as sh o rt as p o s s ib le . I f liga n d tr a n s fe r re a c tio n s (3) p ro c e e d g ra d u a lly in the solu tion , the apparent eoCo reten tio n in c r e a s e s and the apparent sca ven gin g p o w er d e c r e a s e s on standing in the solution. A c c o r d in g ly , the c o rre s p o n d in g point on the c o r r e la tio n c u rv e m ay be lo w e r e d fr o m the b rok en h o riz o n ta l lin e by a m agnitude eq u iva len t to the con trib u tion o f liga n d tr a n s fe r re a c tio n s (3) (c u rv e 3 in F ig . 7). A s we have shown in F ig . 6 , the extent o f re a c tio n s (3) rea ch ed at e q u ilib riu m , a fte r standing fo r a s u ffic ie n tly long tim e , depends on the s ta b ility o f the m e ta l a c e ty la ceto n a te r e la t iv e to that o f tris (a c e ty la c e to n a to )c o b a lt. T h e r e fo r e , the c o r r e la tio n o f the sca ven gin g p o w e r w ith s ta b ility constant o f m e ta l a c e ty la c e to n a te s can be ex p la in ed w e ll by p ostu latin g a m ech an ism in clu din g the re a c tio n s (1), (2) and (3). W ith a v ie w to obtaining .in form ation as to the oxid ation sta tes o f cob alt in the lig a n d -d e fic ie n t 60Co r e c o il s p e c ie s , w e r e c e n tly conducted two kinds o f e x p e rim e n ts [ 14] . T h e f ir s t s et o f e x p e rim e n ts (F ig . 8 ) in d icated that C o (a c a c )3 w as fo rm e d in C o (a c a c ) 2 b en zen e solu tion s on ly in the p re s e n c e o f a tm o s p h e ric oxygen , w h ile the C o (a c a c )3 fo rm a tio n w as c o m p le te ly su p p ressed in the a rgo n a tm o sp h ere. F u rth e rm o re , the C o (a c a c ) 3 fo rm a tio n w as enhanced by the addition o f unstable m e ta l a cety la c e to n a te s such as M g (a c a c ) 2 in the p re s e n c e o f a tm o s p h e ric o xygen . In the second set o f e x p e rim e n ts , w e in v e s tig a te d the e ffe c ts o f a tm o s p h e ric o xygen and M g (a c a c )2 on the b eh a vio u r o f the 60C o r e c o il s p e c ie s in ben zen e solu tion s o f the ir r a d ia te d s o lid C o (a c a c )3 . A lthou gh the 60C o re te n tio n w as in c re a s e d w ith
198
SAIT O and T O M IN A G A
F IG .9.
E ffe c t o f a tm o s p h e ric o x y g e n on
C o r e te n tio n in 0 .2 M solu tions o f irra d ia te d s o lid C o (a c a c )3
in b e n z e n e . no a d d itiv e —■ “
in air
w ith 3 x 10~3 M M g (a c a c )2
" О — no a d d itiv e w ith 3 x 10
M M g (a c a c >2
}
in argon
■
tim e a fte r d isso lu tio n in benzene solu tion s even in the a rgo n atm osp h ere, the in c re m e n t o f the 60C o reten tio n w ith tim e w as enhanced re m a rk a b ly in the p re s e n c e o f a tm o s p h e ric o xygen . M agn esiu m a c ety la ceto n a te as an a d d itive g r e a t ly enhanced the in c re m e n t o f the 60C o re te n tio n w ith tim e both in the a rgo n a tm osp h ere and in a ir . B a sed on the re s u lts fr o m these e x p e rim e n ts (F ig . 9), at le a s t 50% o f the 60C o r e c o il s p e c ie s in the benzene solu tion s m a y be assum ed as 60C o (II) s p e c ie s such as [ 60C o (II) (a c a c )3_n]^n' 1' +, w h ile the r e s e t m a y be 60C o (III) s p e c ie s such as [ 60C o (III) (a c a c )3_n] n+ (n-1, 2, o r 3). A c c o r d in g ly , the fin a l stage o f the 60C o (a c a c )3 re fo r m a tio n m ay be te n ta tiv e ly assu m ed as fo llo w s : °2
60C o (II) r e c o il
s p e c ie s
"*
60C o (III) r e c o il
s p e c ie s
[ 60C o (II) (a c a c )2] + a c a c '- ’- 60C o (a c a c )3 (re c o m b in a tio n and oxid ation ) [ 60C o (II) (a c a c )2 ] + M g (a c a c ) 2—-* 60C o (a c a c )3 (ligan d tr a n s fe r and oxidation , ^ w ith M g (a c a c )2 as an a d d itive) [ 60C o (III) (a c a c )2]+ + acac" -* 60C o (a c a c )3 (reco m b in a tio n ) [ 60C o (III) (a c a c )2]+ + M g (a c a c )2 ->■ 60C o (a c a c )3 (ligan d tr a n s fe r , w ith M g (a c a c ) 2 as an a d d itive)
In con clu sion , w e m a y say that the sca ven g in g e ffe c t o f the m e ta l sa lts w h ich can fo r m sta b le a c e ty la c e to n a te s is a s c rib e d m a in ly to the c o m p etitio n betw een the reco m b in a tio n re a c tio n s ( 1 ) and sca ven gin g re a c tio n s ( 2 ) to
199
IA E A -P L -6 15/11
cap tu re a c e ty la c e to n a te io n s. F o r the m e ta l s a lts w hich fo r m a cety la c e to n a te s o f lo w s ta b ility , h o w e v e r, liga n d tr a n s fe r re a c tio n s (3) m ay p ro c e e d a p p re c ia b ly on standing and a p p a ren tly d im in ish the o v e r a ll sca ven gin g e ffe c t. V e r y s im ila r o b s e rv a tio n s w e r e obtained w ith o rg a n ic solu tion s o f o th e r m e ta l c h ela te c o m p le x e s such as tr is (a c e ty la c e to n a to )c h ro m iu m and tris (n itro s o n a p h th o la to )c o b a lt [ 10, 11, 1 3 ].
REFERENCES [1 ]
K A Y A S , G . , SUE, P . . J. C h im . Phys. 45 (1 9 4 8 ) 188;
[2 ]
A D A M S O N , A . W . , G R U N L A N D , J . M . , J. A m . C h e m . S o c . 73 (1 9 5 1 ) 5508.
[3 ]
ZU BER,
[4 ]
S A IT O , N . ,
[5 ]
K A U F M A N , S . , J. A m . C h e m . S o c . 82 (1 9 6 0 ) 2 9 63.
[6 ]
Z A H N , U . , R a d io c h im . A c ta 7 J 19 67) 1 7 0 .
[7 ]
and B ull. S o c . C h im . (F ra n c e ) ЗЛ (1 9 5 0 ) 1145.
A . , U S A E C D o c . N Y O - 6 1 4 2 B N L (1 9 5 4 ). S A N O , H ., T O M IN A G A , T . ,
YANG, I .e .,
B u ll. C h e m .
S o c . (Japan) 33 (1 9 6 0 ) 20.
W ILE S, D . R . , C a n . J. C h e m . 45 (1 9 6 7 ) 1357.
[8 ]
TO M 1NAG A, T . ,
F U J IW A R A , K . , B ull. C h e m . S o c . (Japan) 43 (1 9 7 0 ) 2279.
[9 ]
T O M IN A G A , T . ,
S A K A I, T . ,
[1 0 ]
T O M IN A G A , T . ,
N ISH 1,
[1 1 ]
T O M IN A G A , T . , S A K A I,T . ,
[1 2 ]
T O M IN A G A , T . ,
N IS H I,
Y .,
ib id . 45 (1 9 7 2 ) 3213.
[1 3 ]
T O M IN A G A , T . ,
H 1SHI,
Y .,
R a d io c h e m . R a d io a n a l. L e tt. 11 (1 9 7 2 ) 289.
[1 4 ]
T O M IN A G A , T . ,
N IS H I,
Y .,
M O T O H A S H I, E ., R a d io c h e m . R a d io a n a l. L e tt. (1 9 7 4 ) in press.
Y .,
F U J IW A R A , K . , ib id . 4 4 (1 9 7 1 ) 3 0 36.
■
R a d io c h e m . R a d io a n a l. L e tt. 8 (1 9 7 1 ) 151. B u ll. C h e m . S o c . (Japan) 45 (1 9 7 2 ) 1237.
D IS C U S S IO N N . G E T O F F : Y o u r r e a c tio n m ech a n ism s e e m s v e r y co m p lic a te d , but i f I m a y put som e kn ow ledge fr o m ra d ia tio n c h e m is tr y into this m ech an ism , one w ould think that two typ es o f e le c tr o n s , the d ry e le c tr o n and the so lva ted e le c tr o n , p a rtic ip a te in the red u ction o f the co m p le x — that m eans in the fo rm a tio n o f co b a lt a c e ty la c e to n a te -II. W hen you have a lco h o l, you have so lv a te d e le c tr o n s ; in p u re ben zen e solu tion , you have on ly d r y e le c tr o n s . But, in both c a s e s oxygen is an e x c e lle n t s c a v e n g e r fo r the e le c tr o n s , it p re v e n ts the red u ctio n o f the c o m p le x , and the re te n tio n b e c o m e s h igh er. D o you think that th is assu m ption about the e le c tro n s could be c o r r e c t? N . S A IT O : I have not paid much attention to the b eh a viou r o f the e le c tr o n . W hat I am thinking is that the 60C o r e c o i l s p e c ie s u n d ergoes reco m b in a tio n w ith a c ety la ceto n a te and, o f c o u rs e , we have som e a c e ty la c e to n a te produ ced by ra d ia tio n e ffe c ts in solu tion . But then w e have a s c a v e n g e r, and this s c a v e n g e r m e ta l r e a c ts v e r y q u ick ly w ith a c ety la ceto n a te ion in solu tion — th is is one o f the typ es o f m ech a n ism I am thinking o f. O f c o u rs e , th e re a r e som e ra d ia tio n e ffe c ts , but in th ese typ es o f re a c tio n w e h a ven 't w o r r ie d to o m uch about the e le c tr o n s . W e have thought m o r e about the re a c tio n s o f a c e ty la c e to n a te w ith the m e ta l, and a ls o about the exchange re a c tio n betw een tw o a c e ty la c e to n a te r a d ic a ls . N . G E T O F F : I a g r e e w ith you, that th ese a re im p o rta n t. But you cannot exclu d e the e le c tr o n s , o r th e ir r e a c tio n s . E le c tr o n s o f both kinds, it is w e ll known, a re r e a c t iv e w ith a c e ty la c e to n a te , w ith aceton e, and w ith
200
SAI TO and T O M IN A G A
a ll th ese com pounds. B e s id e s the r e c o il re a c tio n s in solu tion you w ill also have p u re ly ra d ia tio n c h e m is try re a c tio n s as w e ll. N . S A IT O : D r. T o m in a g a is now c a r r y in g out e x p e rim e n ts w ith an e le c tr o n s c a v e n g e r p re s e n t. N . G E T O F F : O xygen is an e le c tr o n s c a v e n g e r, and acetyla ceto n a te it s e l f is a good s c a v e n g e r . But o xygen is b e tte r fr o m the pu lse r a d io ly s is w o rk . A . G . M A D D O C K : I think that p a rt o f the p e c u lia r itie s o f th ese sy stem s a r is e s fr o m so m e w e ll-k n o w n fe a tu re s o f the c o m p le x e s o f the d iva len t tra n s itio n m e ta ls o f the f ir s t ro w , and w hich turn up w h en ever you a re d ea lin g w ith an o x y g e n -d o n o r b ic h e la te lig a n d . In m ost ca s e s th ese s p e c ie s have been p re p a re d on ly fa ir ly r e c e n tly and w ith c o n s id e ra b le d iffic u lty . F o r ex a m p le, th e re is a g r e a t d ea l o f bogus m a te r ia l in the lite r a tu r e about the fe r r o u s c o m p le x e s , som e o f it w e ll known in re la tio n to som e bogus M o ssb a u er re s u lts , but a ll c le a r e d up r e c e n tly . T h is d iffic u lty a r is e s fr o m two ca u ses: the s p e c ie s a re s e n s itiv e to oxidation , and, although not ligand d e fic ie n t, a re co o rd in a tio n num ber d e fic ie n t. I think that this accounts fo r m any o f the o b s e rv a tio n s , s im p ly in te r m s o f the p e c u lia r r e a c t iv it y o f c o b a lt-a c ty la c e to n a te -II. T h is s o rt o f beh aviou r turns up in a num ber o f s y s te m s , fo r in stan ce in the e x p e rim e n ts that w e re p o rte d w ith S ie k ie rs k a and F e n g e r â&#x20AC;&#x201D; the one w e c a ll the dou ble-dou ble e x p e rim e n t, the one with tw o c o m p le x e s . It w as n e c e s s a r y th ere in o r d e r to g e t r e lia b le fig u re s fo r the reten tio n to in trod u ce a com p etin g ligan d to su p p ress r e fo rm a tio n . One g e ts a bogus re te n tio n by r e - fo r m a tio n in solu tion using sp a re d is s o c ia te d ligan d prod u ced e ith e r by r a d io ly s is o r som e o th er c h e m ic a l p r o c e s s . A f t e r a ll even a v e r y s m a ll amount o f sp a re ligan d w ill be quite e ffe c t iv e . F . S . R O W L A N D : I have a g e n e r a l qu estion that co m e s fr o m m y r e la t iv e la c k o f kn ow ledge o f th is phase o f in o rg a n ic c h e m is try . W hich o f th ese s p e c ie s o r re a c tio n s a re unknown o r v e r y d iffic u lt to study in th e rm a l, m a c r o s c o p ic sy stem s? N . S A IT O : In ou r explan ation w e have p ro p o sed th re e types o f re a c tio n s . F o r g e t about the 60C o - I I s p e c ie s , and ju st ta lk about the 60C o - I I I s p e c ie s . I think that the f i r s t tw o typ es o f re a c tio n a re quite understandable, and much fu ndam ental in fo rm a tio n is a v a ila b le about th ese re a c tio n s . But ligan d tr a n s fe r re a c tio n s a re not so w e ll known. R e g a rd in g C o - III a c e t y l aceton ate com pounds, and a lso ch rom iu m com pounds, much in fo rm a tio n is a v a ila b le fr o m n o n -ra d io a c tiv e ex p e rim e n ts . H o w e v e r, we a re d ealin g with the v e r y unstable s p e c ie s produ ced b y r e c o il o r by ra d ia tio n e ffe c t s , and s o m e tim e s the e x p e rim e n ts a re v e r y d iffic u lt to in te r p r e t, and the con cen tra tio n s a r e e x tr e m e ly s m a ll. T h e d iffic u lty in ou r hot atom c h e m is try is that w e cannot apply standard in o rg a n ic m eth ods. W e cannot apply n o rm a l s p e c tro s c o p y to the study o f the hot atom s p e c ie s b ecau se the co n cen tra tio n is too lo w . F . S . R O W L A N D : I f I understand c o r r e c t ly , you a re sa yin g that in the co b a lt s y s te m y o u r understanding is la r g e ly b ased on th e rm a l re a c tio n s o f known s p e c ie s , and the hot atom c h e m is tr y is ra th e r w e ll hidden. N . S A IT O : T h is p a rt o f m y ta lk w as con cern ed w ith th e rm a l r e a c tio n s . T h e point that I w anted to r a is e is that m any p eo p le s im p ly ir r a d ia te so lid a c e ty la c e to n a te o r som e o th er com pound, and then just d is s o lv e this in som e so lven t, som e o rg a n ic so lven t, without the addition o f m e ta l o r any o th e r a d d itiv e . W e have oth er ex a m p le s o f ir r a d ia te d s o lid s d is s o lv e d in pu re ben zen e g iv in g re te n tio n s o f about 80%, but d is s o lv e d in ben zen e
IA E A -P L-6 1 5/11
201
solu tion w ith som e a d d itive — l e t 's say, a m e ta l salt, iro n o r alu m iniu m — the re te n tio n d e c r e a s e s r e m a r k a b ly to le s s than 20%. W hat I want to s tr e s s is that w e have to su p p ress th e rm a l r e a c tio n s taking p la c e in solu tion im m e d ia te ly a fte r d is s o lu tio n . W ith som e a d d itiv e s , s c a v e n g e rs , w e can e s tim a te the p r im a r y re te n tio n . N o r m a lly , w e o b s e r v e the p r im a r y reten tio n plus the th e rm a l re a c tio n p ro d u cts. F . S . R O W L A N D : M y qu estion is d ir e c te d to w a rd s fin din g out w h eth er th e re a r e so m e ad van tages f o r this hot atom s y s te m fo r the study o f th e rm a l c h e m is tr y , o r w h eth er it can be used on ly fo r id e n tify in g p r im a r y reten tio n , and o th e r fa c ts o f in te r e s t c h ie fly to hot atom c h e m is ts . N . S A IT O : I think th e re is som e p o s s ib ility fo r studying v e r y funda m en ta l solu tion c h e m is tr y , fo r w e can in je c t a s m a ll amount o f a s p e c ie s into solu tion through the use o f r e c o il re a c tio n s . W e can do som e funda m en ta l stu dies o f co o rd in a tio n c h e m is tr y in tr a c e co n c e n tra tio n s . H o w e v e r, th is is m y p e rs o n a l fe e lin g , and m ayb e som e o th e r p eo p le have c o m p le te ly d iffe r e n t v ie w s . S. A M IE L : Is it m o r e o f a to o l to study co o rd in a tio n c h e m is tr y than it is to study hot atom c h e m ic a l p r o c e s s e s ? N . S A IT O : In this c a se, lo ts o f solu tion c h e m is tr y and c o o rd in a tio n c h e m is tr y is in v o lv e d , and som e ra d ia tio n c h e m is tr y . But w e have found that the ra d ia tio n d e c o m p o s itio n o f th ese com pounds is le s s than 1 %. N o r m a lly , w e ir r a d ia t e in the r e a c t o r w ith a neutron flu x o f 5 X 1011 n /cm 2 • s. T h e solu tion is ir r a d ia te d f o r f iv e m in u tes at r o o m te m p e ra tu re , and the g a m m a d ose a s s o c ia te d w ith th is ra d ia tio n is a p p ro x im a te ly 7 X 104 R . W e a lso ir r a d ia te d the s o lid c h ro m iu m a c e ty la c e to n a te -III w ith a s im ila r flu x fo r fiv e m in u tes at the te m p e ra tu re o f d r y ic e , and then w e d is s o lv e d it in b en zen e. W e then ch ecked fo r the d e c o m p o s itio n o f th ese m o le c u le s and found it to be le s s than 1 %. S. A M IE L : P e rh a p s it w ould be b e tte r to m ake a c o m p a ris o n w ith m o n o ch ro m a tic X - r a y s d ire c te d at the К ed ge in cob a lt, to g e t a m o re lo c a liz e d e ffe c t. In th is w ay, m ayb e you can d ir e c t the e ffe c t into the c e n tr a l atom . And then you m ay be able to c o m p a re it w ith r e c o il e ffe c ts in solu tion .
IA E A -P L-61 5/12
HOT ATOM CHEMISTRY OF CARBON A.P. W O L F Chemistry Department, Brookhaven National Laboratory, Upton, Long Island, N.Y., United States of America Abstract H O T A T O M C H E M IS T R Y O F C A R B O N . T h e ch e m is try o f e n e rg e tic carb on atom s is discussed. T h e e x p e rim e n ta l ap p roach t o studies th at h ave been c a rried o u t is d escrib ed and th e m ech an istic fra m e w o rk o f h o t ca rb o n a to m rea ctio n s is c o n sid ered in s o m e d etail. F in a lly , th e d ire c tio n th a t fu tu re w o rk m ig h t take is ex a m in e d , in c lu d in g th e rela tio n sh ip o f e x p e rim e n ta l to th e o re tic a l w ork .
T h e hot atom c h e m is tr y o f carb on is one o f the m ost c o m p le x a re a s o f hot atom c h e m is tr y w h ile b ein g one o f the m o st e a s ily a c c e s s ib le fo r e x p e rim e n ta l study. T h is stem s fr o m the fa c t that the c h e m is tr y o f the com pounds o f carbon is without qu estion the m ost e x te n s iv e ly studied of the compound o f any elem en t in the p e rio d ic tab le. A t the sam e tim e it is p erh aps the m ost co m p le x becau se o f the m u ltiva len t nature o f ca rb on and the fa c t that ca rb on can r e a c t with it s e lf and with many oth er e lem en ts in n e a r in fin ite v a r ie ty . Thus, the r e m a r k s w hich fo llo w can apply in p rin c ip le to o th e r e lem en ts which a re m u ltiva len t and w hose c h e m ic a l re a c tio n s have been studied in the gaseou s, liqu id and s o lid states. One o f the m a jo r d iffic u ltie s in p la c in g the c h e m is try o f e n e r g e tic carb on atom s in p r o p e r focu s in the fie ld o f hot atom c h e m is tr y in g e n e ra l is p erhaps due to the fa ct that m any e ffo r ts h ave been m ade to o v e r s im p lify th ese re a c tio n s and use as m od els the re a c tio n s o f m on ovalen t s p e c ie s such as tritiu m , flu o rin e , c h lo rin e and b ro m in e . W h ile such c o m p a ris o n s have stim u lated much u sefu l w o rk , sw eep in g g e n e r a litie s have been m ade ir. the past which do not stand up to c lo s e r scru tin y in s o fa r as they a n sw ered the b a sic qu estion s o f the n atu re o f the c h e m ic a l in te ra c tio n s o f the p r im a r y r e a c tiv e encounter and how the p r im a r y encou n ters a ffe c te d is o la b le -p ro d u c t d istrib u tion . In studying carb on atom c h e m is tr y a d istin ctio n that is v ita l to any h yp oth esis is a c le a r d elin ea tion o f the c h e m ic a l s y stem . G e n e ra liz a tio n s based on re a c tio n s in one s y s te m do not n e c e s s a r ily apply to a ll sy s te m s as it m ust be c le a r that e le c tr o p h ilic a ro m a tic substitution re a c tio n s d iffe r in kind and c h a r a c te r fr o m n u cleo p h ilic a lip h atic substitution re a c tio n s . So re a c tio n with u nsaturated s y s te m s o ffe r s c o m p le x itie s and d iffe r e n c e s in q u a lity and quantity fr o m re a c tio n s in satu rated sy s te m s . The c h e m is tr y o f e n e rg e tic carb on atom s as c o n s id e re d h e r e d raw s its h ypoth eses fr o m re a c tio n s studied in satu rated s y s te m s . In g e n e ra l the h yp oth eses gen era ted a r e deduced fr o m produ ct a n a ly s is as is the c a s e fo r e s s e n tia lly a ll the w o rk done in the fie ld o f hot atom c h e m is tr y up to the p resen t.
203
204
W OLF
T h e e x p e rim e n ta l approach to th ese stu dies d e s e r v e s som e m ention. E a r ly w o rk on h o t-a to m c h e m is tr y was done u sin g c a r b o n - 14 atom s gen era ted by the n ,p re a c tio n on n it r o g e n - 14. W h ile the e a r ly s ta tis tic a l th e o r ie s o f carb on atom re a c tio n s w e r e based on these r e s u lts , substantial p r o g r e s s w as not a ch ieved u n til gas phase re a c tio n s w e r e studied and a c c e le r a t o r produced c a rb o n - 1 1 was a ccep ted as the b e tte r r a d io a c tiv e s p e c ie s fo r b a sic stu dies. The m o st co m m o n ly used re a c tio n s are 14 N (p , n )n C , 1 2 C (p, p n )u C and the 1 2 C ( 7 , n ) n C. T h e use o f the a c c e le r a to r allo w ed c o n tro l o f con com ita n t ra d ia tio n dose to the s y s te m under study and allo w ed stu dies o f a s in g le s y s te m in a ll ph ases. T h e c o n tro l o f ra d ia tion d ose m ade it p o s s ib le to s e p a ra te ra d ia tio n c h e m is tr y fro m hot atom c h e m is tr y and the c o n tro l o f phase, e s p e c ia lly the fa c ile use o f gas phase s y s te m s a llo w ed in sigh t into the in te r m o le c u la r e ffe c ts on in te rm e d ia te s p e c ie s in addition to the p h y s ic a l e ffe c ts on the r e a c t iv e atom . It is at this stage d iffic u lt to say w h eth er o r not e n e r g e tic ca rb on atom s u n d ergo tru ly unique re a c tio n s when co m p a re d with th e ir th e rm a l c o u n te r p a rts. It has been postu lated that the fo rm a tio n o f H 1 :LC = N in N 2 -H 2 m ix tu res and the fo rm a tio n o f n C H = CH and n C H 2 = CH 2 in h y d ro ca rb o n s a re tru e hot prod u cts. On c a r e fu l c o n s id e ra tio n , h o w e v e r, what m ust be said is that un til the c h e m is tr y o f "B o ltz m a n n d is trib u tio n " carbon atom s in the ground sta te, and perh aps the f ir s t th re e lo w - ly in g e le c tr o n ic states is c o m p le te ly u n d erstood , it is p re m a tu re to sta te that H C N , C 2 H2 and C 2 H 4 a re nonth e rm a l carb on atom prod u cts. R ea so n s fo r this postu late w ill b eco m e apparen t la te r in the text. The c h e m is tr y o f e n e r g e tic carbon atom s m ust be c o n s id e re d in ligh t o f the e le c tr o n ic state o f the atom in the f i r s t r e a c t iv e encounter. E x p e r i m en tal d iffe re n tia tio n s o f the e le c tr o n ic s ta te -c h e m ic a l re a c tio n re la tio n s h ip a re p o s s ib le in gas phase stu dies. The e le c tr o n ic sta tes o f carb on a re as fo llo w s : ground state 3P ; f ir s t e x c ite d state 1 D, 1.26 eV above ground state; 1 S, 2.38 eV above ground sta te, 5 S °, 4.18 eV above ground state. It is p ro b a b le that the 1S and sS° states a re not in v o lv e d in hot atom re a c tio n s , thus w e can co n cen tra te on the 3P and 1D sta tes as b ein g re s p o n s ib le fo r the re a c tio n s we p resu m e take p la ce. A n o th er p h y sica l p a ra m e te r o f n u cleogen ic ca rb on that is w orth y o f note is its c h a rg e state on the f ir s t r e a c t iv e encou n ter. W h ile e s s e n tia lly a ll the studies re p o rte d in the lite r a tu r e to date assu m e re a c tio n by a n eu tra l atom , it m ay be that som e carb on ions a re r e a c tin g in h eliu m and neon m o d era ted s y s te m s . T h is a s p e c t o f n u cleogen ic carb on c h e m is try is as y e t w h o lly u n explored. Studies o f e n e rg e tic c a rb o n -a to m re a c tio n s can b e n e fit fr o m the in c r e a s in g amount o f w o rk which has been done on ca rb on atom c h e m is tr y o v e r the past s ix y e a r s . It has g e n e r a lly been re p o rte d under the heading o f ca rb on atom re a c tio n s but in m o st ca s e s is perhaps m o re p r o p e r ly d e s c rib e d as r a te s o f ca rb on atom d isa p p ea ra n ce in a v a r ie t y o f r e a c tiv e su b stra tes. S e v e r a l s trik in g fe a tu re s appear. T h e postu late ra is e d in e a r ly w o rk on the hot atom c h e m is tr y o f carb on that the 3P carb on atom is e ffic ie n t ly sca ven ged by o xygen w as s tr ik in g ly c o n firm e d by a num ber o f w o r k e r s who m ea su red the ra te o f d isa p p ea ra n ce (re a c tio n ) o f 3P ca rb on in pure oxygen . It is the fa s te s t known re a c tio n o f the 3P state studied to date. T h e s e v a rio u s stu dies a lso showed that the d isa p p ea ra n ce ra te o f carb on atom s in h yd ro ca rb o n s was 104 to 106 s lo w e r. The fa c ts as to the r a te o f d isa p p ea ra n ce o f the state a re som ew hat le s s c le a r . The ra te with oxygen m ay be co m p a ra b le o r an o r d e r o f m agnitude s lo w e r but the ra te
205
IAEA-PL-615/12
with h yd ro ca rb o n s is v e r y much fa s te r than is the ra te o f re a c tio n o f the 3P state w ith h y d ro ca rb o n s and m ay be as fa s t as the r a te o f 3P carb on with oxygen . F r o m th ese o b s e rv a tio n s we can sa y with c e rta in ty that in oxygen sca ven ged hot atom s y s te m s , 3P th e rm a l ca rb on atom s a re not re s p o n s ib le fo r any o f the prod u cts, oth er than и СО , that a re o b s e rv e d . Thu s, in d e v e lo p in g a m ech a n istic b a sis fo r product fo rm a tio n brought about in s y s te m s con tain in g n u cleogen ic ca rb o n s, we need on ly c o n s id e r th re e s p e c ie s : th e rm a l *D ca rb o n s, hot *D carbon s and hot 3P carb on s. In tu rning to the hot atom c h e m is tr y o f carb on it is w e ll known that in m ost s im p le unsaturated oxygen scaven ged s y s te m s th re e produ cts u su ally account fo r o v e r 50% o f the atom s g e n era ted , n CO, n C H = CH , 1 1 CH 2 = C H 2. N u m erou s stu dies have shown that in 0 2 and in s y s te m s con tain in g oxygen ated com pounds, ca rb on m on oxide is the p r im a r y produ ct produ ced by th e rm a l and by hot re a c tio n s . T h e 1 ХС 0 2 o b s e rv e d is due to r a d io ly tic o x id a tio n of 11CO o r oxid ation o f se c o n d a ry in te rm e d ia te s . The h yp oth esis that u n d e rlie s the m ech a n istic fr a m e w o r k o f hot carb on atom re a c tio n s is based on the postu late that the hot ca rb on atom ( 3P and 2D) can in s e r t into a carb on h yd rogen bond and that it ( 3P and *D) can a lso a b s tra c t h yd rogen . T h is h yp oth esis can be used to ex p la in m any o f the p rodu cts that a r e o b s e rv e d . P r e s e n t ev id e n c e a ls o su pports postu lates in v o lv in g s trip p in g o f carb on atom s and s m a ll grou ps fr o m su b stra te m o le cu les u ltim a te ly y ie ld in g products and re p la c e m e n t o f c a rb o n atom s and n itro g e n atom s in "k n o c k -o n " c o llis io n s in v o lv in g su b stra te m o le c u le s le a d in g to la b e lle d paren t com pounds. T h e s e m ech a n ism s, h o w e v e r, at b e s t p rob a b ly p lay r e la t iv e ly m in o r r o le s in ca rb on atom c h e m is try . L e t us exam in e the b a sic re a c tio n . A n e n e rg e tic ca rb on atom r e a c ts with an alkane, e.g . propan e, by e ith e r in s e rtio n I o r a b s tra c tio n A (1) and (2) (r a d ic a l d esign ation s a re om itted ). [n C ]* + CH 3 -C H 2 -C H 3 -> [C H 3 C H 2 CH 2U CH]
I
(1)
[ 11 C ]* + CH 3 -C H 2 -C H 3 ^ [ U C H ]* + C H 3 CH 2 C H 2
A
(2)
The in te rm e d ia te in (1) can d ecom p ose to g iv e a c e ty le n e by a c o n c e rte d ( i f *D in s e rtio n ) o r s te p w is e (ЧЭ o r 3P ) r e a c tio n (3) and (4 ), o r it can d ecom p ose to g iv e CH , (5) (in s e rtio n d e c o m p o sitio n rou te) o r it can r e a c t with C>2 o r su b strate to g iv e a seco n d a ry p rodu ct ( 6 ) [C H 3 CH 2 CH 2U C H ]* - C H 3 CH 2 + n C H = C H + H
(3)
[C H 3 CH 2 C H 2n C H ]* - CH 3 C H 2 + n C H 2 = CH •> ^ c h 2 = n CH -
u CHh c h + h
(4)
j
[C H 3 CH 2 C H 2n C H ]* - CH 3 CH 2 C H 2 + [ U C H ]*
(5)
[CHgCHg CH 2n C H ]* + N - [N ]*
( 6)
E x p e rim e n ta l e v id e n c e u sin g double la b e l techniques s tro n g ly supports the fo rm a tio n o f a c e ty le n e by an in tra m o le c u la r m ech an ism . T h is m eans to say that the a c ety len e is fo rm e d fr o m a s in g le m o le c u le which it en cou n tered in r e a c t iv e c o llis io n .
206
WOLF
T w o p ostu lates have been put fo rth fo r the fo rm a tio n o f eth ylen e. One sta tes that the e x c ite d in s e rtio n in te rm e d ia te (r e a c tio n ( 1 )) u n dergoes u n im o le c u la r d eco m p o sitio n to g iv e the v in y l r a d ic a l which subsequently a b s tra c ts h yd rogen to g iv e eth ylen e, (7 ). The o th e r states that m ethyne [ n CH] in s e rts into a ca rb o n -h y d ro g e n bond to g iv e an in te rm e d ia te which u n d ergoes u n im o le c u la r d eco m p o sitio n to g iv e eth ylen e d ir e c tly , ( 8 ).
[CH3CH2CH21]CH]* - cit, =псн+сн3сн2 CH2=nCH+RH -
ch2=1:ich 2+r
(7)
[nCH] +CH3CH2CH3 - [сн3сн2сн2псн2]* [CH3CH2CH2UCH2]* ->CH3CH2 +CH2=nCH2
(8)
T h e v in y l r a d ic a l m ech an ism is p ro b a b ly not re s p o n s ib le fo r ethylene fo rm a tio n in the hot re g io n . T h is con clu sion is based on e x p e rim e n ta l evid e n c e : (a) V in y l ra d ic a ls r e a c t ra p id ly w ith oxygen and no a p p re c ia b le dim inution o f eth ylen e y ie ld is o b s e rv e d in oxygen sca ven ged s y stem s; (b) o b s e rv e d d eu teriu m iso to p e e ffe c ts in eth ylen e fo rm a tio n a re the op p osite o f what one would p re d ic t if v in y l r a d ic a ls w e r e the im m ed ia te p r e c u r s o r s ; (c ) lit t le o r no v in y l c h lo rid e is o b s e rv e d in ch lorin a ted h yd ro ca rb o n s, the o p p osite o f what one would p re d ic t; (d) d r a s tic a lly red u ced y ie ld s o f ethylene in h aloca rb on s a re not con sisten t w ith the m ech an ism g iven in E q .(7 ); (e ) e v id e n c e based on re s u lts fr o m double la b e l e x p e rim e n ts is not c o n sis te n t with the m ech an ism g iv e n in E q .(7 ). Th e sam e e vid en ce outlined above is co n sisten t with an in te rm o le c u la r m ech a n ism in v o lv in g the fo rm a tio n o f m ethyne in a hot re a c tio n as the p r im a r y step le a d in g to ethylene fo rm a tio n . It has a ls o been p o s s ib le to show that th is in te rm o le c u la r m ech an ism fo r eth ylen e in v o lv e s only m eth yl grou ps as the im m e d ia te p r e c u r s o r o f the product. The gas phase y ie ld o f eth ylen e fr o m com pounds con tain in g m eth ylen e groups on ly (e . g . , c y c lo propan e, cyclob u ta n e, cy clop en ta n e, e tc .) is n e g lig ib le . T h e e th y le n e -n C that is o b s e rv e d m ust a r is e fr o m so m e oth er m ech an ism . Can anything be said about the hot re a c tio n s o f the and 3P carbon a to m s ? A s a w o rk in g h ypoth esis the fo llo w in g sequence is su ggested in oxygen sca ven ged s y s te m s . The p rin c ip a l re a c tio n o f hot JD carbon atom s is to y ie ld m ethyne which then in s e rts in su b stra te to y ie ld ethylene u ltim a te ly . Iso to p e e ffe c t w o rk would in d ica te that m ethyne fo rm a tio n o ccu rs by in s e rtio n -d e c o m p o s itio n in the hot re g io n . The p rin c ip a l re a c tio n o f hot 3P ca rb on atom s is to y ie ld a c ety len e and a m in o r pathway y ie ld s m ethyne. A t lo w e r e n e r g ie s a c e ty le n e m ay a ls o be fo rm e d v ia 1D carbon atom in s e rtio n d eco m p o sitio n , a c e ty le n e fr o m 3P re a c tio n d is a p p e a rs , and a s m a ll com ponent o f 3P ab stra ctio n re a c tio n can lead to m ethyne which can then g iv e eth ylen e. E v id e n c e fo r this h yp oth esis is im p lic it in the p revio u s d iscu ssio n and is stren gth en ed by m o d e ra to r e x p e rim e n ts in h eliu m , neon, and xenon. The b a sic e ffe c t in a ll th re e is to in c r e a s e the y ie ld o f CO as the m o d e ra to r con cen tra tio n in c re a s e s (and the num ber o f carb on s rea ch in g the th e rm a l ra n ge in c r e a s e s ) and to d e c r e a s e the y ie ld o f a c ety len e and eth ylen e which is to be exp ected if th ese a r e p r im a r ily hot produ cts. F u r th e r m o r e , an exam in ation o f the y ie ld o f a c e ty le n e r e la t iv e to ethylene
IA E A -P L-6 1 5/12
207
as th e c o n cen tra tio n o f xenon in c r e a s e s shows a con sta n tly in c r e a s in g ra tio . X en on is an e ffic ie n t n o n -r e a c tiv e spin c o n v e r te r fo r carb on and is c o n v e r t in g carb on to 3P carb on thus enhancing products d e r iv e d fr o m the 3P state and d e c r e a s in g p rodu cts r e s u ltin g fr o m the state. It is c le a r fr o m th ese p ostu lates that the p ro b le m o f shadow ing is v e r y much w ith us in ca rb on hot atom c h e m is tr y . Th e th e o r e tic a l ca lcu la tio n s o f N ew ton and B lin t w e r e not c o n s id e re d h e r e sin ce th e ir im p a ct on an u n derstanding o f carb on atom c h e m is tr y has b een p resen ted s e p a ra te ly at this m eetin g. A su m m a ry o f our p resen t know ledge would su ggest the fo llo w in g fr a m e w o r k .
(1)
A t the u pper end o f the e n e rg y ran ge w h e re the carbon atom s b ec o m e c h e m ic a lly r e a c tiv e (a )
(b)
(2)
A t the lo w e r end o f the e n e rg y ra n ge (a)
(b)
(3)
3P ca rb on in s e rtio n re a c tio n re s u lts in a c e ty le n e fo rm a tio n 3P ca rb o n a b stra ctio n re a c tio n r e s u lts in eth ylen e fo rm a tio n but this is à m in o r pathway fo r eth ylen e fo rm a tio n 1D ca rb on in s e rtio n d eco m p o sitio n re a c tio n re s u lts in eth ylen e fo rm a tio n
3P carb on s in the th e rm a l and n ea r th e rm a l ran ge a r e sca ven ged b y О 2. R a te o f re a c tio n w ith o rg a n ic su b stra tes is so lo w as to be n o n -c o m p e titiv e . 1D carb on s s t ill u n dergo in s e rtio n d e c o m p o s itio n and u ltim a te ly y ie ld eth ylen e. T h e in s e rtio n in te rm e d ia te m ay a ls o b e g in to fra g m e n t to y ie ld a ce ty le n e .
A b so lu te y ie ld depends on stru ctu re and phase. Substantial e vid en ce e x is ts that ca rb o n atom s a re p o w e rfu l e le c tr o p h ile s , that bond e n e r g ie s , e le c tr o n d en sity and bond p o la riz a tio n fa c to r s a ffe c t y ie ld s , that the d e g r e e s o f fr e e d o m o f the in te rm e d ia te (r e fle c t in g the a b ility o f the in te rm e d ia te to d e lo c a liz e the v ib ra tio n a l e n e rg y r e s u ltin g fr o m the r e a c tiv e en cou n ter) a ffe c t the y ie ld , and fin a lly that the la r g e red u ction in y ie ld on change in phase fu rth e r supports the postu late o f a v ib r a tio n a lly ex c ite d in te rm e d ia te o r in te rm e d ia te s . H o w e v e r , bond counting m ethods o r s tru c tu ra l dependence h ypoth eses t e ll us lit t le about d e ta ile d m ech an ism .
It is e s s e n tia l to again s tr e s s that the fr a m e w o r k outlined in (1) and (2) a p p lies to gas phase re a c tio n s in alkanes and a fe w oth er c la s s e s o f s im p le com pounds w hich a r e scaven ged by oxygen . F o r e x a m p le, th e re is e vid en ce in the lite r a tu r e that in som e s y s te m s (e .g ., cy c lo p ro p a n e and b en zen e) m o re than one m ech an ism is o p e r a tiv e in the fo rm a tio n o f a c e ty le n e . (A s much as 40% o f the a c ety len e can be fo rm e d by a second m ech a n ism .) T h is u n d e r lin e s the com m en ts m ade in the in trod u ction that hot atom c h e m is tr y in o rg a n ic s y s te m s m ay indeed not be r e a d ily explain ed by one o r tw o sim p le p ostu lates about 1D and 3P ca rb on atom s.
208
W O LF
CONCLUSION W hat is perhaps m ost im p ortan t at this sta ge is to com m en t on the d ire c tio n that fu tu re w o rk m ight take. N ew e x p e r im e n ta l w o rk on th e rm a l carb on atom s is e s s e n tia l i f w e a re to u n ra v e l and pinpoint th ose re a c tio n s w hich a re uniquely due to k in e tic a lly e x c ite d carb on atom s. T h is is p a r tic u la r ly tru e o f the re a c tio n s o f ï ) (and p o s s ib ly : S) carb on atom s. T h e ir r e la t iv e r e a c t iv it y in a B o ltzm a n n -d is trib u tio n situ ation has as y e t not been d eterm in ed . Thus, knowing the r a te o f d isa p p ea ra n ce o f th ese atom s is on ly a beginning; the produ cts r e s u ltin g fr o m th e ir d isa p p ea ra n ce in a v a r ie t y o f su b strates need to be d eterm in ed . M o r e a ccu ra te va lu es fo r the ra te s o f d isap p earan ce (and re a c tio n ) o f the 3P carb on atom in n on -oxygen con tain in g s y s te m s a re n eeded but a d m itted ly the e x tra o rd in a ry ra te o f 3P + 0 2 m akes such studies qu ite d iffic u lt e x p e rim e n ta lly . E x p e rim e n ta l w o rk on carb on ions should be sta rted . E ffo r ts in v o lv in g carb on ions in s o lid s u n fortu n ately s u ffe r fr o m c o m p le x itie s which m ake it d iffic u lt to d e lin e a te tru e io n -m o le c u le re a c tio n s . P ro d u c t a n a lysis cannot e a s ily p ro v id e a d iffe re n tia tio n betw een ion ic re a c tio n s and n e u tr a l-s p e c ie s re a c tio n s in th ese s y s te m s . Gas phase stu dies w h ere one is c e rta in o f the ch a rg e state of the s p e c ie s u n d ergoin g c h e m ic a lly r e a c tiv e encounter w ould p ro b a b ly be the m o st p ro m is in g approach. C a re to d e te rm in e e le c tr o n ic state should be taken at the outset. Continuing w o rk on the alkan es to stren gth en fu rth e r the hypotheses (o r r e p la c e them w ith new ones) put fo rth is e s s e n tia l. Much can s t ill be done w ith gas phase w o rk in v o lv in g m ix tu re s , m o d e ra to rs , spin c o n v e r to r s and o th e r s im p le c la s s e s o f com pounds such as the h aloalk an es. F o r ex a m p le , the s tr ik in g e ffe c t o f halogen on the y ie ld o f eth ylen e is in need o f fu rth e r study. T h e m in o r products one s e e s as a re s u lt o f carbon atom re a c tio n s a r e a ls o in need o f fu rth e r study. F o r e x a m p le, the fo rm a tio n o f p r o p a n e - n C fr o m ethane, o r to lu e n e - n C fr o m b en zen e, a re explained in te r m s o f the fo rm a tio n o f m e th y le n e - n C [ n C H 2 ] which then in s e rts into a C -H bond to g iv e the product. An a ltern a te m ech an ism in v o lv in g carbon atom in s e rtio n to g iv e (CH 3 -C H 2 -C H ]* (and фп С Н ]*, which in te rm e d ia te then a b s tra cts the n e c e s s a r y h yd ro gen fr o m the su rrou n d in gs, has not been u n e q u iv o c a lly ru led out by any published e x p e rim e n ta l evid e n c e . The gas phase has been s tr e s s e d h e re . L iq u id phase studies a re p a r tic u la r ly ea sy to c a r r y out and m any a re a s a re in need o f study. Som e e a r ly w ork on solid phase re a c tio n s has been re p o rte d but h e r e e x p e rim e n ta l d iffic u ltie s in v o lv ing the is o la tio n o f r a d io c h e m ic a lly pure products make th ese stu dies s o m e what le s s a c c e s s ib le to the use o f c a rb o n - 1 1 and perhaps a re b est c a r r ie d out u sin g c a r b o n - 14. E x te n s iv e com m en t has been m ade on so m e th e o r e tic a l a sp ects of stu dying the hot atom c h e m is tr y o f carb on . T h is is perhaps one o f the m ost e x c itin g new fie ld s . O nly r e c e n tly has th e o r e tic a l w o rk been applied to carb on . T h e o r e tic a l w o rk on tritiu m atom s has been e x te n s iv e not only by hot atom c h e m is ts but by num erous in divid u als who have no d ir e c t in te re s t in the fie ld . Th at this has not been the c a s e with carbon is perhaps e a s ily u n derstood. T h e c o m p le x itie s a r e com pounded en o rm o u sly by the s y s te m s one needs to d ea l with. T h e o r e tic a l w o rk on carb on atom s plus sim p le alkan es is a n e c e s s a r y co m p lem en t to the h ypoth eses about carbon atom s g e n era ted fr o m e x p e rim e n ta l w o rk at p resen t a v a ila b le . C on figu ration
IAE A -P L-61 5/12
209
in te ra c tio n ca lcu la tio n s would m a te r ia lly enhance the a b ility to sep a ra te 3P fr o m *D re a c tio n s and p ro v id e a new b a sis fo r fu rth e r e x p e rim e n ta l w ork . D ynam ic ca lcu la tio n s have as y e t not been c a r r ie d out. T h e im p lic a  tion s o f such ca lcu la tio n s in b ein g able to understand re s u lts in alkan es, a lk y l h a lid e s , a lco h o ls and o le fin s is c le a r . It is hoped that an e ffo r t s im ila r to that in the tritiu m fie ld w ill e v o lv e . E x p e rim e n ta l v e r ific a t io n o r e lim in a tio n o f c u rre n t p ostu lates is s t ill v e r y much a p a rt o f the hot atom c h e m is tr y o f carb on . Much w o rk s t ill needs to be done b e fo r e we understand the c h e m ic a l re a c tio n s o f 3P , *D and 1S ca rb o n atom s both th e rm a l and k in e tic a lly e x cited . H ot atom c h e m is tr y p ro v id e s a unique to o l fo r these stu dies sin ce it can focu s on the m ic r o s c o p ic p r o p e r tie s o f s y s te m s and can h elp us understand what happens on the f ir s t r e a c tiv e en cou n ter o f a carb on atom and a su b strate m o le c u le . T h is basic u n derstanding is e s s e n tia l to a broad a r e a ra n gin g fro m re a c tio n s at the m o le c u la r le v e l to p re d ic tin g the p r o p e r tie s o f m a c ro s c o p ic s y s te m s .
ACKNO W LED G EM ENTS T h e s e com m en ts w e r e m ade p o s s ib le by the dedicated e ffo r ts o f many o f m y c o lle a g u e s who w ork ed with m e in tr y in g to elu cid a te the c h e m is try o f the carb on atom s. S p ecia l m ention is due to H. A c h e , R . A y r e s , D. C h ristm a n , F . Fau citan o, R. F in n , N. Furukaw a, R. L a m b re c h t, P . L ie b e r m a n , G. S tocklin , K . T a y lo r , M . W e lc h and J. Yan g. Som e o f the m ost re c e n t w o rk on isotop e e ffe c ts and m o d e ra to r studies w as done by M e s s r s T a y lo r , A ch e and W o lf and the w o rk on h alocarb on s was done by M e s s r s A y r e s , L a m b re c h t and W o lf.
D IS C U S S IO N G. S T O C K L IN : Y o u r com m en t did not g iv e any d e ta ils about the s p e c tro s c o p ic e x p e rim e n ts on carb on atom c h e m is try , e s p e c ia lly o f the s in g le t states. A .P . W O L F : T h e f ir s t man who attem pted to do th e rm a l s in g le t carbon atom c h e m is tr y was a man nam ed M eaburn in the o r ig in a l paper by M eaburn and P e r n e r . W e then fo llo w e d it by try in g to g e n e ra te carbon atom s in a m ic ro w a v e p lasm a, and tw o y e a r s la t e r H usain and K ir s c h , and then Braun and B ass produ ced carbon atom s in oth er s y s te m s and m ea su red the ra te s o f th ese re a c tio n s . But they didn't m ea su re re a c tio n s fo r fo rm a tio n o f prod u cts, but ra th e r ra te s o f d isap p earan ce. G. S T O C K L IN : A ft e r the M e a b u r n -P e r n e r w o rk on atom ic carb on , th e re w as a second p u blication in w hich P e r n e r was in v o lv e d in which they o b s e rv e d eth ylen e fo rm e d fr o m th e rm a l carb on . T h ey a ls o cited the hot atom w o rk on n C. A . P . W O L F : In that second p ap er th ey g a ve such a sketchy e x p e r i m en tal d e s c rip tio n that th e re is no w ay fo r it to be p o s s ib le to attem pt to re p e a t th e ir ex p e rim e n t. I r e a lly qu estion w h eth er they saw ethylene that had anything to do w ith the ca rb on atom s in th e ir ex p e rim e n t. T h e s y s te m that they used in vo lved pulse r a d io ly s is , and that m eans they had C ( 1 D) atom s p resen t. T h e eth ylen e could v e r y w e ll have com e fr o m the re a c tio n s o f the XD state o f carbon .
210
WOLF
M . N E W T O N : In the fla s h p h o to ly sis o f carb on suboxide, ethylene is a ls o o b s e rv e d , but it m ay not be a p r im a r y product. It m ay co m e fro m a t r ip le t state o f C 2O. A .P . W O L F : R igh t. E th ylen e is not a p r im a r y produ ct in that s y stem . In M eaburn and P e r n e r 's w o rk , th e.situ ation is a lit t le c le a r e r . T h ey c a r r ie d out the pulse r a d io ly s is o f m ethane, and sat s p e c tr o s c o p ic a lly on the C t 1 D) lin e , m ea su rin g its ra te o f disap p earan ce. M . N E W T O N : But they could g en era te m any oth er s p e c ie s in that s y stem . A .P . W O L F : R igh t. T h e r e is no c le a r e vid en ce that the C ( 1 D) which th ey o b s e rv e d in th e ir e x p erim en ts was a c tu a lly a p r im a r y product. T h e re is an e x p e rim e n t that a lm o s t no one e v e r r e f e r s to by R ic h a rd B ersoh n that in v o lv e s the re a c tio n s o f o rie n te d m o le c u le s with 5 S 0 carb on atom s. T h e 5So is perhaps the state o f m ost in te r e s t to the orga n ic ch e m is t, but it is in a c c e s s ib le in th e o ry and in e x p e rim e n ta l p r a c tic e at the p resen t tim e. B a s ic a lly I'm sa yin g that I don 't think that w e should n e g le c t th ese states c o m p le te ly and fo r g e t them . T h ey undoubtedly e x is t when the hot carbon atom c o o ls down. W h eth er o r not they a re re s p o n s ib le fo r any o f the re a c tio n s w e a ctu a lly s e e is quite another question. F .S . R O W L A N D : In r e c o i l tritiu m w o rk th e re is r e la t iv e ly lit t le am b igu ity about an a b stra ctio n produ ct — if you o b s e r v e H T a lm o s t a ll o f it c a m e fr o m an a b stra ctio n p r o c e s s , with v e r y lit t le fr o m d ecom p osition a fte r substitution. The product a n a ly sis situ ation in carb on atom c h e m is try is not so clea n fo r id e n tify in g produ cts fo rm e d by an in itia l a b stra ction and th ose fr o m an in itia l in s e rtio n . A .P . W O L F : One is alw ays tem pted when one s e e s a product to tr y to say that in this p a rtic u la r s y s te m this product is fo rm e d by one p a rtic u la r m ech an ism . But in the ca rb on s y s te m , I think the e vid en ce a ll shows that th ese p rodu cts a re not fo rm e d by a s in g le m ech an ism , but by m u ltip le pathways. A b s tra c tio n as w e v ie w it does not in v o lv e in s e rtio n — un fortu n ately, th ese te rm s a re s o m e tim e s confused. In in s e rtio n , th e re is a C -C -H bond; in a b s tra c tio n , th e re is n e v e r an in te ra c tio n b etw een the r e c o il carbon atom d ir e c t ly with the ca rb on atom s in the su bstrate m o le c u le . T h e in t e r m ed ia te can then d ecom p ose — u su ally in a u n im o lecu la r re a c tio n , o r at le a s t that is the assum ption — by a co n c e rte d re a c tio n f o r -^D in s e rtio n , o r s te p -w is e fo r *D o r 3P re a c tio n s . T h is a ll has to do with spin c o n s e r v a tion, e tc ., and w hether or not th ese ru le s apply at the e n e rg ie s in v o lv e d in th ese re a c tio n s is an open question. H o w e v e r, the o b s e rv a tio n in som e c a s e s that the a c e ty le n e is d e fin ite ly fo rm e d by an in tra m o le c u la r m ech an ism is p rob a b ly one o f the m ost im p orta n t foundations in carbon atom c h e m is try . It se p a ra te s the p resen t fr o m the c la s s ic a l app roach to hot atom c h e m is tr y — the e a r ly su ggestion s by L ib b y and W illa r d . W e 'r e a ll a w a re that w e a re fa r beyond that point, but on the oth er hand, when th e o rie s a re postu lated — I'm u su ally a scep tic in th ese things — u n less you have fa c ts that d is p ro v e them , one shouldn't d is c a rd them out o f hand. You don't g e n e ra te a ll s o rts o f ra d ic a ls that ra n d o m ly com b in e to g iv e prod u cts, but you g e n e ra te s p e c ific , w e ll-d e fin e d in tra m o le c u la r re a c tio n . On a c e ty le n e , th e re is rea s o n a b le a g re e m e n t in the lite r a tu r e — but w ith eth ylen e ...... I r e a lly think that it is often a good thing to o v e r s im p lify th ese re a c tio n s and use as m o d els the re a c tio n s o f m on ovalen t s p e c ie s such as tritiu m ,
IAEA-PL-615/12
211
c h lo rin e , flu o rin e and b ro m in e. I think the pull betw een those p eo p le who v ie w c h e m is tr y as a r e la t iv e ly c o m p le x s c ie n c e , and those who t r y to o v e r s im p lify , is p r e c is e ly what we need. E v e n tu a lly , w e 'l l find som e com m on ground that w ill h elp us to m ake g e n e ra liz a tio n s about the o v e r a ll fie ld . G. S T Ô C K L IN : You m entioned b r ie fly in you r paper that v in y l c h lo rid e is produ ced in v e r y lo w y ie ld fr o m c h lo rin a ted h yd ro ca rb o n s, and that the eth ylen e y ie ld s w e r e d r a s tic a lly redu ced. A .P . W O L F : " A r e not co n sisten t with the m ech an ism g iven in E q .(7 )" , becau se E q .(7 ) s im p ly says that a carbon atom in s e r ts and you get unim o le c u la r d ecom p osition . T h e r e is not a re a s o n in the w o rld why c h lo rin e should su p p ress that re a c tio n . In deed, c h lo rin e should enhance it b ecau se the С - C l bond stren gth is much le s s than that o f the C -H bond. G. S T O C K L IN : Y ou also in d icated that the carb on atom is a s tro n g e le c tr o p h ile . Is n 't it p o s s ib le that in a lk y l h alid e s y s te m s the carbon atom a ctu a lly attacks at the halogen p o sitio n ? F r o m the point o f v ie w o f e le c tr o n d en sity with an e le c tr o p h ile it s e e m s m o st lik e ly that the attack would-be on the h alogen f ir s t and not on the h yd ro ca rb o n m o iety . Then you would not have the kind of in s e rtio n re a c tio n you u su ally o b s e rv e d with h y d ro ca rb o n s, but would have in te ra c tio n betw een the ca rb on and halogen atom s. A .P . W O L F : T h is is the sam e question H a rb o ttle r a is e d — the nature of the p r im a r y c o llis io n even t as the atom c o m e s into the ra n g e o f the ta r g e t m o le c u le . H a rb o ttle was c e r ta in ly c o r r e c t in sa yin g that the distant approach to the m o le c u le is is o tr o p ic . In this c a s e , one has to say that as the carb on atom approach es it is a ttra cted to the c h lo rin e atom . I f you take a m o le c u le such as bu tyl c h lo rid e , and im a g in e what happens as the carb on atom co m es in, you see that the carb on atom has to sw in g o v e r and c o llid e w ith the c h lo rin e i f that is you r assum ption about the nature o f m o st o f the p r im a r y even ts. T h is then gets you into a ll kinds o f d iffic u ltie s — a r e the fo r c e s stro n g enough to cau se this kind o f d evia tio n in path? W e know, fo r ex a m p le , that th e re is v e r y lit t le in tra m o le c u la r m ig ra tio n in ca rb on atom e x p e rim e n ts — i f it hits at one end, it d o esn 't get to the oth er end. Th at r e s u lt c o m e s out o f the d o u b le -la b e llin g e x p e rim e n ts . It 's an in te re s tin g point, and it 's one a re a in which hot atom c h e m is ts m ay be ab le to shed som e ligh t. L . L IN D N E R : Could you e la b o ra te on the statem en t that e x p e rim e n ta l w o rk w ith carb on ions should be s ta rte d ? T h e w o rk on carb on ions in so lid s u n fortu n ately s u ffe rs fr o m c o m p le x itie s as we know. Is th e re an in te r r e la tio n betw een th ese tw o s en ten ces? A r e you thinking in te rm s o f carb on ions fr o m a c h e m ic a l a c c e le r a t o r ? A . P . W O L F : Y e s , those tw o sen ten ces a re re la te d . L e t 's go back to the e a r ly h is to r y o f ca rb on atom c h e m is tr y , a ll done w ith 14 C. T h is w o rk s u ffe re d fr o m tw o d e fe c ts , one m in o r and one m a jo r. T h e m in o r one was that you had to do the e x p e rim e n t in the n u clea r r e a c t o r , and you r sa m p le r e c e iv e d a v e r y la r g e ra d ia tio n d ose. Som e m ethod was n e c e s s a r y to s e p a ra te the ra d ia tio n c h e m is tr y fr o m the hot atom c h e m is tr y in the e x p e r i m en ta l r e s u lts . The oth er p ro b le m was that in s o lid s — fo r the v a s t m a jo r ity o f s o lid s — the num ber o f 14C p rodu cts is in c r e d ib ly la r g e . F o r in stan ce, in an e a r ly p ie c e o f w o rk on am ino a c id s , 40 p rodu cts w e r e is o la te d , and that s t ill accounted fo r le s s than 2 0 % o f the ca rb on atom s that had been produ ced in the s y s te m . It w as s im p ly a v e r y c o m p le x e x p erim en t.
212
W O LF
P r e c is e ly the sam e thing happens when you im p in ge carbon ions on to a ben zen e s u rfa c e . You g e t m any d iffe r e n t prod u cts, and you have a ll so rts o f d iffic u ltie s . F ir s t , as the carb on atom h its the s u rfa c e and slow s down, it is c r e a tin g a la r g e amount o f ra d ia tio n dam age. Secon dly, when it r e a c ts , how do you know it is re a c tin g as an ion ? It m ay be a n eu tral s p e c ie s at the tim e of re a c tio n , and you ca n 't r e a lly an sw er that question ju st by look in g at the kinds o f p rodu cts you find. A l l that you can do is co m p a re the re s u lts you find w ith s im ila r hot atom e x p e rim e n ts . A s M addock pointed out to us the o th e r day, the typ es and nu m bers o f produ cts fr o m carb on ions on a ben zen e s u rfa c e a re s im ila r to those obtained fr o m the carbon atom r e a c tion s. A lthou gh th e re a re d iffe r e n c e s , g e n e r a lly they a re the sam e. I f you want to study the io n -m o le c u le re a c tio n s o f carb on , I think one should do it w ith an io n -c y c lo tr o n reson a n ce m achine w h ere you can is o la te the ion and fo llo w the re a c tio n s p e c ific a lly . You could a ls o tr y it with 14C in the in je c tio n s y s te m , and trap the prod u cts. Then, c o m p a re the k in etics and the s p e c ie s o b s e rv e d with IC R with the products you o b s e rv e . That is o v e r a ll a s im p le r s y s te m fo r the ex p e rim e n t, and you can avoid som e of the c o m p le x itie s that you find with carbon ion re a c tio n s in so lid s. L . L IN D N E R : Did you have L e m m o n 's w o rk in mind s p e c ific a lly ? A .P . W O L F : I had in mind a ll the w o rk in the fie ld s ta rtin g with C a c a c e , and in clu din g L e m m o n 's w ork . M addock has a ls o done a lo t o f w o rk in this fie ld , but he ages his p ap ers in wood, and lik e fin e w in e, they w ill u ltim a te ly appear so m e tim e in the future. I'm not too quick about pu blish in g m y own p a p e rs , fo r that m a tter. I don't think that w e a re c o m p le te ly c le a r fr o m ch a rge state p ro b lem s in the gas phase. The argu m ent is made that th ese n u clea r r e c o il carbon a tom s have to be n eu tra l when they re a c t. It 's not c le a r that this is so, e s p e c ia lly in s y s te m s diluted with h eliu m o r neon. The w ord c h a rg e is a d ir ty w ord in carb on c h e m is try , and in tritiu m c h e m is try . W e tr y to ig n o re it by s tic k in g our heads in the sand, but I'm not con vin ced w e should ig n o re it. M . N E W T O N : A beam e x p e rim e n t has been c a r r ie d out by Mahan with C+ on H 2 , and it p resu m a b ly could be done in a c r o s s e d -b e a m exp erim en t. F .S . R O W L A N D : K o s k i has a lso done a lo t o f ca rb o n -io n w o rk in the gas phase. M . N E W T O N : K o s k i has published tw o n o tes, and Mahan did a d e fin itiv e job on the re a c tio n s with H 2 . H e had enough d eta iled data so that they could d iscu ss the s h o r t-te r m c o m p le x e s and the a b stra ctio n re a c tio n . Such e x p e rim e n ts could a ls o be done with oth er s p e c ie s in addition to H 2. A .G . M A D D O C K : T h e high r e a c tiv ity o b s e rv e d fo r C ( 3 P ) is v e r y in te re s tin g , sin ce it is a v e r y rapid re a c tio n indeed in the gas phase. H o w e v e r , when the atom s get on to a s u rfa c e oth er fa c to rs en ter. A . P . W O L F : T h e high r e a c t iv it y is a good exam p le o f an a re a in which hot atom c h e m is tr y has r e a lly had a fu ndam ental e ffe c t, and it is som ew hat ir r it a t in g , as a m a tte r o f fa c t, to re a d the la te r s p e c tro s c o p ic lite r a tu r e in w hich no m ention is m ade o f it — w h ere c la im s fo r the d is c o v e r y o f the phenom enon a re m ade. I f you rea d the e a r lie r lite r a tu r e o f hot atom c h e m is tr y , the d e fin ite point w as m ade that 3P carb on atom s r e a c t ra p id ly — that th ey m ust r e a c t ra p id ly if one is to explain the re s u lts fo r 1XC re a c tio n s with alkanes. F o u r independent grou ps have m ea su red the ra te o f d isa p p ea ra n ce o f 3P in pure oxygen , and it is one o f the fa s te s t known re a c tio n s — e s s e n tia lly it o c c u rs on e v e r y en cou nter. Th at is both in te r e s t-
IAEA -PL-615/12
213
ing and a t e r r ib le d isadvan tage, becau se it m eans that if you want to w o rk w ith g rou n d -sta te carb on atom s you had b e tte r not have any m o le c u la r oxygen around. In fa c t, if you w o rk with hot atom s y s te m s , even p a rts p er b illio n c r e a te d iffic u ltie s . T h e r e a r e not many re a c tio n s that occu r on e v e r y c o llis io n when you have B oltzm ann d istrib u tio n s o f atom s. T h a t's one o f the things which a re so in te re s tin g about carbon atom re a c tio n s . A .G . M A D D O C K : W e have done som e e x p e rim e n ts with our carbon ion beam m achine w hich have som e in fo rm a tio n about the r e a c t iv it y o f such carb on s p e c ie s . W ith r e s p e c t to the high r e a c t iv it y o f C ( 3 P ), we can say the fo llo w in g . In our m achine when w e produ ce a carbon ion beam and b rin g it o n t o a b a re t a r g e t — that is , a c o o le d , s ta in le s s s te e l s u rfa c e — the carb on can be h eld on the s u rfa c e , and it is not p re s e n t as a carb on oxid e o r carbon suboxide. You can show that it s ta b iliz e s , p o s s ib ly as a c a rb id e , but m o r e lik e ly as som e s o rt o f p o ly m e r ic carbon fo r m — a g ra p h ite nucleus. Som e o f it n e u tra liz e s and s u r v iv e s , but it does not r e a c t with the undoubtedly v e r y lo w p a r tia l p re s s u re of oxygen p re s e n t in the s y s te m . A . P . W O L F : When we w orked with carbon atom s in the m ic ro w a v e p la sm a , w e found a trem en dou s buildup o f s u p e r-p u re carb on . W e cou ld n 't find any oth er ele m e n t in this carb on — it was 99.9+% carbon . T h e r e was oxygen in the s y s te m , o f c o u rs e , but you have to go back to ta lk in g about the a v a ila b ility o f r e a c tiv e s p e c ie s , and the ra te s o f re a c tio n . In the gas phase, if a C ( 3 P ) atom is in the p re s e n c e o f a rea s o n a b le amount o f O2 and any oth er su b stra te, it w ill alw ays r e a c t with the 0 2. On the oth er hand, i f it gets to a s u rfa c e and s its down on it, and then o v e r la p s the pi-bond on a b en zen e r in g fo r s ta b iliz a tio n , the oxid ation o f the re s u ltin g C 7 H 6 s p e c ie s m ay be v e r y slo w , and the carbon atom could be p ro te c te d again st oxidation . But you a re no lo n g e r lo o k in g at the c h e m is try o f an is o la te d ca rb on atom , but at the c h e m is try o f C 7 H 6. N . G E T O F F : D u rin g the la s t tw o o r th ree y e a r s , p eop le have been u sin g la s e r s in a vacuum with m e ta l ta r g e ts (va p o u rs) and have introdu ced o rg a n ic com pounds fr o m m ethane to la r g e r m o le c u le s , and have been lo o k in g at the products which a re fo rm e d . The explanations have been that the m e ta ls in the atom ic fo r m a r e v e r y r e a c t iv e , and can fo r m v a rio u s com pounds. P e rh a p s this technique can h elp to explain som e hot atom re a c tio n s . A . P . W O L F : Q uite a num ber of p a p ers in this a re a a r e fr o m p eop le who have used la s e r s on a v a r ie t y o f o rg a n ic com pounds, and indeed also on gra p h ite. The trou b le w ith th ese e x p e rim e n ts is that you can n e v e r ju st get the carb on atom — what you get is C, C 2, C 3 , C 4 , C 5, and a ll these s p e c ie s re a c t, and r e a c t in d iffe r e n t w a ys, and th e re is no way o f r e la tin g an o b s e rv e d produ ct w ith a p a rtic u la r in itia l s p e c ie s . In the c a se o f la s e r s on g ra p h ite, o r on orga n ic com pounds which is even m o re c o m p li c a ted , th e re is n 't anything in the lite r a tu r e which h elp s. T h e r e m ay be tw o d ozen p a p ers in this fie ld . F .S . R O W L A N D : T h is qu estion is re la te d to you r p revio u s a n sw er, but I want to state the question m o re fo r m a lly . T h e r e has been much w o rk on th e rm a l carbon atom s fr o m carbon a r c s , and th ere d o esn 't seem to be any a g re e m e n t on the p ositio n o f the ca rb on a rc s c ie n tis ts and the hot atom s c ie n tis ts as to what ca rb on atom s a ctu a lly do. Did y o u r p re v io u s an sw er s u m m a rize y o u r fe e lin g s as w e ll that the c a rb o n -a rc e x p e rim e n ts a re not n e c e s s a r ily carb on atom e x p e rim e n ts ?
214
WOLF
A .P . W O L F : T h e an sw er to that qu estion is a h a lf-w a y y e s . The oth er p a rt is that th e re is no o v e r la p betw een the one p erso n who does carbon a rc w o rk and a ll the hot atom c h e m is ts who have done carbon atom c h e m is try . I think that the d iffic u lty in the c a rb o n -a rc w o rk is that the c a rb on is dep osited on a s u rfa c e , and one is studying a d iffe re n t s p e c ie s — y o u 'r e not lo o k in g at a naked carb on atom when it 's on a s u rfa ce. One thing the hot atom c h e m is t can do, and w e s o m e tim e s u n d erestim a te it, is that we know b ecau se o f the nature o f the n u clea r p ro c e s s o f fo rm a tio n that the r e a c tin g s p e c ie s is a s in g le unattached atom , with no co m p lic a tio n s about m u ltip le -a to m s p e c ie s . T h a t's not tru e in the w o rk o f S kell. A .G . M A D D O C K : I w ish to in te rp o s e a con clu sion h e re that I am d raw in g fr o m this w o rk , and I think that it is a re a so n a b le one. The carbon w o rk is e x tr e m e ly im p o rta n t, but one could draw the con clu sion that th ere ought to be m o r e p a r a lle l w o rk on som e oth er m u ltiva len t atom o f lo w e r in trin s ic r e a c tiv ity . Sulphur would be an e x c e lle n t e x a m p le. W e would be in a b e tter p o sitio n i f w e had a c o m p a ra b le , o r even a m o r e m od est body o f data on a m u ltiva len t s p e c ie s o f lo w in trin s ic r e a c tiv ity . G. S T Ô C K L IN : W h ile this is b a s ic a lly tru e , fo r a lm o s t a ll oth er p o ly v a le n t s p e c ie s th e re e x is t good p h otolytic m ethods. F o r ex a m p le, fo r sulphur th e re is this p io n e e rin g w o rk o f Gunning and S trau sz, and th e re is no r e a l need fo r hot atom techniqu es. A .G . M A D D O C K : I s t ill have the fe e lin g that w ith h o tte r sulphur atom s you m igh t o b s e r v e som e a d d ition al re a c tio n s — the p a rt la c k in g in the sulphur atom w o rk is the absen ce o f any hot p ro c e s s e s . G. S T O C K L IN : That would s im p ly be the kinetic e n e rg y argum ent, then? A .G . M A D D O C K : Y e s . F .S . R O W L A N D : W e tr ie d som e p h o to ch em ica l e x p erim en ts w ith 36SCO, and w e did not g e t the sam e re s u lts as Gunning and S trau sz. W ith c a r r ie r SCO in the s y s te m , w e then got a p p ro x im a te ly the sam e re s u lts . The situ ation , I b e lie v e , is that Gunning and S tra u sz w e r e p h oto ch em ica lly p rod u cin g both XS and 3 S, and the 3S was r e a c tin g ra th e r w e ll with OCS to fo r m e le m e n ta l sulphur on the apparatus w a lls , but not in te r fe r in g with the o th e r re a c tio n s . In this w ay, I think that they w e r e gettin g an a r t ific ia lly pure XS which was not in tr in s ic a lly p resen t in the p h otoch em ical system . When w e did the t r a c e r w o rk , w e w e re o b s e rv in g the re a c tio n s o f both 3S and 1 S, and u n fortu n ately it w as a v e r y m e s s y ex p e rim e n t. L . L IN D N E R : T h e p ro b le m w ith sulphur is that th e re a r e trem en d ou s e x p e rim e n ta l d iffic u ltie s b ecau se so fa r the on ly re a s o n a b le e x p e rim e n ta l p ro p o s itio n has been u sin g 35S with the a ccom p an yin g ra d ia tio n dam age. R e c e n tly , in ou r in stitu te we have m anaged to m ake 38S with the 3-h h a lf- life , which is a much b e tte r p rop o sitio n . W e m ake it fr o m a rgo n with gam m a ra d ia tio n — the (y ,2 p ) re a c tio n . One o f our m ain d iffic u ltie s is that w e s t ill s e e the ra d ia tio n dam age to the s y stem . A t le a s t, w e can now c o n tro l this fa c to r , and w e h ave s ta rte d with the s im p le s t s y s te m s — H 2 , C H 4 , and we have obtained so m e p r e lim in a r y re s u lts . W e h ave seen H 2 38S, but w e don't understand why it is s o m e tim e s absent, and s o m e tim e s p re s e n t in y ie ld s o f a lm o s t 100%. W e have seen quite a b it o f in s e rtio n o f 38S into C -H bonds, and that looked ra th e r p ro m is in g . H o w e v e r , one thing ca m e out — e v e ry o n e thought 38SCO would be an im p orta n t produ ct, and it is quite c e rta in that this is not an im p orta n t product.
IAEA-PL-615/12
215
F .S . R O W L A N D : One o f the g r e a t d isadvan tages in w o rk in g w ith 35S in our la b o r a to r y was that you had to d ism a n tle the cou n ter and c le a n it a fte r e v e r y run — the p rodu cts ap p aren tly re a c te d with the in t e r io r su rfa c e s o f the cou n ter. A t le a s t w ith a 3-h h a lf- life in 38S, the cou n ter w ill have c lea n ed it s e lf by the next day. L . L IN D N E R : I should point out, though, that 38S has a 37-m in 38C1 daughter which in trod u ces co m p lic a tio n s o f its own. A .P . W O L F : I'm rem in d ed o f the d iscu ssio n in the la s t s e s s io n on ex p e rim e n ta l d iffic u ltie s . W ith carb on atom s, h eliu m and neon a re ea sy, but if you tr y krypton and xenon you get into in c r e d ib le d iffic u ltie s b ecau se o f the iso to p es that a re g en era ted by the proton re a c tio n s on krypton and xenon th e m s e lv e s . In fa c t, w e r e c e n tly went o v e r som e v e r y e a r ly data w e took in about 1962 and co m p a red our num bers w ith our p re s e n t re s u lts . W e r e a liz e d that what w e did in 1962 was absolu te nonsense becau se what w as happening was that oth er iso to p es w e r e m ix in g in w ith our gaseous prod u cts. W e had s im p ly assum ed that th ese is o to p e s o f oth er elem en ts w ou ldn 't go into the gas phase, and it turned out not to be tru e. It m igh t be w o rth w h ile to say som eth in g about photonuclear c r o s s s e c tio n s . A n y c h e m is t who has the m isfo rtu n e to have to go to the lite r a tu r e fo r a c r o s s - s e c t io n , o r any kind o f e x c ita tio n function, on photonuclear re a c tio n s co m e s up again st an a lm o s t insu rm ou ntable b a r r ie r . You end up doin g the re a c tio n and m e a s u rin g the c r o s s - s e c t io n e x p e r im e n ta lly anyway. I r e m e m b e r lo o k in g up the in fo rm a tio n on n C and th e re is a lo t o f in fo r m a tion on 11С in the p h ysics lite r a tu r e . But as fa r as I'm co n cern ed thé in fo rm a tio n is about as d ecip h e ra b le as E gyp tian h ie ro g ly p h ic s to som eon e such as m y s e lf. W h ile w e 'r e d is c u s s in g e x p e rim e n ta l p ro b le m s , this m igh t be a good tim e to e la b o ra te on the p ro b le m o f spin c o n v e rs io n o f carb on atom s in c o llis io n . F o rtu n a te ly , in the la s t two y e a r s p eop le have m ea su red the e ffic ie n c y o f spin c o n v e rs io n w ith a ll the n oble g a s e s , and it turns out that w ith the excep tion o f xenon the e ffic ie n c y o f spin c o n v e rs io n is quite low . H o w e v e r , that o f xenon is v e r y high — c o n v e rs io n o c c u rs in rou gh ly one out o f e v e r y th re e c o llis io n s . T h at m eans that if you add a la r g e quantity o f xenon to the s y s te m , you a re dum ping a ll you r XD ca rb on atom s into the 3P state — and at the sa m e tim e not r e m o v in g th e ir k in etic en erg y . D .J. M A L C O L M E - L A W E S : One fa c to r w hich co n cern s m e about the n C w o rk is that it is often done by b la stin g a beam o f ch a rged p a r tic le s through the re a c tio n m ix tu re. I f n C can r e a c t w ith O 2 on e v e r y c o llis io n , then it s e e m s e q u a lly p o s s ib le that it m igh t r e a c t on e v e r y c o llis io n with fra g m e n ts le ft behind by the p a ssa g e o f the c y c lo tr o n b eam . B a s ic a lly this is a qu estion o f ra d ia tio n c h e m is try , but the point o f the com m en t is s im p ly that you do not know what s p e c ie s the n C r e a c ts with. A .P . W O L F : W e 'v e published a num ber o f p a p e rs , s ta rtin g w ith the w o rk done with S tocklin , in w hich w e 'v e done r a d io ly tic stu dies. W e 'v e m ea su red the ra d ia tio n dose to the s y s te m , and have co m p a re d the produ ct y ie ld s as a function o f the a b sorb ed d ose. N o w , w e ro u tin e ly run our stu dies at a dose le v e l in the s y s te m b elo w which th e re is no change in the o b s e rv e d d istrib u tio n o f 1]C r a d io a c tiv ity . T h a t's the b est we can do. T h e d oses a re so lo w that even ra d ia tio n c h e m is ts would a rgu e that the n C atom s a r e not goin g to r e a c t w ith ra d ia tio n d e b ris . T h e dose le v e ls a re 10 "4 to 10" 5 eV p e r m o le c u le . T h a t's p re tty low .
216
W O LF
D .J. M A L C O L M E -L A W E S : But it 's s t ill ab ove the p arts p e r b illio n le v e l which you w o r r y about fo r 0 2 . A . P . W O L F : But the r a d io ly tic s p e c ie s which you produ ce a re p resen t at a le v e l fa r b elo w that o f the su b stra te m o le c u le s th e m s e lv e s . In these s y s te m s , 4 out o f e v e r y 100 m o le c u le s a re 0 2, 96 a re m ethane, and 1 out o f e v e r y 1 0 8 o r 1 0 9 is som eth in g else,. D .J. M A L C O L M E - L A W E S : T h is is fin e in a sy s te m in which you d e lib e r a te ly have 0 2 p resen t. A . P . W O L F : T h a t's the only w ay to go. D .J. M A L C O L M E - L A W E S : The exa m p le w hich I was goin g to g iv e is w ith 13N atom s u sin g a c y c lo tro n , and by flo w d is c h a rg e m ethods, both o f which in m y opinion g iv e r is e to s ig n ific a n t amounts o f dam age p rodu cts. T h e p r im a r y y ie ld fr o m both of those sy s te m s has alw ays been H C 13 N. A .P . W O L F : Y e s , you m entioned those re s u lts the oth er day. A s I pointed out, in m ethane o r in propane, a ll you e v e r get is 13 N H 3 . N o H C 13N is fo rm e d . A c tu a lly , th e re a re about h a lf a dozen p ap ers in the lite r a tu r e now, b ecau se this is the method used in the m e d ic a l com m u n ity fo r produ cin g 13 N H 3 fo r h ea rt-sca n n in g. 13 NH 3 tra p s it s e lf in the h ea rt, and it 's a beau tifu l m ethod fo r lo c a tin g dam age to the h ea rt. G. S T O C K L IN : T h e r e is a b a sic d iffe r e n c e betw een 0 2 and the r a d ic a ls . The 0 2 d iffu ses through you r sa m p le a ll o f the tim e , w h ile the ra d ic a ls r e a c t aw ay ra p id ly . So you c a n 't s im p ly c o m p a re co n cen tra tion s. M . N E W T O N : I want to su ggest another use o f io n -m o le c u le re a c tio n s — not to study C+ re a c tio n s , but to study carbon atom re a c tio n s through the u se o f the is o e le c tr o n ic N + ion. W ith you r h ypoth esis that C -H is attacked by in s e rtio n / d e c o m p o s itio n instead o f d ir e c tly , with a p a rtic u la r isotop e e ffe c t — a ll is con tin gen t on you r m ech an ism fo r eth ylen e fo rm a tio n . What about the p o s s ib ility o f studying the re a c tio n o f N + w ith C H 4 and C D 4 , with a d ir e c t o b s e rv a tio n of the iso to p e e ffe c t? A t high kin etic e n e rg ie s the s p e c ia l c h a r a c te r is tic s o f io n -m o le c u le re a c tio n s — the induced io n -d ip o le a ttra c tio n — b eco m es r e la t iv e ly u nim portant. N e x t thing: you could tr y , to push you r lu ck, to get the e x c ite d state o f N + ion as an an alogy fo r the re a c tio n s o f e x cited ca rb on atom s. G. S T O C K L IN : B ack to the b eam w ork : I have tw o qu estion s. I f we would lik e to study the re a c tio n s o f a tom ic ca rb o n , can w e produ ce such a b ea m ? And what would be the life t im e o f the ex p ected in te rm e d ia te s in th ese s y s te m s ? Both these p ro b le m s m igh t m ake the study o f ca rb on atom b ea m s ra th e r d iffic u lt. Y . L E E : One p o s s ib ility fo r p rodu cin g a beam o f carb on atom s would be through the p y r o ly s is o f C3 0 2 m ixed in a r a r e gas — 1% C 3 0 2 w ith 99% H e, A r o r N e. You would have to m ake su re that d is s o c ia tio n was o c c u rrin g in the gas phase, and not recom b in a tion . A .P . W O L F : A s fa r as the life t im e argu m en t g o e s , I think L e e has a lre a d y c o v e r e d th is. It 's a qu estion o f the tim e b etw een a n a ly s is and c o llis io n . I f that tim e is sh o rt com p ared w ith the life t im e o f the in t e r m ed ia te , y o u 'r e in b u sin ess. I f it is n 't, y o u 'r e in trou ble. Y . L E E : N ot r e a lly in tro u b le. S o m etim es w e lik e to m ea su re the life t im e o f the c o m p lex . If it r e a lly liv e s lo n g enough — 10 цт o r lo n g e r — then w e a re in a new bu sin ess, but the e x p e rim e n t is s t ill su ccessfu l. A . P . W O L F : 10’ 9 seconds is p ro b a b ly okay. T h e w o rk that w e did with p re s s u re e ffe c t su ggests that th ere is a co m p le x at le a s t in the in itia l even t, and that c o m p lex m ust be liv in g lo n g e r than 1 0 3 bond v ib ra tio n s .
IA E A -P L-61 5/12
217
Y . L E E : In the b eam w o rk , th e re a r e tw o c lo c k s in a sen se. One c lo c k is the ro ta tio n a l p e rio d as a c lo c k fo r m e a s u rin g the life t im e o f the co m p le x . Th e second c lo c k is the tra n s it tim e o f y o u r apparatus. T h e ro ta tio n a l tim e p e rio d and the tra n s it tim e a r e o r d e r s o f m agnitude apart. A .P . W O L F : I'm p a r tic u la r ly in trigu ed by the p o s s ib ility o f co m p a ris o n o f N ew to n 's th e o r e tic a l re s u lts with beam studies o f carb on atom s on h y d ro g en , p a r tic u la r ly in te r m s o f the o rien ta tio n o f the h yd rogen m o le c u le . L . L IN D N E R : Can you cop e w ith the oxygen p ro b le m ? Y . L E E : In the hot atom e x p e rim e n t you p rod u ce so lit t le carb on that you can n e v e r r e m o v e a ll the oxygen m o le c u le s by re a c tio n . But in this kind o f b eam e x p e rim e n t the amount o f 0 2 p re s e n t is v e r y much le s s than the amount o f carb on atom s you would p rodu ce. A . P . W O L F : H ot atom c h e m is tr y p ro v id e s a unique to o l fo r studying th ese carb on atom s y s te m s sin ce it co n c e n tra te s on m ic r o s c o p ic p r o p e r tie s o f s y s te m s in the sa m e w ay that the w o rk in b eam m ach in es fo c u s e s on another a sp ect o f the m ic r o s c o p ic p r o p e r tie s o f s y s te m s . I think this is what d r iv e s a lo t o f hot atpm ch e m is ts to study hot atom c h e m is try . T h ey approach the su bject fr o m the m ic r o s c o p ic point o f v ie w ra th e r than the m a c ro s c o p ic point o f v ie w in the hopes that they can u ltim a te ly understand the m a c ro s c o p ic s y stem .
IAEA-PL-615/13
MOLECULAR ACTIVATION ANALYSIS P.M. G R A N T , F.S. R O W L A N D Department o f Chemistry, University o f California, Irvine, Calif., United States o f America
Abstract M O L E C U L A R A C T IV A T IO N A N A L Y S IS . T h e p rocess o f n eu tro n a c tiv a tio n p o te n tia lly carries w ith it in fo r m a tio n c o n c e rn in g th e “ s ite ” o f th e origin a l re a c tin g a to m because th e n e w ly cre a te d ra d io a c tiv e n u clid e is fo r m e d at th e p h y sica l lo c a tio n and in th e ch em ica l su rrou n din gs o f th e o rig in a l a to m . C ircu m stan ces u n d e r w h ic h this d isp laced a to m can b e used t o id e n t ify the orig in a l e le m e n ta l lo c a tio n o r th e o rig in a l m o le c u la r c o m p o u n d in w h ic h th e a ctiva ted e le m e n t h ad been b ou n d , are discussed w ith several e x a m p le s p resen ted.
N eu tron a ctiva tio n a n a ly s is in its p re s e n t fo r m is a technique capable o f high s e n s itiv ity e le m e n ta l a n a ly s is w ith good p r e c is io n [ 1 ], but which u s u a lly p ro v id e s no in fo rm a tio n re g a r d in g the c h e m ic a l id en tity o f the m o le c u le in c o rp o ra tin g the p a r tic u la r e le m e n t under a n a ly s is . Such m o le cu la r in fo rm a tio n has been obtained fr o m neutron a c tiv a tio n a n a ly sis on ly through the in trod u ction o f c h e m ic a l sep a ra tio n steps p r io r to neutron ir r a d ia tio n , a p r o c e s s which s a c r ific e s one o f the g r e a t advantages o f neutron a ctiva tio n a n a ly sis fo r tr a c e m a te r ia ls — its fre e d o m fr o m c h e m ic a l handling and p ro c e s s in g o f the sa m p les p r io r to the ir r a d ia tio n it s e lf. H ow e v e r , the p r o c e s s o f neutron a ctiva tio n p o te n tia lly c a r r ie s w ith it in fo r m a tion c o n cern in g the " s i t e " o f the o r ig in a l re a c tin g atom becau se the n ew ly c re a te d ra d io a c tiv e n u clide is fo rm e d at the p h y s ic a l lo c a tio n and in the c h e m ic a l su rrou n din gs o f the o r ig in a l atom . The r e c o il r e s u ltin g fr o m the n u clea r re a c tio n in m an y c a s e s c a r r ie s this produ ct nuclide o n ly a sh ort distan ce (often no m o re than a few A n g s tr o m s ) fr o m the o r ig in a l site , and this p r o x im ity to the o r ig in thus p r e s e r v e s som e in fo rm a tio n about the lo c a tio n o f the a c tiv a tin g re a c tio n it s e lf. U nder d iffe r e n t circ u m s ta n c e s such in fo rm a tio n m igh t id e n tify the o r ig in a l e le m e n ta l lo c a tio n w ithin one o f s e v e r a l com ponents o f a h etero g en eo u s m ix tu re, o r it m igh t id e n tify the o r ig in a l m o le c u la r compound in w hich the a c tiv a te d e le m e n t had been bound. In fa v o u ra b le c irc u m s ta n c e s , the r e c o v e r y o f th is s ite in fo rm a tio n can p e r m it the sim u ltan eou s d eterm in a tio n fr o m a c tiv a tio n a n a ly sis p ro c e d u re s both the e le m e n ta l abundance and addition al in fo rm a tio n about the o r ig in a l c h e m ic a l o r p h y s ic a l lo c a tio n o f the a c tiv a te d elem en t. A n e x a m p le o f " s i t e " in fo rm a tio n fro m a ctiva tio n a n a ly s is w as the d em on stra tion that " e x c e s s 129 X e " and 128X e fr o m 127I(n , 7 ) 128I £~128X e w e re both r e le a s e d at the sam e te m p e ra tu re du ring the h eatin g o f neu tronir r a d ia te d p o rtio n s o f a m e te o r ite sam p le [2 ]. (T w o gaseou s xenon com ponents w e r e found to be g iven o ff du ring p r o g r e s s iv e h eatin g o f u n irra d ia te d m e te o r ite — one w ith a d istrib u tio n o f a ll o f the stable xenon is o to p e s ro u g h ly s im ila r to that o f t e r r e s t r i a l a tm o s p h e ric xenon, and a
219
220
G R A N T and R O W L A N D
second g r e a tly en ric h e d in 129 X e , d e s c rib e d h e re as " e x c e s s 129 X e " , and r e le a s e d in a b u rst n ea r 400-500°C. ) Stable 127I w as assu m ed to be in c o rp o ra te d into the m e te o r ite in a m ic r o s c o p ic io d id e -c o n ta in in g c r y s ta l and the e n e r g y o f r e c o i l fr o m (n, 7 ) a ctiva tio n d oes not tra n s p o rt the daughter 128I produ ct fa r enough to r e m o v e it fr o m th is c r y s ta l. A ft e r the d eca y o f the 25-m inute 128I, the stable 128X e daughter is thus s t ill lo c a te d in the io d id e -c o n ta in in g c r y s t a l, and is o n ly r e a d ily r e le a s e d by h eatin g the m e te o r ite to a te m p e ra tu re such that the io d id e -c o n ta in in g c r y s ta l is m e lte d o r d eco m p o sed . The sim u ltan eou s r e le a s e during p r o g r e s s iv e h eatin g o f the m e te o r ite o f both the " e x c e s s 129X e " and the 1 2 7 I(n , 7 ) produced 128X e then d em o n stra ted that the 128X e w as lo c a te d w ithin the m e te o r ite at a site c h a r a c te r is tic o f the c h e m is tr y o f io d id e -c o n ta in in g c r y s ta ls , and not o f the c h e m is tr y o f ad sorb ed xenon gas. In this ex a m p le , then the activa tion a n a ly s is technique has been used to lo c a te the 129X e in a h eterogen eou s sa m p le, and not fo r a d e te rm in a tio n o f the e le m e n ta l abundance o f iodin e. In the m e te o r ite s y s te m s , the fu rth e r con clu sion w as drawn that the 129X e w as lo c a te d in the iod id e s ite s because it had been produ ced by the b eta d e c a y o f 129 I, an e x tin ct iso to p e no lo n g e r e x is tin g in the n atu ral en viro n m en t (h a lf- life : 1.7 X IO 7 y r ). F o r th is to have o c c u rre d , the m e te o r ite m ust have c r y s t a lliz e d in its p resen t fo r m at a tim e c lo s e enough to the tim e o f n u cleo syn th esis that e le m e n ta l iodin e then con sisted o f two is o to p e s , 127 I and 129I, i. e. w ith in about 10 8 y r a fte r n u cleosyn th esis, p re s u m a b ly about 5 X IO 9 y r ago. The 129 I could then subsequently d ecay to 129X e w ith the la tte r p h y s ic a lly lo c a te d in the m e te o r ite w ithin the sam e 12 Q m ic r o c r y s t a l as the o r ig in a l I. M o le c u la r abundance in fo rm a tio n can be sought by attem p tin g to com bine e le m e n ta l abundance in fo rm a tio n w ith som e type o f site in fo rm a tio n . W e have e m p lo y ed the condensed phase re c o m b in a tio n e ffe c ts known in hot atom c h e m is tr y to g e th e r with a sim p le p o s t-ir r a d ia tio n c h e m ic a l sep a ra tio n to d e te rm in e the fe a s ib ilit y o f such a m o le c u la r a c tiv a tio n a n a ly s is p ro ced u re on la b o ra to ry -p ro d u c e d sa m p les. O ur re s u lts have shown s u c c e s s fu l ana ly s is at the e le m e n ta l abundance le v e l o f about 1 0 ‘ 6 , and have dem on strated that the b a sic id ea o f m o le c u la r a ctiva tio n a n a ly s is is fe a s ib le . W e have, h o w e v e r, not y e t m ade any a p p lica tion o f this m ethod to any sa m p les m o re ty p ic a l o f r e a l a n a ly tic a l p ro b le m s . The con d en sed -p h ase enhancem ent o f y ie ld s r e lie s upon the p r o x im ity o f the atom at the end o f its r e c o i l path to the fra g m e n ts le ft behind fro m its in itia l fo rm a tio n . In our e x p e rim e n ts , c h lo r a n ilic acid (2 ,5 -d ic h lo ro 3 ,6 -d ih yd ro x yb en zo q u in o n e) w as d is s o lv e d in w a te r at v a rio u s con cen tra tion s, and then ir r a d ia te d in a th e rm a l neutron flux o f 0.7 X IO 12 n cm ' 2 s " 1 at the T R IG A r e a c to r fa c ilit y o f the U n iv e r s ity o f C a lifo r n ia Ir v in e . S am p les w e re ir r a d ia te d at both am bient te m p e ra tu re (about 20°C) and at -196°C in a s p e c ia l c ry o g e n ic apparatus 13]. R e c o il 38C1 w as thus c re a te d fro m the e le m e n ta l c h lo rin e in c h lo r a n ilic acid in both the aqueous and ic e m a tr ic e s , and w as su bsequ ently sought in qu an titative a n a ly s is . A ft e r irr a d ia tio n , a solven t e x tra c tio n techniqu e w as used to sep a ra te o r gânic ally-b ou n d 38 C l fr o m in o rg a n ic 38C1, the la tte r p re s u m a b ly p re s e n t as 38 C1~ [4 ]. Both the o rg a n ic and in o rg a n ic fra c tio n s w e r e a ssa yed by g a m m a -r a y s p e c tro s c o p y fo r the 1.64- and 2 .1 7 -M e V gam m a r a y s fr o m 38C1. The r e s u lts a re e x p re s s e d in T a b le I in te r m s o f the p e rc e n ta g e reten tio n o f 38C1 in o rg a n ic fo rm . The o rg a n ic re te n tio n o f 38 C1 in irr a d ia tio n s at ro o m te m p e ra tu re in aqueous solu tion is e s s e n tia lly z e r o ex c e p t at the h igh est con cen tration ran ge.
IAEA-PL-61 5/13
221
T A B L E I. P E R C E N T O R G A N IC Y IE L D F O R 38C1 F R O M N E U T R O N -IR R A D IA T E D C H L O R A N IL IC A C ID - W A T E R
Chloranilic acid concentration (ppm by veight)
Irradiation temperature (°C)
Organic retention (°¡o)
1100
20
4.4 ± 0.1
1100
-196
12.5 ± 0.2
94
20
94
-196
9.7 ± 0.6
8.5
20
0.0 ± 0.1
8.5
-196
10.0 ± 0.5
0.74
-196
0.1
9
± 0.1
±2
O rg a n ic re te n tio n can be an ticip ated fro m th ree p o s s ib le s o u rc e s : (a )F a ilu r e to ru ptu re the o r ig in a l C - 37 C l bond in c h lo r a n ilic acid d u rin g the 37 C l(n , 7 )38 C 1 p r o c e s s ; (b) the r e a s s e m b ly into an o rg a n ic m o le c u le o f the 38 C1 atom w ith som e fra g m e n t le ft fr o m the m o le c u le con tain in g the o r ig in a l C - 37 C1 bond; o r (c ) the fo rm a tio n o f a new m o le c u le by r e a c tio n with a n eigh b ou rin g s p e c ie s . The 4.4% o rg a n ic re te n tio n found with 1100 ppm o f c h lo ra n ilic acid in aqueous solu tion p re s u m a b ly re p r e s e n ts re a c tio n o f r e c o ilin g 38 C1 atom s w ith n eigh b ou rin g m o le c u le s o f c h lo ra n ilic acid , sin ce on ly (c ) o f the th ree p r o c e s s e s above should be p a r tic u la r ly s e n s itiv e to the c o n c e n tra tion o f c h lo ra n ilic a cid . The re s u lts at lo w e r con cen tra tion s at 20°C in d icate that n eith er (a ) n or (b) is an im p o rta n t rou te fo r fo r m in g 38C l- la b e lle d o rg a n ic s p e c ie s . S u rv iv a l o f the in itia l bond has been m ea su red to be n e g lig ib le fo r C - 3 7 C1 bonds in the gas phase (S 0.02% fo r CH 3 CHC1CHC1CH 3 [5 ], and is shown by th ese ro o m -te m p e r a tu r e e x p e rim e n ts to be s 0.1% fo r c h lo ra n ilic acid in aqueous solution. In the sa m p les ir r a d ia te d as ic e , the o b s e rv e d o rg a n ic y ie ld is su b sta n tia lly h ig h e r, and is e s s e n tia lly constant at about 1 0 % o v e r a 1500 -fold d e c re a s e in co n cen tra tion o f c h lo ra n ilic acid. The y ie ld fro m the m o st con cen trated fr o z e n solu tion is s lig h tly h ig h e r, and p ro b a b ly r e fle c t s a s m a ll con trib u tion fr o m 38C1 re a c tio n s with n eigh b ou rin g m o le c u les, as o b s e rv e d in the aqueous sa m p les at c o rre s p o n d in g co n cen tra tio n s. The 10% o rg a n ic y ie ld fr o m sa m p les con tain in g o rg a n ic s p e c ie s at the ppm le v e l d e m o n stra tes a v e r y stro n g p r e fe r e n c e fo r re a c tio n w ith o rg a n ic f r a g m ents o r s p e c ie s in p r e fe r e n c e to the HzO m o le c u le in ic e , con sisten t w ith the 38 C l atom s h avin g lo ca ted and r e a c te d w ith o rg a n ic fra g m e n ts fr o m the o r ig in a l m o le c u la r s ite . In th is c a s e , the s e n s itiv ity o f the m o le c u la r
222
G R A N T and R O W L A N D
a c tiv a tio n a n a ly s is to w a rd s c h lo rin e in c h lo ra n ilic acid is e s s e n tia lly on etenth that o f the e le m e n ta l a n a ly s is fo r ch lo rin e alon e, sin ce on ly 1 0 % o f the 38 C l atom s have re g a in e d an o rg a n ic fo rm . C om p a ra b le background e x p e rim e n ts u sin g r e la t iv e ly dilu te solutions o f o rg a n ic compound to g e th e r w ith la r g e r amounts o f Cl" in the aqueous solu tion d em on stra te that 38C1 atom s fr o m 3 7 C l" have a p r o b a b ility o f le s s than 0.5% fo r fo r m in g an o r g a n ic a lly bound m o le c u le in solu tions fr o z e n to -196°C and ir r a d ia te d as ic e . F o r r e a lly u sefu l a n a ly tic a l m ea su rem en ts o f o r g a n ic a lly bound c h lo rin e in the p re s e n c e o f in o rg a n ic c h lo rin e , it is h ig h ly d e s ira b le to have even g r e a te r s p e c ific it y in the r e - fo r m in g o f an o rg a n ic s p e c ie s when the c h lo rin e atom w as in it ia lly o r g a n ic a lly bound, and fo r g r e a te r s p e c ific it y in re a c h in g an in o rg a n ic con dition when the ch lo rin e atom w as in it ia lly p resen t as C l . In s im ila r e x p e rim e n ts with tr ic h lo r o a c e tic acid and with 7 - benzene h e x a c h lo rid e (lin d a n e), o rg a n ic reten tio n s o f a p p ro x im a te ly 2 0 % w e re obtained fr o m -196°C ir r a d ia tio n o f aqueous solu tions o v e r the sam e con cen tra tio n ran ge [4 ]. The o rg a n ic reten tio n s w e r e again n e g lig ib ly s m a ll fo r the lo w e r con cen tra tion s when irr a d ia te d at 20°C. In su m m a ry, the concept o f m o le c u la r a ctiva tio n a n a ly sis has been shown to be v a lid fo r c e rta in s p e c ific com pounds under c o n tro lle d la b o ra to ry con d ition s. W h eth er the technique can be app lied to fie ld sam p les o f r e a l a n a ly tic a l in te r e s t re m a in s to be seen.
REFERENCES [1]
[2] [3] [4] [5]
GUINN, V.P., in Treatise on Analytical Chemistry. Parti. Theoryand Practice, (KOLTHOFF,I.M., ELVING, P.J., Eds), 9, Wiley-Interscience, New York (1971) 5583-5641; KRUGER, P., Principles of Activation Analysis, Wiley-Interscience, New York (1971). REYNOLDS, J.H., Ann. Rev. Nucl. Sci. 1]_ (1967) 253. ROWLAND, F.S., MILLER, G.E., GRANT, P., STEINKRUGER, F.J., in Irradiation Facilities for Research Reactors, (Proc. Symp. Teheran, 1972), IAEA, Vienna (1973) 281. GRANT, P.M., Ph.D. Thesis, University of California, Irvine (1973). WAI, C.M., ROWLAND, F.S., J. Phys. Chem. 71 (1967) 2752.
D IS C U S S IO N
S. A M IE L : I understand that the su ccess o f this p a rtic u la r m ethod r e lie s on a pure solu tion, o r a w e ll-s tu d ie d solution. W e have ju st h eard fr o m D r. Saito that w h a te v e r a d d itives you throw in, you have a w hole s e r ie s o f p o s s ib le r e a c tio n s . Since the p r a c t ic a l substances you u su ally have to an alyse contain a ll kinds o f im p u ritie s , how w ill you then use this kind o f m ethod? W ith solu tion s which contain s c a v e n g e rs , m e ta ls , ion s, e tc .? F .S . R O W L A N D : The o n ly an sw er I g iv e is that one would have to tr y any p a rtic u la r sy s te m to see what the in te r fe r e n c e s w ill be fr o m s c a v e n g e rs . If you r p ro b le m is a n a lysin g r i v e r w a te r, m aybe you don 't have d iffic u lty fr o m the im p u ritie s that a re in it. A s I say, I have no w o rk a b le exam p le. T h e r e is som e w o rk which Don W ile s has done, ap p aren tly b ein g published v e r y s h o rtly , in which with fis h sa m p les he is able to show that th ere is re te n tio n fo r m e r c u r y in m e th y lm e rc u ry .
IAEA-PL-615 /13
223
S. A M IE L : Y e s , th is has been known fo r a lon g tim e . F .S . R O W L A N D : W h eth er h is m ethod is a p r a c t ic a lly u sefu l w a y fo r d istin gu ish in g betw een unbound m e r c u r y and m e th y lm e rc u ry re m a in s to be seen. A . G. M A D D O C K : I 'v e been thinking about th is, and I can think o f one s y stem which m igh t show som e p r o m is e , and that w ould be v ita m in B -12 in tis s u e . I f you could do that e x p e rim e n t in fr o z e n tis s u e , and th e re is a h ig h ly s e le c tiv e so lven t e x tra c tio n with c a r r ie r m a te r ia l — i f you had a rea s o n a b le re te n tio n , it m ight be an im p ro v e d m ethod ra th e r than the b io a s s a y w hich I think is n o r m a lly u sed. S. A M IE L : Can cob alt be p resen t in tissu e in oth er c h e m ic a l fo r m s ? A . G. M A D D O C K : T h e re w ill be som e oth er cob alt, y e s , but I don't see why that should in t e r fe r e . F o r one thing, the chances o f any oth er cob alt b ein g n ea r the v ita m in B -12 is p r a c tic a lly n il. S. A M IE L : Then you can ju st m ake cob alt a n a ly sis. A . G. M A D D O C K : N o. You would o n ly be lo o k in g at the cob alt bound in the v ita m in B -12. A . P . W O L F : How do you know the co b a lt is n 't o r g a n ic a lly bound in oth er w ays? A . G. M A D D O C K : It d o esn 't m a tte r i f i t 's o r g a n ic a lly bound in oth er w a ys. You add a bit o f c a r r ie r v ita m in B -12 a fte r ir ra d ia tio n . Then you put it through a m u ltip le so lven t e x tra c tio n unit v e r y e a s ily — w hich can be e x t r e m e ly s e le c tiv e fo r a m o le c u le lik e v ita m in B -12. F .S . R O W L A N D : W hat s o rt o f le v e ls a re ty p ic a lly b ein g an alysed? W hat is the cob alt le v e l in tissu e fo r v ita m in B-12 fo r th ese m ea su rem en ts? A . G. M A D D O C K : I have no idea. T h ese le v e ls a re p re tty lo w , but I don't see why it shouldn't w ork . F .S . R O W L A N D : I f the le v e ls a re p a rts p e r b illio n , then it 's much m o r e d iffic u lt. It sounds as though it m igh t be a sy s te m w hich would be w orth lo o k in g into, but p a rts p e r b illio n le v e l sa m p les m a y be hard ones to w o rk with. A . G. M A D D O C K : I think that it is s o m ew h ere betw een p a rts p e r m illio n and p a rts p e r b illio n . F . S. R .O W LAN D : The k ey question has to do w ith w h eth er any oth er cob a lt g ets into a c h e m ic a l fo rm which is in sep a ra b le fro m v ita m in B -12. L . L IN D N E R : I think I w ould ra th e r turn th is argum ent upside down. I think the hot atom ch e m is ts have som eth in g to t e ll to the a ctiva tio n an alysts — m aybe th e re is m o r e that you can do. The p ro b le m in a ctiva tio n a n a ly s is is that you m ust r e c o v e r the r a d io a c tiv ity a fte r irr a d ia tio n , and if you don't know the c h e m ic a l fo r m , then you can be in tro u b le. I think th is is an ex a m p le , and th ere a re thousands o f o th e rs , I gu ess, in w hich you have to be v e r y much a w a re o f the c h e m ic a l states o f you r fin a l prod u cts. R ow lan d has g iven an exa m p le o f how c a r e fu l you need to be. I think that it is im p orta n t to a ctiva tio n a n a ly s is fr o m that point o f v ie w . N. S A IT O : I think that th is kind o f e x p e rim e n t m a y be u sefu l in som e ca s e s fo r ex a m in in g the c h e m ic a l state o f som e e le m e n t under study, but I think that th ere a re two im p orta n t fa c to r s to m e a s u re . One is the r e te n tion , but another is the en rich m en t fa c to r. W e have to check both. F .S . R O W L A N D : I'm not sure what you m ean in th is ca se by the en rich m en t fa c to r . N . S A IT O : W hen you ir r a d ia te with neutrons o r gam m a ra d ia tio n , y o u 'll c e r ta in ly have som e kind o f ra d ia tio n d e c o m p o sitio n a ccom p an yin g
224
G R A N T and R O W L A N D
the (n, 7 ) p r o c e s s . O f c o u rs e , w e can d e te rm in e the reten tio n e x p e rim e n ta lly , but in addition w e can d e te rm in e the en rich m en t fa c to r fo r the elem en t. F r o m th is you can find w h eth er the s p e c ie s a re r e c o il atom s o r in te ra c tio n s p rodu ced by ra d ia tio n d eco m p o sitio n . In the case o f ra d ia tio n d ecom p osition , I think th e re w ill be no en rich m en t. F .S . R O W L A N D : In our ty p ic a l ir r a d ia tio n th ere is a n e g lig ib le amount o f ra d ia tio n d eco m p o sitio n . M aybe i f you w e r e goin g down to p a rts p er b illio n le v e ls , you m igh t have to le a v e the sam p le in the r e a c to r fo r a long tim e , and then have som e ra d ia tio n d eco m p o sitio n tro u b les. H. M IG G E : Som e e x p e rim e n ts have been c a r r ie d out in our institute w hich w e re d ire c te d to w a rd s d e te rm in in g the lo c a tio n o f lith iu m atom s in the la ttic e : F o r ex a m p le , in germ an iu m o r niobium . The lith iu m was im plan ted into th ese m e ta ls with an a c c e le r a to r . A fte r w a r d s , the sam p les w e r e p la ced in a v e r y low th e rm a l neutron flu x in the r e a c to r , and the tritiu m fr o m the 6 L i ( n , a ) T re a c tio n w as then m ea su red . The aim w as to get som e in fo rm a tio n on the depth d istrib u tio n o f the lith iu m in the la ttic e , and this r e q u ir e s an e s tim a te o f the stopping p o w e r. On the oth er hand, one can a lso look fo r the angular d istrib u tio n o f the tritiu m , and get som e in fo rm a tio n about the la ttic e lo c a tio n o f the lith iu m . H o w e v e r, I have to say that th e re a re som e d iffic u ltie s . I f you have a g g re g a te s , as s o m e tim e s o c c u rs when the co n cen tra tion gets h ig h e r, then you ca n 't get any in fo r m a tion about the la ttic e lo c a tio n o f the lith iu m . And you have to put in the stopping p o w e r ca lcu la tio n , too. F . S. R O W L A N D : In that p a rtic u la r e x p e rim e n t, since the range o f the tritiu m atom fr o m 6L i ( n , a ) T is v e r y lon g, I would think that you are ta lk in g about dim en sion s w hich could be an alysed ju st by s lic in g o ff the c r y s t a l w ith a m ic ro to m e . H. M IG G E : Y e s , the depth d istrib u tio n is in the m ic ro n dim en sion s. S. A M IE L : A s a m a tte r o f fa c t, th e re a re quite a num ber o f site lo c a tio n tech n iqu es, and this is on ly one o f them . D avid Sam uel at the W eizm a n n In stitu te is u sin g 170 - la b e lle d com pounds, and is look in g at the s u b -c e llu la r d istrib u tio n o f 170 - la b e lle d com pounds through the (n , a) re a c tio n with th e rm a l neu tron s. Since the alpha p a r tic le has much lo w e r e n e r g y than the 2 .7 -M e V tritiu m and 2 -M e V alpha fr o m the lith iu m re a c tio n , the ran ge is v e r y s m a ll, and he can r e a lly lo ca te the 170 in s u b -c e llu la r lo c a tio n s . But th e re a re m any e x a m p le s o f this kind o f w o rk — the fis s io n ■tracks in uranium through which one look s at the u ranium lo ca tio n within a c e rta in m in e r a l, and so on. B y the w ay, th is is not a site lo ca tio n technique, but a sou rce lo ca tio n technique — it cannot d e te rm in e the s ite , but it can d e te rm in e the so u rce. But I w ould say that th e re a re num erous techniques which can d eterm in e the s ite and the c h e m ic a l id en tity o f a c e rta in tra c e as w e ll. But I don't think that th is is w ithin the scope o f our ses s io n u n less p eop le w ould lik e to d is c u s s it fu rth e r. D. M. R IC H M A N : I would lik e to retu rn to the com m en t o f D r. L in d n er. I f I 'm in te r p r e tin g h im c o r r e c t ly , he w a s m akin g the point that m any people who p r a c tic e a ctiva tio n a n a ly s is a re e ith e r in stru m en ta l p e o p le , who count w ith as high e ffic ie n c y as p o s s ib le but no c h e m ic a l sep a ra tion , o r those who m ake u se o f c h e m is tr y to e lim in a te in te r fe r e n c e s so that you can get the h igh est s e n s itiv ity . If I fo llo w th is point, perh aps the p eop le who do c h e m ic a l sep a ra tio n fo r a n a ly s is should not s tr iv e to m a x im iz e the sep a ra tion , but t r y to obtain fra c tio n s o f a g iven c h e m ic a l com ponent in a m o re so p h is ti cated w a y to p ro v id e som e a d d ition al c h e m ic a l in fo rm a tio n .
IAEA-PL-615/13
225
L . L IN D N E R : Y e s , I think you a re rig h t. If you do not know about the fin a l c h e m ic a l state o f the ra d ion u clid e you a re in te re s te d in, you a re ev e n tu a lly bound to m ake som e m is ta k e s , e s p e c ia lly in inhom ogeneous m a te r ia ls . W e a re d ea lin g d a ily in our in stitu te w ith m e te o r ite s , and they a re n e v e r v e r y hom ogeneou s. W e a re lo o k in g fo r ch lo rin e th e re , too, and it w o r r ie s m e. W e need to do the p ro p e r c h e m ic a l a n a ly sis fo r one p a rtic u la r ch e m ic a l state. A . P . W O L F : I w ould lik e to m ake a com m ent and ask a question. I'm not sure that I understand how the R eyn o ld s m e te o r ite e x p e rim e n t which you d e s c rib e d b e a rs on the oth er e x p e rim e n ts you a re re p o rtin g . W hy does the bu rst o f xenon that is o b s e rv e d s im p ly be a function o f the sudden r e le a s e o f xenon fr o m som e p a rt o f the c r y s ta l? W h y does it have anything to do w ith w h ere the iod in e is lo ca ted ? F .S . R O W L A N D : The sim ultaneous r e le a s e at the sam e te m p e ra tu re in d ica tes that the 129X e in the m e te o r ite and the 128X e in trodu ced by the neutron ir r a d ia tio n a re both trapped in the sam e kind o f c r y s ta llin e lo ca tio n in the m e te o r ite . A . P . W O L F : I don't see that it says that at a ll, and I ' l l g iv e you an ex a m p le o f w hy I don 't think that is tru e. I f you bom bard tellu riu m with alpha p a r t ic le s , you get m o re o r le s s an is o tr o p ic d istrib u tion o f xenon through the tellu riu m . N ow , i f you plot the r e le a s e o f xenon fr o m te llu riu m as a function o f te m p e ra tu re du ring h eating, and as a function o f the s tru c tu re o f the te llu riu m , you get th ese sudden b u rst e ffe c ts a ll o f the tim e . I f you use spongy te llu riu m , the burst o c c u rs v e r y e a r ly at low te m p e r a tu res. I f you use c r y s ta llin e te llu riu m , the xenon d rib b le s out, and then suddenly th ere is a bu rst o f xenon, and then it continues to d rib b le out again. In th is p a rtic u la r illu s tra tio n , the xenon is m o re o r le s s is o tr o p ic a lly d istrib u ted throughout the te llu riu m — th ere is a d en sity e ffe c t in the te llu riu m , to be su re — but it d o esn 't t e l l you anything about w h ere the xenon is w ithin the tellu riu m . F .S . R O W L A N D : But th a t's becau se the is o to p ic com p o sitio n o f the xenon fr o m te llu riu m in you r ca se is constant. A . P . W O L F : Sure. F .S . R O W L A N D : But it is n 't fr o m the m e te o r ite . The xenon that is c o m in g out a ll alon g has ro u gh ly the sam e is o to p ic com p o sitio n o f atm o s p h e ric xenon, and the xenon that co m es out in the bu rst can be as much as 50-60% 1 Z9I. A . P . W O L F : O kay, righ t. S. A M IE L : I think it is a m is -d e fin itio n . A l l he is tr y in g to say is that w h e r e v e r the 129I is com in g fro m — that both o f them com e fro m the sam e lo ca tio n . W ithout te llin g what the site is , i t 's on ly the sam e o r ig in . T h e re a re c e rta in tech n iqu es w hich in d ica te s ite s , fo r e x a m p le, fis s io n tra c k s . And o th e rs w hich t e l l you e x a c tly w h ere a m o le c u le lie s in a p a rtic u la r c r y s ta l. F .S . R O W L A N D : I t 's not fr o m a site in the sen se o f a s p e c ific p ositio n w ithin a c r y s ta l. But the h yp oth esis is that th e re is an iod id e c r y s t a l w ithin the m e te o r ite w hich has ju st d ecom p osed at the s p e c ific te m p e ra tu re , r e le a s in g e v e ry th in g that is trap p ed w ithin it, g iv in g this sudden burst. The site o f in c o rp o ra tio n o f the o r ig in a l atom w as the iod id e c r y s ta l — the o r ig in , i f you w ish. A . P . W O L F : G ettin g back to u sin g this as an a n a ly tic a l technique. Do you v ie w th is as anything oth er than a technique w hich r e q u ir e s ra th e r e x te n s iv e c a lib ra tio n in w h a te v e r s y s te m you w ish to study b e fo r e you can
226
G R A N T and R O W L A N D
apply it? F o r e x a m p le, in neutron a ctiva tio n , i f you a re ju st doing bulk a n a ly s is o f som e is o to p e , you r e a lly don't have to know a g re a t d ea l about the nature o f the sa m p le. But, in this c a s e , to use M a d d ock 's ex a m p le, suppose I want to an alyse blood as opposed to stom ach w a ll tissu e fo r the content o f v ita m in B -12. Now stom ach w a ll tissu e has a low supply o f blood, v e r y lit t le iro n ; blood has a g re a t d ea l o f iro n . I f the v ita m in B -12 is lo ca ted in the b lood , and the a ctiva tio n and r e c o il p r o c e s s e s o ccu r, it is tru e that you can then is o la te the v ita m in B-12 that has been activa ted , and re ta in e d , in the blood; add c a r r ie r ; p u rify ; se p a ra te ; and count. But th a t's goin g to be v e r y d iffe re n t fro m the a n a ly sis o f stom ach w a ll tissu e becau se th ere is v e r y lit t le iro n around, and as a consequence the re te n tio n m a y be e n tir e ly d iffe re n t — Saito ju st showed how the reten tio n in h is system depended upon the con cen tration o f m e ta l ions that a re p resen t. The re te n tio n m a y be s e n s itiv e to the w ay in which you d is s o lv e the stom ach w a ll tis s u e , the te m p e ra tu re , e tc. , and you have to r e c a lib r a te the w hole system again. So the qu estion is — do you v ie w th is technique as a g e n e ra l one, o r a p p lic a b le o n ly to v e r y s p e c ific c a s e s in w hich you have to set up the w h ole c a lib ra tio n b eforeh an d , and then ap ply it ju st to that p a rtic u la r sy s te m with no v a ria tio n ? F .S . R O W L A N D : I think that it would be m o r e lik e the la tte r . The num ber o f situ ation s fo r which you want to know both e le m e n ta l and m o le cu la r abundance — both e le m e n ta l abundance and the d istrib u tion o f the e le m e n t am ong v a r io u s m o le c u la r fo r m s — is not v e r y g re a t. But perhaps one o f the re a s o n s that the num ber o f such situ ations is s m a ll is that the e x p e rim e n ts a re hard to do. D .J . M A L C O L M E - L A W E S : I'm not quite con vin ced about th is site qu estion, and now you b rin g in the w o rd m o le c u la r abundance. I t 's not r e a lly te llin g you anything m o re than you knew in the f ir s t p la ce — that you have a c e rta in amount o f iodin e th ere. It 's not te llin g you w h ere it is in p h y s ic a l te r m s . F .S . R O W L A N D : W h ich e x p e rim e n t do you m ean? The xenon in the m e te o r ite ? D .J . M A L C O L M E - L A W E S : Y e s . The e x p e rim e n t is n 't te llin g you in p h y s ic a l t e r m s , and it is n 't te llin g you in c h e m ic a l te r m s w h eth er it is an iod id e o r som eth in g e ls e . F .S . R O W L A N D : W hat it is te llin g you is that the 129X e and 128X e a re in e s s e n tia lly the sam e lo ca tio n w ithin the m e te o r ite , and that th ere are oth er xenon is o to p e s p re s e n t in d iffe r e n t lo ca tio n s. D .J . M A L C O L M E - L A W E S : T h a t's fa ir enough. But it s t ill d o e s n 't t e ll you anything about the p h y s ic a l o r c h e m ic a l o r ig in o f the iod in e. The re a s o n that it co m e s out in a bu rst could ju st be an a c tiv a tio n . e n e rg y fo r d iffu sion ra th e r than anything e ls e — it d o esn 't have to be a c r y s ta l o f iod id e b lo w in g up. F .S . R O W L A N D : The fa c t that the n eu tron -p rod u ced 128X e co m es out in the sam e bu rst as the trap p ed e x c e s s 129X e d o e s n 't p ro v e that they a re both contained in an iod id e c r y s ta l, but it does in d ica te that the 128X e m ust be n ear the 127 I fr o m which it sta rted , and that the e x c e s s 129X e m ust be in a v e r y s im ila r lo ca tio n . D .J . M A L C O L M E - L A W E S : I am not e x p la in in g m y s e lf w e ll. I am not denying that the e x c e s s xenon cam e fr o m the iodin e lo c a tio n , and that when you ir r a d ia te fu rth e r, that m o re xenon is p rodu ced that a lso co m e s fr o m the iod in e.
IAE A -P L-61 5/13
227
F .S . R O W L A N D : T h a t's a ll R eyn old s wanted to know. D .J . M A L C O L M E - L A W E S : W hen you used the w o rd o r ig in , I think e ith e r in p h y s ic a l o r c h e m ic a l te r m s . F .S . R O W L A N D : F o r the p u rp oses o f his e x p e rim e n t, a ll R eyn old s w anted to know is th is: w as the m a te r ia l in c o rp o ra te d into the m e te o r ite when it w as 129X e o r when it w as 12 9 1. I f it w as in c o rp o ra te d when it w as 129 I, then the m e te o r ite m ust have e x is te d and c r y s t a lliz e d w ithin about 1 0 0 m illio n y e a r s o f n u cleosyn th esis — b e fo r e the 129I had d ecayed aw ay — and that t e lls you som eth in g about the e a r ly h is to r y o f the u n iv e r s e . I f the in c o rp o ra tio n took p la ce when the atom s had a lre a d y d ecayed to 129X e then the e x p e rim e n t says som eth in g v e r y d iffe r e n t about this e a r ly h is to ry . S. A M IE L : P e rh a p s A d lo ff can m ake som e com m en ts about p h y s ic a l tech n iqu es fo r d e te rm in in g the c h e m ic a l nature o f irr a d ia te d m a te r ia l. Can such tech n iqu es as M ossb a u er e ffe c ts shed ligh t on the c h e m ic a l state o f the ir r a d ia te d e le m e n t fo r the p u rp oses o f a ctiva tio n a n a lysis? J . P . A D L O F F : In fo rm a tio n can be obtained on the su rrou n din gs and on the c h e m ic a l state o f the im p u rity , but not on the lo ca tio n w ith in the sa m p le. S. A M IE L : In the sense o f m o le c u la r a ctiva tio n a n a ly s is , then, it is m o r e in the state ra th e r than in the site. J . P . A D L O F F : M aybe in a lim ite d ca se it w ould be p o s s ib le , but up to now I have no e x p e rie n c e . H o w e v e r, re g a r d in g the contribu tion o f hot atom c h e m is try to a ctiva tio n a n a ly s is , th e re w as a p ap er tw o o r th ree y e a r s ago in A n a ly tic a l C h e m is try , studying the a ctiva tio n a n a ly s is o f s e d i m en ts in ro c k s . He used a lo t o f in fo rm a tio n fr o m S z ila r d -C h a lm e r s c h e m is tr y in connection w ith th is a ctiva tio n a n a ly s is . I don't re m e m b e r the d e ta ils o f the w o rk , but th e re w as a v e r y stro n g con tribu tion of hot atom c h e m is tr y to a c tiv a tio n a n a lysis. S. A M IE L : T h e re is the v e r y c la s s ic a l ex a m p le o f perm an gan ate which goes into M n 0 2 , u sin g the S z ila r d -C h a lm e r s p r o c e s s in this sen se. Y . N IS H IW A K I: I am fr o m the D iv is io n o f N u c le a r S a fety and E n v ir o n m en ta l P r o te c tio n . T h is r e p o r t c o n cern in g the d iffe r e n c e s in y ie ld b etw een in o rg a n ic and o rg a n ic c h lo rin e at d iffe r e n t te m p e ra tu re s s eem s to be v e r y in te r e s tin g in connection with the in te rp re ta tio n o f som e o f the d e ta ils o f the b io lo g ic a l e x p e rim e n ts . A s D r. F ein en d egen exp lain ed y e s te rd a y , in a num ber o f e x p e rim e n ts u sin g m ic r o o r g a n is m s exp osed to rad io n u clid es o r io n iz in g ra d ia tio n , the sa m p les a re often d e e p -fr o z e n in o r d e r to avoid v a rio u s m e ta b o lic d istu rb a n ces. Do you fo r e s e e any sig n ific a n t d iffe r e n c e s i f w e ex p ose the d e e p -fr o z e n state, and i f w e t r y to e x tra p o la te the re s u lts obtained fro m d e e p -fr o z e n tis s u e s to the p o s s ib le e ffe c ts at am bient n o rm a l te m p e ra tu re , u sin g ra d io n u c lid e s ? T h is could be a v e r y im p orta n t point. F .S . R O W L A N D : Y o u r qu estion is w h eth er o r not the reten tio n o f lo c a tio n is d iffe r e n t in the d e e p -fr o z e n s y stem fr o m that found in the aqueous solu tion s y stem — in e x p e rim e n ts in which the n u clear change is r a d io a c tiv e d ecay. I r e a lly don't know o f any e x p e rim e n ts w hich g iv e a n sw ers to you r question. It is som eth in g w hich could be in v e s tig a te d in sim p le s y s te m s to see w h eth er any changes o f this s o rt o c c u r. Then you would ju st have to go back to the o r ig in a l e x p e rim e n ts to see w h eth er th is fa c to r w as the one that w as m ak in g the d iffe re n c e . Y . N IS H IW A K I: W hen we did s im ila r e x p e rim e n ts tw en ty y e a r s ago, w e f ir s t exp osed som e ra d ion u clid e in the m edium ; w e then incubated; then, a fte r the in c o rp o ra tio n to the gen etic m a te r ia l o f D N A , w e d e e p - fr o z e to
228
G R A N T and R O W L A N D
keep it fo r a c e rta in tim e to avoid co m p lica tio n s fr o m the v a rio u s m e ta b o lic p r o c e s s e s du ring exp osu re to these e ffe c ts o f d eca y o r ra d ia tion . A ft e r the exp osu re p e rio d , w e in c re a s e the te m p e ra tu re again, and count the plate a fte r som e c o lo n ie s have been fo rm e d in the m ediu m . F r o m the e x p e rie n c e under d e e p -fr o z e n con d ition s, w e u su a lly e x tra p o la te that the e ffe c ts o f tran sm u tation w ill be p rop agated to c e rta in d ista n ces, ra n g e s , o r ch ro m o 足 som a l a b e rra tio n s . F r o m you r c h e m ic a l e x p e rim e n ts , would you fo r e s e e that this m igh t be m is le a d in g i f w e t r y to e x tra p o la te the re s u lts obtained under the d e e p -fr o z e n con dition s to the e ffe c ts that m igh t be goin g on under the n o rm a l am bient con dition s o f the liv in g c e ll? F . S. R O W L A N D : I have no fe e lin g about it one w ay o r the oth er, ex cep t that th ere a re som e e x p e rim e n ts which could be done to see w hether th e re is any p ro b le m in v o lv e d in such e x tra p o la tio n s to d iffe r e n t te m p e r a 足 tu re s . The e x p e rim e n ts com e under the h eading o f annealing o f sam p les in o rg a n ic m e d ia , and th e re have been su ggestion s a ll d u rin g this w eek that m aybe we should be doing m o re e x p e rim e n ts o f this kind. S. A M IE L : W hat you a re tr y in g to say is : can w e c o n tro l reten tion ? O nce w e can c o n tro l re ten tio n , then this technique can be found to be u sefu l. But the tro u b le is , can w e c o n tro l reten tion ? A . P . W O L F : I think the an sw er to N is h iw a k i's question is y e s . What you a re ta lk in g about o f co u rse is the con ven tion al la b e llin g technique, and th e re is a g re a t d eal o f evid en ce in the lite r a tu r e to in dicate that the b io 足 lo g ic a l a c tiv ity o f m a te r ia l la b e lle d at high le v e ls v a r ie s g r e a tly , dependent upon the m ethod o f sto ra g e . W h eth er o r not that is the case fo r m a te r ia ls la b e lle d by r e c o il h asn 't been studied, but, fo r ex a m p le, w ith tritiu m la b e llin g in our own la b o r a to r y and in m any o th e rs , it has been known that the m ethod o f s to ra g e fo r th ese com pounds at high a c tiv ity le v e ls v e r y s e r io u s ly a ffe c ts what you then see. One exa m p le is the la b e llin g o f A C T H . I f you la b e l it as m u ltic u rie le v e ls , and s to re it at v e r y low te m p e ra tu re s in a dilu te m ediu m , the b io lo g ic a l a c tiv ity o f the A C T H stays at a m axim u m fo r a v e r y lon g tim e , as te s te d by la te r r e s to r in g it to ro o m te m p e ra tu re and u sin g it. On the oth er hand, if you s to re this m a te r ia l at ro o m te m p e ra tu re , the b io lo g ic a l a c tiv ity o f this m a te r ia l g oes down e x t r e m e ly ra p id ly as the re s u lt o f r a d io ly tic dam age to the A C T H . F .S . R O W L A N D : Has the m ech an ism been esta b lish ed that this o ccu rs b ecau se o f the su p p ression o f r a d io ly tic dam age, and not, p erh ap s, som e tran sm u tation e ffe c t? A . P . W O L F : N o, it h asn 't been esta b lish ed . Y . N IS H IW A K I: M ay I ask D r. F ein en d egen w h eth er th ere a re som e c o m p a ra tiv e stu dies o f the tran sm u tation e ffe c t o f the m ic ro o rg a n is m s under the d e e p -fr o z e n con d ition s, and under the n o rm a l am bient te m p e r a 足 tu re o f the liv in g c e lls ? And what w ould be the re s u lt? L . F E IN E N D E G E N : I w ill cite on ly one e x p e rim e n t that has been published r e c e n tly by the group o f P e r s s o n at P en n sy lva n ia State U n iv e rs ity . T h e y o b s e rv e d a p a rtic u la r d iffe r e n c e in m u ta g en icity o f the d eca y o f tritiu m o f the 5 -p o s itio n o f the p y rim id in e r in g v e rs u s d eca y in the 6 -p o s itio n . T h e s e e x p e rim e n ts w e r e done both at n o rm a l te m p e ra tu re s , and in the deepfr o z e n state fo r s to ra g e , w ith id e n tic a l re s u lts . T h e re w e re a p p ro x im a te ly six tim e s m o r e m utations p rodu ced p e r d eca y fro m the 5 -p o sitio n than w as o b s e rv e d fr o m d eca y in the 6 -p o s itio n , independent o f the te m p e ra tu re at which these m a te r ia ls w e r e s to re d du ring the p e rio d o f accum ulation o f the dam age. One group w as d e e p -fr o z e n and one w as in liqu id fo rm .
1АЕА-РЬ615/14
ROLE OF NUCLEAR TECHNIQUES IN THE STUDY OF GAS-PHASE IONIC REACTIONS F. C A C A C E University o f Rome, Rome,
Italy
Abstract R O L E O F N U C L E A R T E C H N IQ U E S I N T H E S T U D Y O F G A S -P H A S E IO N IC R E A C T IO N S . A c r itic a l re v ie w o f th e th e o r y , th e a p p lic a tio n s a n d th e ro le o f th e n u c le a r a p p ro a c h t o th e c h e m is tr y o f g aseous io n s is p re s e n te d . T o p ic s d e a lt w it h in c lu d e m e th o d s o f in v e s tig a tio n s o f io n ic r e a c tio n s in th e gas p hase, s tu d ie s in v o lv in g d e c a y o f t r i t i u m a n d c a r b o n - 14 -la b e lle d m o le c u le s a n d fo r m a tio n s o f ra d io a c tiv e h a lo g e n io n s fr o m n u c le a r tr a n s fo r m a tio n s . L in e s o f fu tu r e d e v e lo p m e n t in th is fie ld a re su ggested.
1.
IN T R O D U C T IO N
E v e n a c o n s e rv a tiv e evalu ation o f the p resen t status of hot atom c h e m is tr y lea d s to the g r a tify in g con clu sion that the th e o r e tic a l and e x p e rim e n ta l e ffo r ts d evoted in the past th re e decades to the study o f c h e m ic a l e ffe c ts of n u clear tra n s fo rm a tio n s a r e c u rre n tly y ie ld in g a highly re w a rd in g retu rn . N ot only, in fa c t, has hot atom c h e m is try e v o lv e d into an independent, th e o r e tic a lly w e ll-fo u n d e d branch o f c h e m is try , c h a r a c te r iz e d by o r ig in a l p ro c e d u re s and re s u lts , but ty p ic a l hot atom tech n iqu es have been d evelo p ed into r e s e a r c h to o ls o f g e n e r a l valu e to fundam ental and app lied c h e m is try . T h e cu rre n t situ ation su ggests that th is panel, ra th e r than p resen tin g a s p e c ia lis tic r e v ie w d ir e c te d to w o r k e r s in the fie ld , should be p r im a r ily r e g a r d e d as a tim e ly o cca sio n fo r ex p o sin g a w id e r c h e m ic a l audience to the r o le and p o te n tia lity o f hot atom tech n iqu es in m a jo r a re a s o f c h e m is try . T h is approach ap p ears p a r tic u la r ly a p p ro p ria te fo r the s p e c ific su bject of th is r e p o r t, i.e . the re a c tio n s of ch a rged s p e c ie s fo rm e d fr o m n u clear tra n s fo rm a tio n s , sin ce th e ir th e o r e tic a l and e x p e rim e n ta l study has le d to the d evelop m en t o f a new technique that in its e a r ly , and s t ill lim ite d , a p p li ca tion s has shown g r e a t p r o m is e as a to o l fo r the in v e s tig a tio n o f ion ic re a c tio n s in the gas phase. A c c o r d in g to the c h a ra c te r and the g e n e ra l o b je c tiv e s o f the panel, this r e p o r t p r im a r ily a im s to p ro v id e a c r it ic a l r e v ie w of the th e o ry , the a p p li cation s and the r o le o f the n u clea r approach to the c h e m is try o f gaseou s ion s, o m ittin g any s y s te m a tic account o f the many s ig n ific a n t con trib u tion s that h elped to elu cid a te the r o le o f io n ic p r o c e s s e s in hot atom c h e m is try . The r e a d e r in te re s te d in the en ligh ten in g h is to r ic a l background is d ire c te d to the a v a ila b le s p e c ia lis tic r e v ie w s [1 (a )- (g )].
2.
IO N IC R E A C T IO N S IN T H E GAS P H A S E : IN V E S T IG A T IO N
C U R R E N T M E TH O D S O F
A n y c u r s o r y in sp ectio n o f the c u rren t c h e m ic a l lite r a tu r e r e v e a ls the im p r e s s iv e su rge o f in te r e s t in g a s-p h a se ion ic re a c tio n s . In p art, this
229
230
CA C AC E
a r is e s fr o m the fu ll r e a liz a tio n o f the b a sic r o le o f io n -m o le c u le re a c tio n s in m any in te re s tin g phenom ena, inclu ding u pper a tm osp h ere c h e m is try , d is c h a rg e s , fla m e s , ra d ia tio n c h e m is tr y o f g a s e s , e tc ., coupled with the in c re a s in g a v a ila b ility of new e x p e rim e n ta l tech n iqu es, inclu ding highp r e s s u r e m ass s p e c tr o m e tr y , io n -c y c lo tr o n reso n a n ce m a ss s p e c tro m e try , r a d io ly tic and p h o to -io n iza tio n m eth ods. H o w e v e r, the m o st fundam ental c h e m ic a l in te re s t to a b e tte r understanding o f the p r o p e r tie s and the r e a c t i v ity o f gaseou s ions undoubtedly a r is e s fr o m the in h eren t s im p lic ity o f the gaseou s en viron m en t, fr e e fr o m the c o m p lic a tin g e ffe c ts of solva tion , c o u n te r-io n s , e tc ., w hich a re in v a ria b ly m e t in condensed ph ases. In p a r tic u la r , only the g a s -p h a s e in v e s tig a tio n s g iv e the ch em ist a chance to subm it to a d ir e c t e x p e rim e n ta l scru tin y the r e s u lts o f the in c re a s in g ly s o p h istica ted and r e lia b le th e o r e tic a l ca lcu la tio n s, co n cern in g the stru ctu re and the r e a c t iv it y o f a g r e a t v a r ie ty o f ions, which n e c e s s a r ily r e f e r to u n solvated, gaseou s s p e c ie s [ 2 ]. T h e a c tiv e in te r e s t in the c h e m is try o f gaseou s ions has a lre a d y p ro v id e d a w ealth o f fundam ental re s u lts , such as the con stru ction o f a g a s-p h a se a c id ity s c a le , the m ea su rem en t o f the proton a ffin ity o f a la r g e num ber of gaseou s m o le c u le s , the d em on stra tion o f g a s-p h a se re a c tio n s , including h y d rid e -io n t r a n s fe r , a lk yla tion , condensation, p o ly m e riz a tio n , e lim in a tio n , h alogén ation , a ro m a tic -s u b s titu tio n s , which exh ib it basic a n a lo g ies and s ig n ific a n t d iffe r e n c e s fr o m the corresp o n d en t p r o c e s s e s o c c u rrin g in solu tion . D e sp ite its a c h ievem en ts, the study o f io n ic c h e m is try in the gas phase is s e v e r e ly r e s t r ic t e d by in stru m en ta l lim ita tio n s , and the need fo r a d iffe re n t e x p e rim e n ta l approach is d eep ly fe lt. F o r a lon g tim e , m a ss s p e c tr o m e tr y has p ro v id e d the only ex p e rim e n ta l technique to the in v e s tig a tio n s o f gaseou s ion s, and its scop e and u sefu ln ess have been g r e a tly extended by re c e n t d evelo p m en ts, in p a rtic u la r ch e m ic a l io n iza tio n , io n -c y c lo tr o n res o n a n c e , and tandem m ass s p e c tr o m e te r s . H ow e v e r , even in its m o s t re c e n t and sop h istica ted a p p lica tio n s, a p u re ly m a ss s p e c tr o m e tr ic approach is in m any r e s p e c ts inadequate, i f evalu ated by s o lu tio n -c h e m is try stan dards. In fa c t, apart fr o m the s e v e r e ly r e s tr ic te d p re s s u re ran ge g e n e r a lly a c c e s s ib le , and the r e la t iv e ly high e n e rg y o f the io n ic re a g e n ts , the m a ss s p e c tr o m e tr ic study o f io n -m o le c u le re a c tio n s p ro v id e s lit t le m o r e than a s to ic h io m e tr ic outline o f the p r o c e s s in v e s tig a te d , g iv in g s c a r c e in fo rm a tio n , i f any, on the m o le c u la r stru ctu re o f the ions, th e ir s te r e o c h e m ic a l fe a tu re s , the nature o f th e ir n eu tra l re a c tio n s , etc. w hich re p r e s e n t e s s e n tia l data, g e n e r a lly a c c e s s ib le in solu tion. In the next p a ra gra p h s it w ill be shown that, at le a s t in a fe w re p re s e n ta tiv e ca s e s , tech n iqu es based on su itab le n u clea r tra n s fo rm a tio n s have p ro v e d h elp fu l to advance the study o f gaseou s io n ic re a c tio n s fr o m the e s s e n tia lly s to ic h io m e t r ic s ta g e, ty p ic a l o f the m a ss s p e c tr o m e tr ic m ethod, to the m o r e co m p lete k in e tic , s tru c tu ra l and s te r e o c h e m ic a l approach that c h a r a c te r iz e s the m o d ern solu tion c h e m is tr y o f o rg a n ic ions. 3.
GASEOUS C A T IO N S F R O M T H E D E C A Y O F T R IT IA T E D M O L E C U L E S
3.1. O utline o f the m ethod T h e cation o f in te r e s t is obtained fr o m a p r e c u r s o r containing at le a s t tw o T atom s in the sa m e m o le c u le , which is a llo w e d to d ecay w ithin a gaseou s s y s te m at any con ven ien t p r e s s u r e [3].
IAEA-PL-615/14
231
T h e decay of one o f the tr itiu m atom s lea d s to the fo rm a tio n o f a la b e lle d daughter cation , w h ose abundance, nature and e n e r g e tic s m ust be known in advan ce fr o m p r e lim in a r y th e o r e tic a l and e x p e rim e n ta l in v e s tig a tio n s (v id e in fr a ), e.g.: T 2------- ►3 H e T + +(3" C T 4----- ► C T 3+ + 3He + 3 ‘ , etc. T h e re a c tio n s o f the daughter ion can be tra c e d , and th e ir fin a l p rodu cts id e n tifie d , ow in g to the p re s e n c e of the oth er T a to m (s ) w hich acts as a la b e l. T h e fin a l p rodu cts can be d ir e c t ly d e te rm in e d using e ffic ie n t a n a ly tic a l tech n iqu es, such as ra d io gas ch ro m a to gra p h y, o r is o la te d and p u rifie d , fo llo w in g the addition o f in a c tiv e c a r r ie r s , by p r e p a r a tiv e ch rom atograp h y and su b jected to c h e m ic a l d egra d a tion p ro c e d u re s in o r d e r to d e te rm in e the p o s itio n o f the T atom in th e ir m o le c u le s . T h e y ie ld s a re ca lcu la ted fr o m the r a tio of the a c tiv ity contained in each produ ct to the to ta l a c tiv ity o f the d ecay ion s fo r m e d within the s y s te m during the ex p e rim e n t. Suitable r a d ic a l s c a v e n g e rs , io n ic in te r c e p to r s , gaseou s b a ses o r in e rt g a s e s can be in trod u ced into the s y s te m , whose re a c tio n s can be in v e s tig a te d by a ll c la s s ic a l techniqu es o f g a s-p h a se k in e tic s , including co m p e titio n e x p e rim e n ts , p r e s s u r e dependence, te m p e ra tu re changes and m o d e ra to r e ffe c ts . 3.2.
T h e o r e t ic a l re s u lts con cern in g the d eca y-in d u ced fra g m en ta tio n o f t r itia te d m o le c u le s
T h e c h e m ic a l consequ ence o f the j3 d ecay o f T have been the su bject o f s ig n ific a n t th e o r e tic a l w o rk [4 ]. R e fe r r in g f ir s t to the decay o f is o la te d T atom s, th e re a re tw o m a jo r cau ses o f ex c ita tio n , i.e . the m om entum tr a n s fe r to the daughter 3He+ ion fr o m the em itted /3 p a r tic le and an ti-n eu trin o , and the "s h a k in g " o f the e le c tr o n cloud due to the sudden in c r e a s e o f the n u clear c h a rg e , w h ile oth er c o n c e iv a b le so u rc e s o f e x c ita tio n (inclu ding the d ir e c t c o llis io n o f the e m itted ¡3 p a r tic le w ith an o r b ita l e le c tro n , va ca n cy c a s ca d es, b re m s s tra h lu n g c o n v e rs io n , e tc .) a r e in s ig n ific a n t. T h e r e c o il e n e rg y im p a rte d to the is o la te d 3He+ ion has been ca lcu la ted by v a rio u s authors, using s im p lify in g assum ptions as to the angular c o r r e la tio n o f the tw o p a r tic le s , and its m axim u m valu e is set around 3.6 eV , w h ile o v e r 80% o f the daughter ions r e c e iv e r e c o il e n e rg ie s < 0.08 e V [5]. T h e "sh a k in g e ffe c t " is m o r e s ig n ific a n t, and has been th e o r e tic a lly tr e a te d by evalu atin g the p ro b a b ility o f each e le c tr o n ic tra n s itio n fr o m the ground s ta te, o f T to e ith e r the ground state o r the v a rio u s e x c ite d sta tes of the daughter 3H e+ ion, using h y d ro g e n -lik e w ave functions. T h e p ro b a b ility is com puted fr o m the squ are o f the o v e r la p in te g ra ls : 2
w h ere n ,l a r e the p rin c ip a l quantum nu m bers r e f e r r in g to the ground state of p a ren t, n ' l 1 those r e f e r r e d to each o r b ita l o f the daughter ion, dr is the vo lu m e elem en t.
232
C A C AC E
T h e r e s u lts [ 6 ] in d ica te that the decay o f an is o la te d atom lead s p r e dom in an tly to the fo rm a tio n o f 3H e+ in its ground sta te, but a s ig n ifica n t fr a c tio n o f the decays y ie ld s e x c ite d daughter ions: 3He+ ( I s , grou nd)
"
(2 s) (3 s)
7 0% 25% 1.3%
T ( l s , grou n d )-
1.7% I f the T atom is c o v a le n tly bound in a m o le c u le , the s ig n ific a n c e of r e c o i l ex c ita tio n is s t ill lo w e r , sin ce only p art o f the r e c o il e n e rg y is a v a ila b le fo r bond ru p tu re, as shown on a sim p le c a se by the e x p re s s io n v a lid fo r d ia tom ic m o le c u le s : _ 5
l_ x
w h ere E¡ is the e n e rg y a v a ila b le fo r bond ru ptu re, pfl is the m om entum o f the
¡3 p a r tic le , M j and M 2 the m a s s e s o f the r a d io a c tiv e and the stab le atom . The e le c tr o n ic e x c ita tio n o f m o le c u le s due to the "sh a k in g e ffe c t " has been evalu ated by d iffe re n t th e o r e tic a l ap p roach es. T h e rig o r o u s trea tm en t, due to W o lfs b e r g [7 ], r e q u ir e s know ledge o f the w ave functions o f v ib ra tio n a lly and r o ta tio n a lly e x c ite d sta tes, and can be t h e r e fo r e used only fo r v e r y s im p le m o le c u le s , lik e H T and T 2. A p p ro x im a te m o d e ls m ust be used fo r m o r e c o m p lic a te d m o le c u le s , in p a rtic u la r the " lo o s e " m o d e l o f W e x le r and H ess [ 8 ], a r b it r a r ily r e s t r ic t in g the "s h a k in g " e ffe c ts to the bond e le c tro n s o f the r a d io a c tiv e atom , which lea d s to con clu sion s in q u a lita tiv e a g reem en t w ith the e x p e r im e n ta l r e s u lts . T h e o v e r a ll con clu sion o f the th e o r e tic a l ca lcu la tio n s is that a v e r y la r g e fra c tio n of the decay ion s, ran gin g fro m 75 to 95 a c c o rd in g to d iffe re n t authors, a re fo rm e d in th e ir ground state, r e c e iv in g a n e g lig ib le amount o f r e c o il ex cita tion . It should be noted, how e v e r , that the v e r y change o f the c h e m ic a l id en tity o f the r a d io a c tiv e atom cau sed by the d ecay m ay r e p re s e n t a v e r y e ffe c tiv e fa c to r fo r m o le c u la r d is s o c ia tio n . F o r in stan ce, when the T atom was bound to C , the in trin s ic w eakn ess of the С -H e bond, c le a r ly p re d ic ta b le on th e o r e tic a l grounds, can be in v a ria b ly exp ected to cause the d is s o c ia tio n of the daughter d ecay ions. 3.3. M a ss s p e c tr o m e tr ic studies o f the p r im a r y , d eca y-in d u ced fra g m e n ta tio n S p e c ia l in stru m en ts (the "c h a r g e " m ass s p e c tro m e te rs and, m o r e re c e n tly , m o d ifie d IC R s p e c tr o m e te r s ) have been used to study the d ecay-in d u ced fra g m e n ta tio n o f is o la te d tr itia te d m o le c u le s under u n im o lecu la r d is s o c ia tio n con d ition s, i.e . under p r e s s u r e b elo w 10‘ 5- 10 ’ 7 t o r r [4, 9]. T h e re s u lts can be s u m m a rize d as fo llo w s : (i)
O v e r 90% o f the T 2 (H T ) decays lea d to the fo rm a tio n of stable 3 H e T +( 3H eH +) daughter io n s, which s u r v iv e in d e fin ite ly in the c o llis io n - fr e e space.
233
IA E A -P L-6 1 5/14
T A B L E I. Y IE L D S O F M A JO R D A U G H T E R IONS F R O M T H E D E C A Y O F T R IT IA T E D H Y D R O C A R B O N S _______ Y ie ld Precursor
( ii)
Ion (% )
.
82
C H 3T
CH3 +
C 2H 5T
c 2H 5
80
c 6H 5T
c 6H 5 +
72
o - C H 3- C 6H 4T
c 7H ?
78
m - C H 3- C 6H 4T
C ,H ?
79
p - C H 3- C 6H4T
C 7H Ă?
76
C 6H 6- C H 2T
C jH *
79
C -C 4H 7T
C 4H 7
c a . 80
C -C 5H 9T
C SH 9 +
c a . 75
L e s s than 10% o f the d eca ys le a d to fra g m e n ta tio n o f the daughter ion s, w ith fo rm a tio n o f 3H e2+, T +(H +) ions. T h e decay o f tr itia te d h yd ro ca rb o n s is in v a r ia b ly fo llo w e d by the lo s s of n eu tra l 3H e fr o m the daughter ion. A la r g e fra c tio n , (o v e r 80%) o f the o rg a n ic cation s fo r m e d es c a p e s fu rth e r fra g m en ta tio n , as illu s tra te d in T a b le I, su ggestin g the la c k of any a p p re c ia b le ex c ita tio n e n e rg y . A few in v e s tig a tio n s c a r r ie d out by IC R s p e c tr o m e tr y on the e a r ly io n -m o le c u le r e a c tio n s o f the a lk y l cation s fr o m the /3 decay o f tr itia te d h yd rocarb on s have fa ile d to p ro v id e ev id e n c e fo r the in te rv e n tio n o f e x c ite d s p e c ie s [9].
A s a w h ole, the m a s s s p e c tr o m e tr ic stu dies la r g e ly support the th e o r e tic a l con clu sion s illu s tr a te d in the p re c e d in g S ection c o n cern in g tw o im p orta n t points: (i) (ii)
M o st (o v e r 80%) o f the daughter ions a r e fo rm e d in th e ir ground state. T h e change o f c h e m ic a l id en tity undergone by the T atom is the m ost s ig n ific a n t cause o f m o le c u la r fra gm en ta tio n : thus, the th e o r e tic a lly w e ll-k n o w n [10] s ta b ility o f the H eH + m o le c u la r ion is fu lly c o n firm e d by the e x p e rim e n ta l re s u lts , w h ile the th e o r e tic a lly p re d ic ta b le in trin s ic w eak n ess o f the ĐĄ -H e bond lea d s to the lo s s o f 3H e fr o m 100% o f the daughter ion s p rodu ced fr o m the d ecay o f tr itia te d h yd ro ca rb o n s.
3.4. E x p e rim e n ta l a sp ects A s d iscu ssed in s e c tio n 3.1, la b e lle d daughter ions fr o m the decay of m u lti- tr itia te d p r e c u r s o r s m ust be used in the study o f g a s -p h a s e ion ic r e a c tio n s in sy s te m s at a tm o sp h eric p r e s s u r e . T h e n e c e s s ity of a ra d io a c tiv e la b e l a r is e s fr o m the e x tr e m e ly s m a ll ra te o f fo rm a tio n o f the decay ions, sin ce only ca. 0.5% o f the tr itia te d p r e c u r s o r u n d ergoes /3 decay in one month.
234
CA C AC E
Thus, ow ing to the lim ita tio n s to the p r e c u r s o r a c tiv ity p osed by s e lf 足 r a d io ly s is p ro b le m s (v id e in fr a ), the num ber o f decay ions fo rm e d within any rea s o n a b le p e rio d o f tim e is too s m a ll to a llo w d etection o f th e ir produ cts, u n less som e r a d io a c tiv e la b e l is used. T h e syn th esis o f p r e c u r s o r s containing at le a s t tw o tritiu m atom s in eq u iva len t p o sitio n s w ithin the sam e m o le c u le is fa r fr o m ea s y , and p rob a b ly re p r e s e n ts the m o s t s e rio u s e x p e rim e n ta l d iffic u lty a s s o c ia te d with the use o f the decay ion s. E s p e c ia lly d evelo p ed syn th etic, p u rific a tio n , and a n a ly tic a l techniqu es m ust be used fo r the p re p a ra tio n o f c h e m ic a lly , r a d io 足 c h e m ic a lly and is o to p ic a lly pure m u ltila b e lle d p r e c u r s o r s such as C T 4, re a c h in g a s p e c ific a c tiv ity in e x c e s s o f 10 6 C i p er m o le , which m ust be obtained in the c a r r i e r - f r e e state, b e fo r e d ilu tion w ith a v e r y la r g e e x c e s s o f the corresp o n d en t in a c tiv e compound can take p la ce [ 1 1 ]. A n o th er p ro b le m a r is e s in con n ection with the fa ct that decay o f one o f the T atom s contained in a m u ltila b e lle d m o le c u le in trod u ces into the s y s te m , to g e th e r w ith a tr itia te d daughter ion , a (3 p a r tic le w ith a m ean e n e rg y of 5.6 k eV . It is in fa c t c o n c e iv a b le that, i f a fra c tio n o f the la b e lle d p r e c u r s o r u n d ergo es r a d io ly s is , tr itia te d produ cts a re fo rm e d w hich would be in d is 足 tin gu ish a b le fr o m those a r is in g fr o m the re a c tio n s of the decay ions. The p ro b le m has been s o lv e d by keep in g the co n cen tra tion o f the tr itia te d m o le 足 c u les to a s u ffic ie n tly lo w le v e l to m ake th e ir r a d io ly s is r a te in sig n ifica n t in c o m p a ris o n w ith the r a te o f fo rm a tio n of the decay ions. It can be shown by k in etic c o n s id e ra tio n s , and it has been re p e a te d ly d em on stra ted in blank e x p e rim e n ts c a r r ie d out with m o n o tritia te d p r e c u r s o r s , w h ere only ra d ia tio n -in d u ced p r o c e s s e s can be o p e r a tiv e , the fo rm a tio n of la b e lle d produ cts v ia decay ions b ein g a p r io r i su p p ressed , that s e lf- r a d io ly s is p o ses no p ro b le m , p ro v id e d that the s p e c ific a c tiv ity o f the gaseou s sam p le does not e x ceed 0.1 - 0.5 m C i p e r m m o le. In the actu al e x p e rim e n ts , the s p e c ific a c tiv ity can be fu rth e r red u ced by a fa c to r o f at le a s t 1 0 , without s a c r ific in g the a c c u ra c y o f the products a n a ly s is , by using s u ffic ie n tly lon g d ecay p e rio d s (2 to 6 m onths) to a llow the accu m u lation o f the prod u cts, and by em p lo yin g e ffic ie n t counting tech n iqu es. 3.5.
A p p lic a tio n s
T h e stu dies on the g a s -p h a s e io n ic re a c tio n s c a r r ie d out with d ecay ions has been m a in ly co n cern ed w ith a lk y l (c y c lo a lk y l) cation s and w ith the gaseou s H e T + a cid fr o m the d ecay o f m o le c u la r tritiu m . L a b e lle d a lk y l and c y c lo a lk y l ions have been obtained fr o m the d ecay o f the co rre s p o n d e n t m u lti- tr itia te d h yd roca rb on s: C T j ions fr o m m e th a n e -T 4 [12], C 2 H4 T + ions fr o m 1 ,2 -C 2 H4T 2 [13], an e q u im o le c u la r m ix tu re o f 1-C 3 H 6T and 2 -C 3 H 6T + fr o m 1 ,2 -C 3H6T2[14], c -C 4 H 6T + fr o m c-C 4 H 6T 2, c - C 5 H 8T + fr o m c - C sH 8T 2 [9, 15]. T h e r e a c t iv it y o f th ese io n ic s p e c ie s has been in v e s tig a te d o v e r a w ide ra n ge o f p re s s u r e s in a la r g e v a r ie ty o f s y s te m s , in clu d in g the c o rresp o n d en t h y d ro ca rb o n s, ben zen e, tolu ene and x y le n e s [16], h alogen d e r iv a tiv e s [17], w a te r vapou r and the lo w e r alip h atic a lc o h o ls [1 8 ,1 9 ], am m on ia, a lk y la m in e s and n itro b en zen e [ 2 0 ], d ia lk y le th e rs , o le fin s [ 2 1 ]. T h e H e T + m o le c u la r ion, an e x c e e d in g ly s tro n g B r^ n sted a cid con ven ien tly obtained fr o m the decay o f T2 , has been la r g e ly used as a p o w erfu l proton (tr ito n ) donor to a v e r ie t y o f su b s tra te s , in clu din g alkanes (m ethane, ethane [22], propane, n- and i-b u tan e [2 3 ]), a lk y l h a lid es
235
IAEA-PL-615/14
(1 ,2 - d iflu o r o - 1 ,2 -d ich loroeth a n e [2 4 ]), c y clo a lk a n es (cyclob u tan e, cyclop en tan e, c y clo h ex a n e [23], 1, 2 -d im e th y l c y clo p ro p a n e [2 6 ]), b ic y c lo a lk a n e s (b ic y c lo [2 .1.0] pentane, b ic y c lo [3 .1 .0 ]h e x a n e , b icyc lo [4 .1 .0 ]h e p ta n e [27]), a ro m a tic e and th e ir m ix tu re s (ben zen e, toluene and th e ir deu terated cou n ter p a rts [28], t-b u ty lb en zen e [29], f lu o r o - , c h lo r o - , and b ro m o b en zen e [30], a n is ó le , b e n z o n itrile arid a - tr iflu o r o to lu e n e [2 9 ]). A d e ta ile d a n a ly s is o f the in d ivid u al co n trib u tion s, and th e ir d iscu ssio n , is beyond the scop e o f the r e p o r t. F r o m the a v a ila b le r e s u lts , the p ertin en t con clu sion can be drawn that the d ecay approach has p ro v id e d s ig n ific a n t in fo rm a tio n , u n availab le w ith oth er tech n iqu es, on the stru ctu re and the r e a c t iv it y o f gaseou s cation s, in clu din g the unam biguous e x p e rim e n ta l evid e n c e fo r the o c c u rre n c e of c y c lo p e n ty l, cy c lo b u ty l, and c y clo p ro p a n iu m ion s as sta b le s p e c ie s in the gas phase, the s te r e o c h e m ic a l c o u rs e o f one attack o f a c h a rg e d e le c tr o p h ile to a satu rated carb on , the d eterm in a tio n o f the p o s itio n a l and su b strate s e le c t iv it y o f g a s -p h a s e a ro m a tic substitu tions by ch a rged e le c tr o p h ile s.
4.
D E C A Y O F 14C - L A B E L L E D M O L E C U L E S So fa r , no s p e c ific a p p lica tio n o f daughter ion s fr o m the d eca y o f
14C - la b e lle d p r e c u r s o r s to the study o f g a s -p h a s e io n ic re a c tio n s has been
r e p o r te d , y e t the su bject is of in te r e s t to the p resen t r e p o r t, sin ce th e o r e tic a l, m a s s s p e c tr o m e tr ic and ra d io c h e m ic a l stu dies have d em on stra ted that the 13 d ecay o f 14C m ay re p r e s e n t a u sefu l so u rce o f gaseou s cation s con tain in g a n itro g e n atom . A th e o r e tic a l tre a tm e n t o f the m o le c u la r consequ en ces fo llo w in g the d ecay o f 14С has been c a r r ie d out by W o lfs b e r g [7] and, m o r e r e c e n tly , by S k orob oga tov [31] and by R a a d s c h e ld e r s -B u ijz e , R o o s and R o s [32]. The re s u lts o f th ese c a lcu la tio n s in d ica te that the extent o f bond ru pu tre by r e c o il, although not p r e c is e ly known, is v e r y low and that the m a jo r so u rce o f m o le c u la r e x c ita tio n is due to e le c tr o n "s h a k in g " e ffe c t. A c c o r d in g to the m o st r e c e n t re s u lts of IN D O ca lcu la tio n s [32], the t h e o r e tic a l reten tio n va lu e s fo r d iffe r e n t m o le c u le s a re high and independent o f the p a r tic u la r stru ctu re o f the p r e c u r s o r , as shown below : P a re n t m o le c u le M e th a n e -14C E th a n e-14C P r o p a n e - 14C E th y le n e - 14C C y c lo p r o p a n e -14C B e n z e n e -14C 14C 0 9
T h e o r e t ic a l reten tio n (%) 66.5 66.5 61.0 63.5 62.7 61.0 67.1
T h e only m a ss s p e c tr o m e tr ic study so fa r a v a ila b le [33] has shown that 81% o f the daughter N 0 2 ions fr o m the decay o f 14С О г s u rv iv e d is s o c ia tio n , in fa ir a g re e m e n t w ith the th e o r e tic a l valu e. T h e r e a r e only a fe w r a d io c h e m ic a l in v e s tig a tio n s on the c h e m ic a l e ffe c ts o f the 14C d ecay. E a r ly stu dies on ethane- 14C [34], b e n z e n e -14C [35], to lu e n e -14C ,[3 6 ], su ccin ic a c id - 14C [37], and a re c e n t, c a r e fu l in v e s tig a tio n
236
C A C AC E
on s e v e r a l 14 C -h y d ro c a rb o n s [38], lea d to the con clu sion that the decay o f a 14C - la b e lle d p r e c u r s o r y ie ld s , in a la r g e (> 50%) fr a c tio n o f the tra n sitio n ,
a daughter ion which s u r v iv e s fu rth e r fra g m en ta tio n , e.g.: I4 C H 2 R: 2R 2
В decay" R 1 R 2N H 2 +
^
T h is con clu sion u n d erlin es the p o ten tia l valu e o f a m ethod based on the /3 d ecay o f 14C - la b e lle d p r e c u r s o r s in the study o f the g a s-p h a se r e a c tiv ity o f io n ic s p e c ie s con tain in g n itrogen .
5.
H A L O N IU M IONS F R O M N U C L E A R T R A N S F O R M A T IO N
M any n u clea r tra n s fo rm a tio n s , including (n, 7 ) re a c tio n s , is o m e r ic tra n s itio n s , e le c tr o n captu re, p o s itro n e m is s io n e tc ., and the a s s o c ia te d p ertu rb a tio n o f the o r b ita l e le c tr o n b ro a d ly d e s c rib e d as "in n e r s h e ll io n iz a tio n " lea d to the fo rm a tio n o f ra d io a c tiv e h alogen ion s in v a rio u s sta tes of c h a rg e , e le c tr o n ic and tra n s la tio n a l ex cita tion . D e sp ite the fa ct that the r o le o f io n ic s p e c ie s in the c h e m ic a l p ro c e s s fo llo w in g the n u clea r tra n s fo rm a tio n s of h alogen atom s has lon g been re c o g n iz e d , and has been c o n firm e d by re c e n t in v e s tig a tio n s [3 9 -4 5 ], only in re c e n t y e a r s has an e x p e rim e n ta l technique fo r the study o f the r e a c tiv ity o f gaseou s halonium ions been d e s c rib e d [46]. T h is approach is based on a p a r tic u la r ly w e ll- d e s c r ib e d n u clear tra n s fo rm a tio n , the is o m e r ic tra n s itio n o f 80g r m ^ which is known fr o m th e o r e tic a l [4, 47-49] and m a ss s p e c tr o m e tr ic [48] evid en ce to lead , v ia A u g e r p r o c e s s e s and va ca n cy ca s c a d e s , to m u ltip ly ch a rged , hot 80B r + daughter ion s. A c c o r d in g to the technique d e s c rib e d , a p r e c u r s o r lik e C H g ^ B r 115, o r CF 3 8sB r m is a llo w e d to d ecay w ithin a gaseou s s y s te m containing a s u ffic ie n tly la r g e e x c e s s o f a noble g a s, h avin g an io n iza tio n p oten tia l in t e r m e d ia te betw een the f ir s t and the secon d io n iza tio n p oten tia l o f B r , in the p re s e n c e o f a low con cen tra tio n o f the d e s ir e d o rg a n ic su b strate. T h e c o llis io n s o f the 80B r n+ ions w ith the in e r t m o d e ra to r ra p id ly redu ce th e ir c h a rg e to the + 1 va lu e by fa s t c h a rg e exchange p r o c e s s e s , and re m o v e the e x c e s s k in etic e n e rg y , b e fo r e the attack o f brom on iu m ions on the sub s tra te can take p la ce. T h e p r o c e s s can be ou tlined as fo llo w s :
(the m o s t lik e ly c h a rg e o f the daughter ion is +7, its k in etic e n e rg y s e v e r a l eV).
M a n y c o llis io n s
80B r+ + RH
P ro d u c ts
T h e a n a ly sis o f products is c a r r ie d out by ra d io g a s ch ro m a to gra p h y, and th e ir y ie ld is r e f e r r e d to the to ta l num ber o f the brom on iu m ions fo rm e d w ithin the sy s te m .
IA E A -P L-61 5/14
237
T h e a n a ly sis o f p rodu cts is c a r r ie d out by ra d io g a s c h ro m a to gra p h y, and th e ir y ie ld is r e f e r r e d to the to ta l num ber o f the brom on iu m ions fo rm e d w ithin the s y stem . F r o m the e x p e rim e n ta l standpoint, the I .T . o f 80B r m p re s e n ts o v e r the /3 decay o f T and 14C the re m a rk a b le advantage that the daughter ion being a r a d io a c tiv e s p e c ie s p e r se, the use o f doubly la b e lle d p r e c u r s o r s is not n e c e s s a r y . A d m itte d ly , the e le c tr o n ic state o f the 80B r + ions when th e ir re a c tio n w ith the su b stra te takes p la ce is not as e x a c tly defin ed as in the c a s e o f t r itia te d s p e c ie s , sin ce lo n g - liv e d e x c ite d sta tes o f B r + could con c e iv a b ly be in v o lv e d . N e v e r th e le s s , a c a r e fu l k in etic a n a ly s is of the ion ic re a c tio n s can h elp red u cin g th is unwanted e ffe c t. The n u clea r technique ou tlined r e p r e s e n ts a m o st u sefu l s o u rce o f gaseou s b rom in iu m ion s, as shown by its e a r ly a p p lica tio n s, inclu ding so fa r the f ir s t in v e s tig a tio n of the g a s -p h a s e a ro m a tic b rom in a tion by fr e e B r+ ions [46, 50], and the f ir s t d e te rm in a tio n o f the s te r e o c h e m ic a l c o u rs e o f a g a s -p h a s e e le c tr o p h ilic b ro m in a tio n [51]. V e r y r e c e n tly [50], the scop e o f the techniqu e has been c o n s id e ra b ly extended using the 125X e (E C Я251 n u clea r tra n s itio n as a s o u rce of hot, m u ltip ly c h a rg e d [45] (m o st lik e ly c h a rg e state fr o m +8 to +9) iod in e ca tion s, which a re m o d e ra te d and red u ced to the c h a rg e state + 1 by a la r g e num ber o f c o llis io n s with A r o r X e , b e fo r e b ein g a llo w e d to r e a c t with the orga n ic su b stra te. T h is technique has been s u c c e s s fu lly ap p lied to the study of g a s -p h a s e e le c tr o p h ilic iod in ation o f a re n e s and th e ir halogenated d e r iv a tiv e s [50].
6.
C O N C LU S IO N S A N D PRO G N O SIS
In the decade ela p s e d sin ce it was f ir s t p ro p o sed [52], the n u clear approach to the study o f gaseou s ion s has e v o lv e d into a u sefu l r e s e a r c h to o l, w hose valu e is at p re s e n t e s p e c ia lly ack n ow ledged [53] in connection w ith the p o s s ib ility it o ffe r s o f a d ir e c t e x p e rim e n ta l co m p a ris o n with the re s u lts o f th e o r e tic a l ca lcu la tio n s. In the opinion o f the author o f this r e p o r t, h o w e v e r, the s u rfa c e o f the m a jo r fie ld o f a p p lica tion , i.e . the study o f the is o m e r iz a tio n and r e a rra n g e m e n t o f gaseou s p rodu cts and the s t e r e o c h e m ic a l c o u rs e o f gaseou s io n ic re a c tio n , has h a rd ly been scra tch ed . C o n c e rn in g , in p a r tic u la r , the d ecay o f tr itia te d p r e c u r s o r s , the fo llo w in g lin e s o f d evelop m en t can be an ticipated: (i)
T h e m a s s s p e c tr o m e tr ic d eterm in a tio n o f the p r im a r y , d ecay-in d u ced fra g m e n ta tio n p a ttern o f a la r g e r num ber o f p r e c u r s o r s would open to in v e s tig a tio n a c o rre s p o n d in g ly g r e a t e r v a r ie t y o f gaseou s ions. ( i i ) T h e tr ito n tr a n s fe r fr o m H e T + to oth er gaseou s s p e c ie s (e .g ., noble g a s e s ) would p ro v id e a w id e r c h o ic e o f Brçinsted acids o f d iffe re n t stren gth , as su ggested by p r e lim in a r y re s u lts [54]. ( i i i ) T h e author o f th is r e p o r t su ggests that a fo r e s e e a b le im p ro v e m e n t of the cu rre n t a n a ly tic a l technique., and a c a r e fu l c h o ic e o f the e x p e rim e n ta l con d ition s in o r d e r to p re v e n t s e lf- r a d io ly s is , m a y even tu a lly a llo w the u se o f m o n o tritia te d p r e c u r s o r s , thus tre m e n d o u s ly e n la rg in g the scop e o f the tech n iqu e, w h ose m o st v e x in g e x p e rim e n ta l d iffic u lty , the u se o f m u ltip ly -la b e lle d p r e c u r s o r s , w ould be elim in a te d . D ouble la b e llin g by d iffe r e n t n u clid es m ay a lso be c o n s id e re d .
238
CA C AC E
O th er fo r e s e e a b le lin e s o f d evelop m en t con cern , ap art fr o m the use o f WC d ecay, fu rth e r ap p lica tion s o f n u cleogen ic halonium ions: in this connec tion , h o w e v e r, s p e c tr o s c o p ic stu dies on the c o llis io n a l quenching o f e le c t r o n ic a lly e x c ite d daughter ion s would be h igh ly d e s ira b le . F in a lly , the C ou lom b e x p lo s io n o f h alogen ated m o le c u le s fo llo w in g the p h o to e le c tr ic a b so rp tio n o f m o n o ch ro m a tic X - r a y s a ffo rd s another u sefu l ro u te to gaseou s halonium ion s, as su ggested by the p ro m is in g re s u lts of c u rre n t in v e s tig a tio n s [55].
REFERENCES [1 ]
(a )
M ADDOCK, A .G .,
W O L F G A N G , R ., in N u c le a r C h e m is try (Y A F F E , L . , E d .), A c a d e m ic Press,
N e w Y o r k (1 9 6 8 ).
[2 ]
(b )
S T O C K L IN ,
(c )
M ATSUURA, T .,
G .,
(d )
M cK A Y , H .A .C .,
(e )
W O LFG ANG ,
(f)
NEFEDOV. V . D . ,
(g )
W O L F , A . P . , A d v . P hys. O rg. C h em ■ 2_ (1 9 6 4 ) 201.
S ee fo r in stan ce :
C h e m is try o f H o t A to m , V e r la g C h e m ie , W e in h e im (1 9 6 9 ). K ogak u (K y o t o )
25 (1 9 7 0 ) 24.
P rin c ip le s o f R a d io c h e m is try , Butterw orths, London (1 9 7 1 ) 4 4 6 .
R . , P ro g . R e a c t. K in e t. 3 (1 9 6 5 ) 97. Z A IT S E N , V . M . , T O R O P O V A , M . A . ,
RADOM , L .,
POPLE, J . A . ,
R u s s .C h e m .R e v .
SC H LEYER, P . v o n R . ,
32 (1 9 6 3 ) 604.
J. A m e r . C h e m . S o c .
94,
(1 9 7 2 ) 5935, and r e fe r e n c e s th e re in . [3 ]
For a r e v ie w t o 1 9 7 0 :
[4 ]
For an e x te n s iv e r e v i e w :
c f. c f.
( H A ÏS S IN S K Y ,
8, Masson Paris (1 9 6 5 ) 107.
М . , E d .)
C A C A C E , F . , A d v .P h y s .O r g .C h e m . 8 (1 9 7 0 ) 7 9 . WEXLER, S . ,
J .C h e m T p h y s .
in A c tio n s C h im iq u e s e t B io lo g iq u e s des R adiation s
[5 ]
CARLSO N, T . A . ,
[6 ]
M IG D A L , A . , J .P h y s . (U SSR ) 4 (1 9 4 1 ) 4 4 9 .
32 (1 9 6 0 ) 1234.
[7 ]
W OLFSBERG, М . , J .C h e m .P h y s . 24 (1 9 5 6 ) 24.
[8 ]
WEXLER, S . . HESS, D . C . ,
J .P h y s .C h e m . 62 (1 9 5 8 ) 1382.
[9 ]
POBO, L . G . ,
C A R O N N A , S .,
W EXLER, S .,
R a d io c h im .A c ta .
[1 0 ]
W O L N ΠW IC Z , L . , J .C h e m .P h y s . 43 (1 9 6 5 ) 1087.
[1 1 ]
For a re p r e s e n ta tiv e e x a m p le , c f . C IR A N N I,
[1 2 ]
C A C A C E , F .,
C IR A N N I,
A L IP R A N D I, B ., C A C A C E , F . ,
[1 4 ]
C A C A C E , F .,
BABERNICS, L . , NEFEDOV, V . D . ,
J .L a b e lle d C o m p d s. 2 (1 9 6 6 ) 198.
G U A R IN O , A . , J .C h e m .S o c . (B ) (1 9 6 7 )
C A R O S E LLI, М . ,
[1 6 ]
G U A R IN O , A . ,
5.
G . , G U A R IN O , A . , J. A m . C h e m . S o c . 88 (1 9 6 6 ) 2903.
[1 3 ]
[1 5 ]
G .,
19(1 9 7 3 )
G U A R IN O , A . ,
519.
J. A m . C h e m . S o c . 89 (1 9 6 7 ) 4 5 8 4 .
C A C A C E , F . , J. C h e m . S o c . (B) (1 9 7 1 ) 2313. et a l.,
Z h . O r g . K h im . (1 9 7 0 ) 1214.
[1 7 ]
N E F E D O V , V . D . , S IN O T O V A ,
E .N ., A K U LO V ,
G . P . , R a d io k h im iy a 10 (1 9 6 8 ) 609.
[1 8 ]
N E F E D O V , V . D . , S IN O T O V A ,
E .N ., AK U LO V,
G . P . , S Y R E IS C H C H IC O V , V . A . ,
R a d io k h im iy a 10
(1 9 6 8 ) 60 0 . [1 9 ]
N E FE D O V , V . D . , S IN O T O V A ,
E .N ., A K U LO V ,
G . P . , SASS, V . P . ,
[2 0 ]
N E FE D O V , V . D . , S IN O T O V A ,
E .N ., A K U LO V ,
G . P . , M e to d I z t o p . In d ik a to ro v N a u c h .- Is s le d .P r o m .
R a d io k h im iy a
10 (1 9 6 8 ) 7 61.
P r o iz v o d . (1 9 7 1 ) 346. [2 1 ]
NEFEDOV, V . D . ,
S IN O T O V A , E . N . , A K U L O V , G . P . ,
KORSAKOV,
M .V ..
Z h . O r g . K h im . 9 (1 9 7 3 ) 6 2 9 ;
see a lso R a d io k h im iy a 1^5 (1 9 7 3 ) 286. [2 2 ]
CACACE,
[2 3 ]
C A C A C E , F . , C A R O S E L L I.
[2 4 ]
C A C A C E , F . , G U A R IN O , A . ,
[2 5 ]
F . , C IP O L L IN I, R .. C IR A N N I, М .,
G . , J . A m . C h e m . S o c . 90 (1 9 6 8 ) 1122.
C IP O L L IN I, R . , C IR A N N I,
G ., J. A m . C h e m . S o c . 90 (1 9 6 8 ) 2222.
P O S S A G N O , E ., J . A m . C h e m . S o c . 91 (1 9 6 9 ) 3131.
R e fe r e n c e n o t g iv e n .
[2 6 ]
CACACE,
F . , G U A R IN O , A . ,
SPERANZA,
М . , J. A m . C h e m . S o c . 93 (1 9 7 1 ) 1088.
[2 7 ]
CACACE,
F . , G U A R IN O , A . ,
SPERANZA,
М . , J . C h e m . S o c . , P erk in T ra n s . 2 (1 9 7 3 )
[2 8 ]
CACACE,
F . , C A R O N N A , S ., J . A m . C h e m . S o c . 89 (1967 ) 6 848.
[2 9 ]
C A C A C E , F . . C IP O L L IN I, R . , C IR A N N I, G . , J . C h e m . S o c . (B ) (1 9 7 1 ) 2089.
[3 0 ]
CACACE,
F . , PEREZ, G . , J .C h e m .S o c .(B ) (1 9 7 1 ) 2085.
[3 1 ]
SKOROBOGATOV,
[3 2 ]
R A A D S C H E L D E R S -B U U Z E , C . ,
G . A . , T e o r . i E h k s p .K h im . 2 (1 9 6 6 ) 20.
[3 3 ]
SNELL, A . ,
ROOS, C . L . ,
ROS, P . ,
C h e m . Phys. 1_ (1 9 7 3 ) 4 6 8 .
P L E A S O N T O N , F . , J .P h y s .C h e m . 62 (1 9 5 8 ) 1377.
66.
239
IA E A -P L-6 1 5/14
[3 4 ]
W O L F G A N G . R ., A N D E R S O N . R . C . , D O D S O N . R . W . , J .C h e m .P h y s . 24 (1 9 5 6 ) 1 6.
[3 5 ]
SKOROBOGATOV,
G . A . , N E FE D O V , V . D . ,
[3 6 ]
M A N N IN G . P . G . ,
M O N K . C . B . , J .C h e m .S o c . (1 9 6 2 ) 2573.
[3 7 ]
NEFEDOV. V . D . ,
e ta l.,
[3 8 ]
R A A D S C H E L D E R S -B U U Z E , C . ,
Z h .O b s h c h e i K h im . 36 (1 9 6 6 ) 995.
Z h . O b sh ch ei K h im . 33 (1 9 6 3 ) 339. T h esis, V r i j e U n iv e r s íte it te A m s te rd a m , B ro n d e r-O ffs e t В . V . ,
A m s te rd a m (1 9 7 4 ). [3 9 ]
H O R N IN G , J . F . , L E V E Y , G . ,
[4 0 ]
RAC K, E .P .,
W IL L A R D , J . E ., J. C h e m . P hys. 25 (1 9 5 2 ) 904.
[4 1 ]
N IC H O L A S , J .B ., R A C K , E . P . . J .C h e m .P h y s . 48 (1 9 6 8 ) 4 0 8 5 .
G O RDUS, A . A . ,
J. C h e m . Phys. 34 (1 9 6 1 ) 1855.
[4 2 ]
T A C H IK A W A ,
[4 3 ]
P E T T U O H N , R .R .,
E ., B u ll. C h e m . S o c .J a p a n . 43 (1 9 7 0 ) 1293, and Refs th e re in .
[4 4 ]
LOBERG,
[4 5 ]
S C H R O T H , F . , A D L O F F , J . P . , J .C h e m .P h y s . 61 (1 9 6 4 ) 1373.
M .D .,
RACK,
E .P .,
J .P h y s .C h e m . 76 (1 9 7 2 ) 3342.
W E LC H , M . J . , J . A m . C h e m . S o c . 95 (1 9 7 3 ) 1075.
[4 6 ]
C A C A C E , F . , S T O C K L IN .
[4 7 ]
L IS T E N G A R T E N ,
[4 8 ]
WEXLER, S . , A N D E R S O N ,
[4 9 ]
GORDUS, A . A . ,
[5 0 ]
K N U S T , E .J .. H A LPE R N , A . ,
S T O C K L IN , G . ,
[5 1 ]
D A N IE L ,
S T O C K L IN ,
[5 2 ]
C A C A C E , F .,
S .H .,
M .A .,
G . , J. A m . C h e m . S o c . 94 (1 9 7 2 ) 2518.
I z v .A k a d .N a u k .S S S R 24 (1 9 6 0 ) 1050. G . R . , J .C h e m .P h y s . 33 (1 9 6 0 ) 850.
W IL L A R D , J .E ., J . A m . C h e m . S o c . 79 (1957 ) 4 6 0 9 .
ACH E, H .J .,
J. A m . C h e m . S o c . (in p ress).
G . , J .P h y s .C h e m .
(in press).
in P ro c . C o n f. M eth od s o f P rep arin g and S torin g M a rk ed M o le c u le s , E uratom , B ru x elles
(1 9 6 4 ) 7 19. [5 3 ]
C f. :
[5 4 ]
C A C A C E , F .,
A n n .R e p s P r o g r .C h e m . ( C h e m . S o c . ) 69 (B ) (1 9 7 2 ) 108.
[5 5 ]
S T O C K L IN ,
C IP O LL1 N I, R ., C IR A N N I,
G .,
R a d io c h e m . R a d io a n a l. L e t t . 4 (1 9 7 0 ) 5 1.
G . , e t a l . (p r iv a t e c o m m u n ic a tio n ).
1АЕА-РЬ615/15
BETA DECAY AND ISOMERIC TRANSITION T. S H IO K A W A D epartm ent o f Chem istry, T oh ok u U niversity, Sendai Japan
Abstract B E T A D E C A Y A N D IS O M E R IC T R A N S I T I O N . H o t a to m s p ro d u c e d b y b eta d e c a y and is o m e ric tra n s itio n can be e ffe c t iv e ly u tiliz e d t o stu d y th eir ch em ical e ffe c ts . E x p e rim e n ta l proced u res, in c lu d in g m eth o d s o f analysis, e m p lo y in g h o t a to m s p ro d u c e d in these w ays, are p resen ted and w o rk th at has been carried o u t in th e gas phase, in th e liq u id phase, and in th e s o lid phase, is discussed in so m e detail.
1.
IN T R O D U C T IO N
T h e study o f hot atom c h e m is tr y using the c h e m ic a l e ffe c ts a s s o c ia te d w ith b eta d ecay, e le c tr o n captu re and is o m e r ic tra n s itio n has m any advan ta g e s , one im p o rta n t one b ein g that it is e a s y to a rra n g e the e x p e rim e n ta l con dition s to avoid m any o th er fa c to r s w hich fre q u e n tly in flu en ce the produ ct y ie ld in sy s te m s in v o lv in g the fo rm a tio n o f p rodu cts by n u c le a r re a c tio n s . I f n e c e s s a r y , e x tr e m e lo w te m p e ra tu re s and c o llis io n - fr e e dilu te gaseou s s y stem s can be used in the e x p e rim e n ts . H o w e v e r, the hot atom s fo rm e d by th ese d eca y p r o c e s s e s alw ays have such a lo w r e c o il e n e rg y that th e ir re a c tio n s often in v o lv e c o m p lic a tin g fe a tu re s through in te ra c tio n w ith surrounding m o le c u le s . In is o m e r ic tra n s itio n d eca y s, e s p e c ia lly , the r e s id u a l atom s have m u ltip le c h a rg e s w hich le a d to ra p id ch a rge tr a n s fe r o r io n -m o le c u le re a c tio n s w ith n eigh bou rin g m o le c u le s . M any a d d ition al p a p ers have been published in this fie ld sin ce the e x c e lle n t r e v ie w and fin e e x p e rim e n ts o f W e x le r in 1965 [ 1 ], and the ca lcu la tio n s o f H siung and G ordu s, a ls o in 1965 [ 2 ] . M o r e re c e n t r e v ie w s have been g iv e n by M addock and W o lfg a n g in 1968 [ 3 ] , and by N ew ton [4 ] in 1972, d e s c rib in g the addition al d evelo p m en ts. T h e m ain c h a r a c te r is tic s o f beta d ecay fo r c h e m ic a l p u rp o ses a re (a) the change in n u clea r c h a rg e , i . e . change in c h e m ic a l elem en t; (b) the "s h a k in g " o f the e le c tr o n s h e ll by th is abrupt change in n u c le a r ch a rge; (c) e le c tr o n ic , v ib r a tio n a l and ro ta tio n a l excita tio n ; and (d) the m om entum o f the r e c o il p r o c e s s . E le c tr o n -c a p tu r e and is o m e r ic tra n s itio n a re accom p an ied by m u ltip le ch a rgin g fo llo w in g va ca n cy p rodu ction , in te rn a l c o n v e rs io n , and the A u g e r con sequ en ces plus som e e ffe c ts o f r e c o il m om en tu m . Thus, the c h a r a c te r is tic c h e m ic a l beh aviou r is that o f ch a rged atom s w ith m o r e than th e rm a l m om entum in te ra c tin g w ith the surrounding m o le c u le s . M any e x p e rim e n ts have b een p e r fo r m e d to elu cid a te this c h e m ic a l b eh a vio u r in both g a seou s and condensed p h ases. F r o m quite another point o f v ie w , th ese d e c a y p r o c e s s e s a re a lso a v a ila b le as an e x c e lle n t to o l fo r the syn th esis o f new c h e m ic a l com pounds,
241
242
S H IO K AW A
p a r tic u la r ly th ose o f the n oble g a s e s . T h is a sp ect o f the d eca y p r o c e s s e s is o f p a r tic u la r v a lu e , and m uch r e s e a r c h has been c a r r ie d out along these lin e s . (See A d lo ff's p a p er Ń&#x2026;. )
2.
E X P E R IM E N T A L P R O C E D U R E S
In g e n e r a l, a fte r su itab le p e rio d s o f s to ra g e under c o n tro lle d con dition s p e rm ittin g d eca y and tra n s fo rm a tio n , the d istrib u tio n o f the produ cts should b e d e te rm in e d using s e v e r a l kinds o f a n a ly tic a l tech n iqu es. M any r e s e a r c h w o r k e r s have r e c e n tly attem pted to d e te rm in e the in te rm e d ia te p r o c e s s e s and p rod u cts through the addition o f s c a v e n g e r m o le c u le s and oth er r e a c tiv e com pounds, deducing the m ech a n ism o f the p r o c e s s fr o m the o b s e rv e d v a r ia tio n s in th ese a n a ly tic a l r e s u lts . T h e s e c h e m ic a l a n a ly tic a l p ro c e d u re s inclu de m any kinds o f c h ro m a to g ra p h ic tech n iqu es, so lv e n t e x tra c tio n , ion exch an ge, e le c tr o p h o r e s is e t c . , c a r r ie d out both w ith and without added c a r r i e r du ring the sep a ra tio n . H o w e v e r, m o r e in fo rm a tio n is needed about the in itia l p r im a r y state in o r d e r to understand the s ig n ific a n c e o f the c h e m ic a l p r o c e s s e s a s s o cia ted w ith th ese n u c le a r d eca y r e a c tio n s . F o r this pu rp ose, s e v e r a l oth er tech n iqu es have been applied, including c h a rg e s p e c tro m e try , n u clea r re so n a n ce flu o re s c e n c e [5 , 6 ], M ô s s b a u e r s p e c tro s c o p y , p ertu rb ed angular c o r r e la tio n s , and e le c tr o n spin reso n a n ce [ 7 ]. In addition to th ese p h y s ic a l tech n iqu es, in fo rm a tio n about the shake o f f [ 8 , 9] and io n iza tio n [ 10-12] o f s h e ll e le c tro n s during beta decay, as w e ll as the accu m u lated in fo rm a tio n on the p h y sics o f th ese d eca y p r o  c e s s e s , should a s s is t g r e a t ly in elu cid a tin g the c h e m ic a l fe a tu re s a s s o c ia te d w ith th em . T h e r e c e n t re m a rk a b le advances in the d evelo p m en t o f p h y s ic a l d e te c tio n s y s te m s w ill a llo w the co n stru ctio n o f s p e c ia l d e v ic e s fu rn ish in g e ven m o re d e ta ile d in fo rm a tio n about these p r o c e s s e s .
3.
GASEOUS S Y S T E M S
T w o ty p ic a l m ethods have been used fo r a n a ly sis in gaseou s s y s te m s . T h e f i r s t is m a s s s p e c tro s c o p y , and the second is produ ct a n a ly s is , em p loyin g gas ch ro m a to g ra p h y o r oth er c h e m ic a l p ro c e d u re s capable o f sep a ra tin g the fin a l p rod u cts. 3.1. M ass s p e c tro s c o p y (ch a rg e s p e c tro s c o p y ) The e a r lie r , e x c e lle n t r e s e a r c h w o rk using m a s s s p e c tr o m e tr y had p ro v id e d a qu an titative c o n firm a tio n by W e x le r in 1960 [1 3 ] o f th e o r e tic a l p r e d ic tio n s con cern in g the e ffe c ts o f tritiu m d eca y in H T and T 2. T h e fra g m e n t r a tio ( 3 H e +/H +) fr o m H T in d icated that the fra g m e n ts w e r e p rodu ced by the d is s o c ia tio n o f an e x c ite d state o f 3 H eH +. S im ila r e x p e rim e n ts by Shiokaw a et a l. [ 14] g a v e v e r y s im ila r re s u lts to th ose o f the e a r lie r w o r k e r s , both w ith m o le c u la r h yd rogen containing tr itiu m (T a b le I) and w ith the ex p lo s io n m o d e l o f C a rls o n [1 5 ], as illu s tra te d in T a b le II f o r m eth y l io d id e.
1 IA E A - P L - 6 1 5 / 5 , th ese P ro c e e d in g s .
243
IAEA-PL-61 5/15
T A B L E I.
D IS S O C IA T IO N O F T 2 A N D H T B Y 0 - D E C A Y
T2
Probability of occurrence (% )
Ion
Molecule
Shiokawa
Wexler
(T3He)+
89.H O . 8
94.5+0.6
T++3He+
10.9+0.8
5.5±0.6 89.5±1.1
(3HeH)+ HT
Snell
93.2±1.9
(3He)+
8.2±1.0
5.1±0.3
H+
2.3±0.4
1.55Ю.1 0.14±0.01
(3He)+
T A B L E II. F R A G M E N T A T IO N F O L L O W IN G T H E S ' D E C A Y O F C H * 31I
Abundance (%) Ion Shiokawa et a l . CH3Xe+
63.0
CH2Xe+
2.0
CHXe
4.6
CXe+
4.6
Xe+
13.8
Xe++
Carlson et a l . 69 4
14.6 ± 0.6 2.7 ± 0.3
CH„+
2.8
2.0 ± 0.2
c h 2+
3.2
2.4 ± 0.3
CH+
4.2
2.2 + 0.4
C+
1.8
1.9 ±0.3
H+
—
1.4 ±0.2
U n le s s th e re is su bstantial r e c o i l e n e rg y , 60-80% o f the m o le c u la r ion s s u rv iv e the b e ta -d e c a y p r o c e s s and the h y d ro g e n -d e fic ie n t ions (CH2X e , C H X e , C X e ) a re a ls o found to be p re s e n t. F o r the is o m e r ic tra n s itio n p r o c e s s , C a r ls o n has p re s e n te d the "e x p lo s io n " m o d e l in w h ich the p a ren t m o le c u le a fte r is o m e r ic tra n s itio n is p ostu lated to d e co m p o se c o m p le te ly into fra g m e n t s p e c ie s through the co u lom b ic re p u ls io n o f m any ch a rged atom s fo rm e d by the m u ltip le ch a rgin g o f the A u g e r c a s c a d e . H o w e v e r, Shiokaw a et a l. [ 16] have found that the fra g m e n t d istrib u tio n a fte r is o m e r ic tra n sitio n in CH 3 80B r m p re s e n ts a r a th e r d iffe r e n t p a ttern fr o m the "e x p lo s io n " m o d e l, and shows the e x is te n c e
244
SH IO K A W A
o f a f a ir amount o f p a ren t and h y d ro g e n -d e fic ie n t ion s w hich a re not c o m p le te ly fra g m e n te d into atom s by the ex p lo sio n . On the oth er hand, the fra g m e n t d is trib u tio n o f m eth y l iod id e and e th y l b ro m id e [1 3 ] a fte r the usual e le c tro n im p a ct o r c h a rg e exch an ge e x c ita tio n p r o c e s s e s show no e x is te n c e o f such h y d ro g e n -d e fic ie n t ion s. T h e e le c tr o n im p a ct p r o c e s s is usu ally co n s id e re d to g iv e re s u lts s im ila r to the phenom ena o b s e rv e d w ith c h a rg e exchange [ 1 7 ]. In th is c a s e , then, the fo rm a tio n o f h y d ro g e n -d e fic ie n t s p e c ie s in the decay p r o c e s s e s d e m o n s tra te s som e n ew re a c tio n paths c h a r a c te r is tic on ly o f the c h e m ic a l con sequ en ces o f the A u g e r p r o c e s s . M u ltip ly -c h a rg e d atom s le f t fo llo w in g the A u g e r p r o c e s s e s se e m to le a d to C -H bond ru ptu re through io n iza tio n o f the H atom in the m o le c u le . T h e s e o b s e rv a tio n s o f h yd ro gen d e fic ie n t s p e c ie s in such d eca y p r o c e s s e s should be pursued in m o re qu an titative d e ta il in fu rth e r stu dies. 3.2.
C h e m ic a l s e p a ra tio n e x p e rim e n ts
In th ese s y s te m s , the m o th er n u clid es should have a su itable h a lf- life p e rm ittin g the p re p a ra tio n o f the la b e lle d com pounds in w hich d eca y is to o c c u r. Such com pounds n o r m a lly have fa ir ly high va p o u r p r e s s u r e under usual la b o r a to r y con d ition s, and the dau gh ter n u clide should be e a s y to d e te c t. W h ile m any e x p e rim e n ts have been c a r r ie d out in such s y stem s, the n u clid es em p lo y ed so fa r tend to be ra th e r lim ite d : 3 H, 80 B r m, 82B r m, 123X e ( E . C . ), i 30 F>. A f t e r the d eca y even t o c c u rs in a v e s s e l containing the la b e lle d m oth er m o le c u le plus h yd ro ca rb o n o r oth er rea cta n t g a s e s , the fin a l p rodu cts a re g e n e r a lly an alysed by gas c h ro m a to g ra p h ic o r oth er tech n iqu es. A s e r ie s o f stu dies on the is o m e r ic tra n s itio n o f 80B r m and 82B r m has been p e r fo r m e d by Y a g i et a l. [ 1 8 -2 2 ]. F o u r kinds o f la b e lle d com pounds, 80B r m B r , H 80B r m, 82 B r m B r and H 82B r m, have been used, p ro v id in g a ran ge o f r e c o il e n e r g ie s estim a te d as, r e s p e c tiv e ly , 49.3, 0.85, 48.6, and 0.83 e V . A d is tin c t iso to p e e ffe c t w as o b s e rv e d b etw een the re a c tio n s o f 80 B r " 1 and 82B r m [2 0 ], and should be attribu ted to d iffe r e n c e s in the g a m m a -ra y tra n s itio n s o b s e rv e d w ith the r e s p e c tiv e n u clid es. T h e d eca y o f 82B r m to ground state 8ZB r o c c u rs w ith a o n e -s te p M3 tra n s itio n , w h ile 80B r m decays to ground state 80B r through a tw o -s te p p r o c e s s in v o lv in g s u c c e s s iv e M3 and E l tra n s itio n s through an in te rm e d ia te 4.7 X 10~9 second l e v e l o f 80B r . B oth M3 tra n s itio n s a re 100% in te rn a lly co n v e rte d , but the E l tra n s itio n fr o m the in te rm e d ia te le v e l in 80B r is on ly 61% c o n v e rte d . T h e ch e m ic a l e ffe c t o f the is o m e r ic tra n s itio n in 80B r m can then be an alysed as the sum o f 39% behaving s im ila r ly to the o n e -s te p tra n s itio n in ÂŽ B r m and 61% fo llo w in g the tw o -s te p tra n s itio n p r o c e s s . In addition, th e rm a l io n ic re a c tio n s o ccu r in both the 80B r m and 82B r m m ix tu re s w ith h yd ro c a rb o n s . W hen k rypton gas is added in e x c e s s as a m o d e r a to r , p rodu ct a n a ly sis in th ese sy s te m s in d ica tes that the produ cts a r e m a in ly fo r m e d by th e rm a l io n ic re a c tio n s o f B r * (F ig . 1), the fin a l c h a rg e d state le f t fo llo w in g c h a rg e exchange o f B r n+ w ith the surrounding m o le c u le s . C a c a c e and S tock lin [2 3 ] have r e c e n tly r e p o rte d a s im ila r re a c tio n m ech a n ism in the C H 3 80B r m- C gH 5X s y stem , and have d e s c rib e d the prod u cts in te r m s o f the re a c tio n s o f the b ro m in iu m ion, B r +. In th e ir e x p e rim e n ts , b ro m in iu m ion s r e a c t w ith the phenyl h a lid es to p rod u ce an e x c ite d c h a rg e co m p le x , C 6 H 6 X 80B r +, w ith the in d ivid u al o b s e rv e d p rodu cts
IAEA-PL-61 5/15
245
MOLE FRACTION OF Kr F IG . 1.
E ffe c t o f K r a d d it iv e in H 82Brm - C H 4.
fo r m e d by the fra g m en ta tio n o r s ta b iliz a tio n o f the c o m p le x . N ic h o la s and R a ck [2 4 ] and T a ch ik a w a and Y a n a i [2 4 ] have a ls o su ggested the p re s e n c e o f a p r o c e s s independent o f k in etic e n e rg y in the 82B r m-h y d ro c a rb o n s y s te m s em p lo y in g v a r io u s m o d e r a to r s . In addition, L o b e r g and W e lc h [2 5 ] have d e v e lo p e d a m o d e l in v o lv in g m o le c u la r ion c o m p le x e s o f A I + in w hich the s p e c ie s A can be CH^, X e , N e, A r , K r and N 2. T h e ir data w e r e obtained in 123X e ( E . C . ) 123I d e c a y p r o c e s s e s w ith s im p le h y d ro ca rb o n s. P r o g r e s s can be e x p ected in such r e s e a r c h w o r k w ith the in c re a s in g kn ow ledge o f io n -m o le c u le r e a c tio n s . A s e r ie s o f p a p e rs by C a c a c e and c o - w o r k e r s [2 6 -2 9 ] p r o v id e s va lu a b le in fo rm a tio n on a ro m a tic substitution re a c tio n s o f the H e T + ions w ith v a rio u s h y d ro c a rb o n s . H e itz et a l. [3 0 ] have pointed out that r a d io ly tic p r o c e s s e s can occu r in an "a u to - ir r a d ia tio n z o n e ", w ith o b s e rv e d y ie ld s s tro n g ly dependent on the m o le fr a c tio n o f iod in e in 130I m-butane m ix tu re s w ith A r , K r and X e . T h e y have o b s e rv e d y ie ld s o f 1.1% fo r hot r e a c tio n and 0.59% fo r io n -m o le c u le re a c tio n .
4.
L IQ U ID P H A S E
T h e e x c ite d atom s, r a d ic a ls o r m o le c u la r ions fo rm e d fo llo w in g |3-decay o r is o m e r ic tra n s itio n then r e a c t w ith the su rrou nding m o le c u le s in the liq u id p h ase. S u ccessfu l in te rp re ta tio n s o f the m ech a n ism s in v o lv e d is not p o s s ib le by a n a ly sis o f fin a l p rod u cts a lo n e. A d d itio n a l in fo rm a tio n can be gain ed through the use o f v a rio u s kinds o f s c a v e n g e rs o r rea c ta n ts fo r r a d ic a ls and ion s, as done in p h oto- o r ra d ia tio n c h e m is try , and such e x p e rim e n ts should be p e r fo r m e d to elu cid a te the c h e m ic a l con sequ en ces fo llo w in g th ese d e c a y s . T h e s e le c tio n o f s o lven t in the p a r tic u la r s y s te m can a lso be quite im p o rta n t. F r o m this point o f v ie w , the em p lo ym en t o f
246
S H IO K AW A
F e C l 3 , 6 H 2 0 in ethanol by T o m in a g a et a l. [3 1 ] as a s c a v e n g e r fo r the cob alt a c e ty la c e to n a te s y s te m , and b en zen e as a so lv e n t fo r C o (a c a c ) 3 [3 1 ] and C r (a c a c ) 3 [3 2 ] a re o f c o n s id e ra b le in te r e s t. T h e d e c a y r e a c tio n s in liq u id s y s te m s a re the a sp ect o f hot atom c h e m is try w h ich c o r r e la t e s m o s t c lo s e ly w ith g e n e tic o r m utagenic p ro b le m s in b io lo g ic a l m a te r ia ls [ 3 3 ] . M o r e p r e c is e e ffo r t s should be c a r r ie d out to f i l l up the gaps b etw een the standard hot atom r e s e a r c h and the b io lo g ic a l c o n c e rn s . L o w -te m p e r a tu r e e x p e rim e n ts by Shaw et a l. [3 4 ] in the 80B r mbrom oeth an e s y s te m have been in te rp re te d a cco rd in g to a m o d el which assu m ed that d is s o c ia tio n and n e u tra liza tio n o c c u r at co m p a ra b le ra te s . T h e nature o f the fin a l p rod u cts is then d e te rm in e d b y the fa s te r d is s o c ia tio n p r o c e s s e s and by the f r e e r a d ic a ls fo rm e d in th em . R e s e a r c h w o rk by Mohan and Iy e r [ 35-37] has shown the e ffe c ts o f the is o m e r ic tra n s itio n o f 80B r m fo r v a rio u s s o u rce m o le c u le s (N H 4 B r , M g B r 2, C u B r2 , Z n B r 2 , S r B r 2 o r H g B r 2) in a lip h a tic o r a ro m a tic h yd ro ca rb o n s. T h e resu lta n t p o s itiv e c h a rg e on 80B r at the tim e o f its b re a k fr o m its o r ig in a l p a rtn e r, and not the k in etic e n e rg y o f 30B r , d e te rm in e s the fin a l c h e m ic a l s ta b iliz a tio n o f the ra d io a c tiv e atom . Thu s, the data a v a ila b le in the lite r a tu r e illu s tr a te that even in condensed phases the b rea k in g o f bonds can be exp ected to c o rre s p o n d som ew hat to the d e ta ils o f the d eca y p ro cess. E a r l i e r w o r k [3 8 ] show ed no evid e n c e o f bond ru ptu re in 144C e a c e ty la c e to n a te by /3-d e c a y , e ith e r in the s o lid state o r in liq u id solu tion s. R e c e n tly , h o w e v e r, this s y s te m has been r e - in v e s tig a te d s e p a ra te ly by G len tw orth et al. [3 9 -4 1 ] and O m o r i e t a l. [4 2 - 4 5 ], and d e fin ite e vid en ce o f bond ru p tu re has now been found. T h e c h e m ic a l b eh a vio u r o f 144P r , c r e a te d by the 0 -d e c a y o f 144C e in c o m p le x e s w ith p o ly a m in o -p o ly c a rb o x y lic a c id ch ela tin g agen ts, in d ic a te s that the s p e c ie s 144 P r 3+ o r its p r e c u r s o r is prod u ced as a r e s u lt o f in te rn a l c o n v e rs io n o r e le c tr o n s h a k e -o ff in con n ec tion w ith the th erm o d yn a m ic in s ta b ility o f the daughter m o le c u la r ion s. A d e ta ile d r e a c tio n sch em e [4 3 ] is shown in F ig . 2, in w hich in tra m o le c u la r e le c tr o n tr a n s fe r p la y s an im p o rta n t r o le in the s ta b iliz a tio n o f 144 P r . T h e ra tio o f bond ru ptu re o f liga n d (a = 0.25) in DC T A c o m p le x c o rre s p o n d s w e ll to the va lu e e x p ected th e o r e tic a lly fr o m the co u lom b ic re p u ls iv e fo r c e o f the m u ltic h a rg e d ions prod u ced by in te rn a l c o n v e rs io n [ 4 4 ] . In the ca se o f 143C e D T P A co m p le x , "th e b re a k -u p " p e rc e n ta g e o f 89± 10% c o rre s p o n d s to its in te rn a l c o n v e rs io n c o e ffic ie n t [4 1 ]. In the c a s e o f is o m e r ic tra n s itio n s o f 129 X e m, 131 X e m and 133 X em , high re te n tio n s w e r e o b s e rv e d , p ro b a b ly becau se o f the la r g e change o f spin [ 4 6 ] . T h e bond ru ptu re by is o m e r ic tra n s itio n o f 133X e m in aqueous solu tion s o f X e 0 3 is 0.85% ± 0.5, w h ich c o rre s p o n d s v e r y c lo s e ly to the in te rn a l c o n v e rs io n c o e ffic ie n t. S e v e r a l in v e s tig a tio n s o f te llu riu m s y s te m s have been c a r r ie d out by L e b e d e v et a l. [4 7 - 4 9 ], U llr ic h and V in cen t [5 0 ] and Jones and W a r r e n [5 1 ], w ith the fr o z e n aqueous solu tion s m o s tly b ein g o b s e rv e d b y M o ssb a u er s p e c tro s c o p y . I27ipem and 129T e m have a ls o been in v e s tig a te d [5 2 , 53] in aqueous solu tion . In th ese e x p e rim e n ts it w as shown that a fr a c tio n o f the dau gh ter atom s o f 127T e fr o m 127 T e m is s ta b iliz e d in the fo r m o f T e (V I ), w h ich p e r m its an e ffe c t iv e s e p a ra tio n o f the n u c le a r is o m e r s o f Т е [ 53 ]. It a ls o d e m o n s tra te s that the m o le c u le s in aqueous solu tion s a re not c o m p le te ly d e s tro y e d by the A u g e r e ffe c t [5 2 ] .
247
IA E A -P L-61 5/IS
I44_ 34Ce
F IG .2.
5.
CeY"'
Ce
W "
S t a b iliz a tio n process o f 144Pr fo r m e d by th e d e c a y o f 144C e Y n " .
S O LID P H A S E
S p e c ia l c a r e is r e q u ire d in the in te rp re ta tio n o f the data obtained fr o m d eca y p r o c e s s e s in the so lid p h ase. F ir s t , a u to ra d io ly s is should be taken into account, e s p e c ia lly in E . Х . d eca y o r is o m e r ic tra n s itio n . Second, the dau gh ter a tom s o f 0 -d e c a y in the p a ren t s o lid should be c o n s id e re d as a v e r y dilu te im p u rity in a n o n -is o to p ic m a tr ix . T h e a u to -ra d ia tio n e ffe c t has been c o n s id e re d to be an im p o rta n t m ech a n ism in the s ta b iliz a tio n o f a high c h a rg e state, as pointed out by F r ie d t e t a l. [5 4 , 55] and L la b a d o v [5 6 ], and o fte n cau ses p o ly m e r iz a tio n du rin g th e rm a liz a tio n fo r the h ig h e r r e c o il e n e r g ie s [ 5 7 ]. Jones and W a r r e n [ 51] show ed that th e re is a d is tin c t d i f f e r  ence in the annealing b eh a vio u r b etw een 132T e and 131T e by fo llo w in g the in te rn a l c o n v e rs io n in 132T e . T h e daughter atom s o f 0 -d e c a y a re found to be e x tr e m e ly s e n s itiv e to e ven a s m a ll content o f la ttic e d e fe c ts fo rm e d d u ring the p re p a ra tio n and p u r ific a tio n o f the p a re n t c r y s ta llin e com pounds. C on sequ en tly, the e x p e r im e n te r is often fa c e d w ith a tro u b le s o m e d is p e rs io n o f data d esp ite h is e ffo r t s to m ain tain unchanged e x p e rim e n ta l con d ition s. T h is " s e n s it iv it y to s tr u c tu r e ", as w e ll as p o s s ib le c h e m ic a l changes du ring the d isso lu tio n o f s o lid p r io r to the a n a ly s is , m u st be c a r e fu lly c o n s id e re d f o r each s y s te m . U n d er th ese c irc u m s ta n c e s , m an y new n o n -d e s tru c tiv e d etectio n tech n iqu es have been ap p lied in o r d e r to in v e s tig a te p o s s ib le e a r l i e r sta tes o f daughter atom s a fte r 3 -d e c a y . M ô s s b a u e r s p e c tro s c o p y and p ertu rb ed an gu lar c o r r e la tio n stu dies m a y be the m o s t p ro m is in g o f such tech n iqu es. 5.1. H ig h e r oxid a tio n sta tes a fte r b eta d e c a y E s s e n tia lly , beta d eca y should be an o x id iz in g p r o c e s s , but in fa c t th e re a re v e r y fe w e x a m p le s o f s im p le o xid a tion fo llo w in g 0 -d e c a y . S in g ly ch a rged ion s, e x c ite d atom s o r e x c ite d m o le c u la r ions fo rm e d a fte r the
248
S H IO K AW A
T A B L E III. E F F E C T O F H E A T IN G T E M P E R A T U R E ON TH E Y IE L D O F 5 7 C o (III)-S P E C IE S
Compound
Initial Yield
(%)
< r C <*>
< l c W
[Ni(NH3)6]S208
41.8
32.8
36.5
[Ni (NH3 )6](IM03 )2
34.8
15.2
11.3
[Ni(NH3 )6]Cr04
18.1
10.4
26.0
[Ni(NH3)6](C104 )2
36.6
0.5
0Đ&#x203A;
[Ni(NH3 )6]I2
29.6
1.6
-3.6
[Ni(NH3 )6]Br2
27.3
0.4
2.0
[Ni(NH3)6]C12
27.0
0.2
11.2
AYT ; YT (Co(III)) - I.Y. (Co{III )) in itia l d eca y m ay in te ra c t e a r ily w ith surrounding m o le c u le s in the condensed ph ase, and be s ta b iliz e d into the o b s e rv e d sta b le oxid ation sta te. A n a ly s is o f the fin a l produ ct d istrib u tio n shows on ly the a ft e r - e ffe c t s in th is system , as c o n firm e d [5 8 ] by M o s s b a u e r s p e c tro s c o p y . A s an exa m p le o f th is type o f ex p e rim e n t, O m o ri et a l. [59., 60] have in v e s tig a te d the j3+ a n d E .C .d e c a y o f|57N i in the la b e lle d hexaam m ine c o m p le x e s . A s shown in T a b le III, su bstan tial amounts o f 57 C o (III) a re found, with y ie ld s m uch in flu en ced by the o x id iz in g a b ility o f the o u te r-s p h e re anions. In th is c a s e the ra te o f annealing a lso depends on the o u te r-s p h e re anion. T h e b eta d eca y o f 77Ge to 77A s is a lso o f in te r e s t fr o m this point o f v ie w . A f t e r the f ir s t w o rk by B a ro and A ten in 1960 [6 1 ], the s y s te m has been studied fu rth e r by s e v e r a l in v e s tig a to rs [6 2 - 6 5 ]. T h e v a le n c e d i s t r i bution o f 77A s (III and V ) is found to depend c r it ic a lly on the co n cen tra tion o f the a rsen io u s c a r r i e r in the s o lven t p r io r to d isso lu tio n o f the germ a n iu m com pound. The in itia l y ie ld o f A s (V ) in 77 G e 0 2 sto red in liq u id n itro g e n w as m ea su red to be 60% by G en et and F e r r a d in i [6 3 -6 5 ] and 24% by H alp ern and S a w le w ic z [ 6 2 ] . T h e th e rm a l annealing c o n v e rs io n o f 77A s (I I I ) to 77A s (V ) is v e r y s im ila r in both e x p e rim e n ts . In o th e r e x p e rim e n ts , the /3-d e c a y s o f 125Sn to 125Sb and 121 Snm to 121Sb have been in v e s tig a te d s e v e r a l tim e s [6 6 - 6 9 ], a lw a ys show ing a s ig n ific a n t fr a c tio n o f ra d io a n tim o n y in the pen tavalen t state fo r p ota ssiu m and am m onium h e x a c h lo ro s ta n a te . T h e y ie ld o f Sb5+ in potassiu m h exach lorostan ate depends upon the p re p a ra tio n and p u rific a tio n p ro c e d u re s , w ith a y ie ld o f 1 0 % in crude c r y s ta ls and above 90% in a h igh ly p u rifie d one [ 6 7 ] . T h e )3-decay o f 99M o to 99 T e m has a ls o been studied by s e v e r a l in v e s tig a to rs [ 70, 7 1 ]. T h e h igh est o x id a tion state (V II) o f T c is r e g u la r ly found, independent o f the con dition s o f d eca y [7 0 ], w h ile T c (V I ) and T c (V ) a re a lso p o s s ib ly p re s e n t in a lk a lin e solu tion [ 7 1 ].
249
IA E A -P L-61 5/15
T h e e x c ite d atom s r e le a s e d fr o m m o th er m o le c u le s o r e x c ite d m o le c u la r ion s a re a lso s ta b iliz e d into th e ir fin a l state by c h e m ic a l re a c tio n s w ith the su rrou n d in gs. N e fe d o v e t a l. [7 2 ] have an alysed t r ia r y l d e r iv a tiv e s o f bism uth in the study o f the c h e m ic a l con sequ en ces o f (3 -d e c a y o f R a E , w h ich in v o lv e s c o m p a r a tiv e ly s m a ll changes o f e le c tr o n d en sity . T h e y have shown that the d iffe r e n c e in the y ie ld s o f the s ta b iliz a tio n prod u cts o f the p r im a r y m o le c u la r ion s can be exp la in ed by a change in the e n e rg y o f th e B i- C bond in the c o rre s p o n d in g d e r iv a t iv e s . 5.2.
Is o m e r ic tra n s itio n
In th is c a s e , unlike e le c tr o n cap tu re and b eta d eca y, the dau gh ter atom s a re p rod u ced in a m a tr ix co m p o sed o f the sam e ele m e n t. M u lle r has c a r r ie d out a s e r ie s o f e x p e rim e n ts on m ix ed c r y s ta ls o f K 2 R e B r 6 - K 2 S n C l 6 [ 7 3 -7 5 ]. H is e x p e rim e n ts have shown that a ll the r e c o il atom s do not m o v e fa r fr o m th e ir in itia l s ite s , fo r d eca y in both the c e n tra l atom rheniu m , and in the b ro m in e liga n d . T h is fa c t c o n fir m s the d is o r d e r m o d e l p ostu lated e a r lie r , but d oes not ap ply to r e a c tio n s such as (7 ,n ), (n ,a) and (n, 2 n), f o r w h ich the r e c o il e n e r g ie s a re c o n s id e ra b ly h ig h e r. H o w e v e r, bond ru ptu re by co u lom b ic re p u ls io n a lso tak es p la c e in condensed p h a ses. Jones [ 76] has found a p r o b a b ility o f bond ru ptu re up to 65% f o r 80B r in N a 80B r m 0 3 . A r n ik a r and R ao [ 77] have found that the re te n tio n s at -8043 a re 25, 21 and 21%, r e s p e c t iv e ly , fo r K, C a and Zn b ró m a te s. A s e r ie s o f in v e s tig a tio n s b y L a z z a r in i [ 78, 79] has shown an is o m e r ic e ffe c t in 60Com (III) c o m p le x e s . T h e r e is a c o r r e la tio n o f the r a tio o f 60C om r e te n tio n / 60CoS re te n tio n w ith the lig a n d - fie ld sp littin g , b u tth is c o r r e la tio n d is a p p e a rs at h ig h e r te m p e ra tu re . O u te r-s p h e re c o -o rd in a tio n e x e r ts on ly a s e c o n d -o r d e r e ffe c t on the r a tio . A s d e s c r ib e d in the se c tio n on (3-d e c a y , the c h e m ic a l c h a r a c te r is tic s o f the su rrou n din gs also p la y an im p o rta n t r o le in the s ta b iliz a tio n p r o c e s s e s fo r the r e c o ils fr o m is o m e r ic tra n s itio n . A study b y Sasaki e t a l. [8 0 , 81] o f 80B r r e c o il fo llo w in g is o m e r ic tra n s itio n in a ll the a lk a li- m e ta l b ro m a tes show s that the d iffe r e n c e s in re te n tio n betw een the tw o te m p e ra tu re s -196 and 184°C a re re la te d to the f i r s t io n iza tio n p o ten tia ls o f the a lk a li-m e ta l a tom s as shown in T a b le IV . T A B L E IV . Y IE L D S F O R 80éBrO¿ A T 77 A N D 457°K
Compound
Yield (%)
AY
{%)
77°K
457°K
Li*Br03
25.7
40.5
14.8
Na*Br03
31.4
47.2
15.8
K*Br03
32.5
43.6
11 .1
Rb*Br03
24.5
34.8
10.3
Cs*Br03
32.2
35.5
3.3
250
6.
S H IO K A W A
CONCLUSIONS
Much excellent re s e a rc h has been c a rrie d out by m eans of M ossbau er spectroscopy, on the p rim a ry state of the solid sam ple p rio r to dissolution. E xperim en ts on the annealing behaviour of solid sam ples and the re c o il synthesis of new compounds by decay p ro c e sse s have both been c a rrie d out. D iscu ssio n s o f these investigations a re being conducted in other sessions of this m eeting. I have m ade a short rev iew within the scope of the p rim ary and initial states, and of stabilization p ro c e sse s and chem ical consequences of 0 -d ecay and iso m e ric transition. D uring the past ten y e a rs, much valuable inform ation has been accum u lated in this a re a . F o r exam ple, therm al ionic reactions have been introduced into the postulated chem ical consequences o f iso m eric transition, and many important chem ical ch aracteristics have been found which play an important role in the stabilization p ro cess fo r decay re c o ils. I hope that investigators in this field w ill em phasize the aim s o f their investigations in choosing suitable system s for study, such that our under standing of the link between p rim a ry physical events and the final chem ical consequences of the decay w ill be furthered. Up to now our understanding is rath er shadowy, and im proved knowledge w ill req u ire better physical and chem ical techniques. Fin ally, I believe that investigators in this field must give attention to the decay event and its consequences in bio lo gical m aterials because of the frequent occurrence and important influences on human beings.
ACKNOW LEDGEM ENTS The author thanks P r o fe s s o r J . P . A d lo ff for his annual bibliograph ies of "C h em ical E ffect of N u clear T ran sfo rm atio n s", P r o f. A . G . Maddock, P r o f. R . W olfgang, and P r o f. G . W . A . Newton fo r their valuable books, and D r . A . H alpern fo r his excellent m anuscript.
REFERENCES [1 ]
W E X L E R , S . , “ P r i m a r y p h y s i c a l a n d c h e m i c a l e f f e c t s a s s o c i a t e d w i t h n u c l e a r p ro c e s s e s ” , A c t i o n s
[2 ]
c h i m i q u e s e t b i o l o g i q u e s des R a d i a t i o n s ( H A I S S I N S K Y , М . , E d . ) 8 ( 1 9 6 5 ) 1 0 5 . H S IU N G , С . , G ORDUS, A . A . , i n C h e m i c a l Effects o f N u c le a r T ra n s fo rm a tio n - 1 9 6 4 (P ro c. S ym p. V i e n n a , 196 4) 2 , I A E A , V i e n n a ( 1 9 6 5 ) 4 6 1 .
[3 ] [4 ] [5 ] [6 ] [7 ] [8 ] [9 ]
M A D D O C K , A . G . f W O L F G A N G , R . , N u c l e a r C h e m i s t r y ( Y A F F E , L . , E d . ) 2 , N e w Y o r k ( 1 9 6 8 ) 1 86 . N E W T O N , G . W . A . , " C h e m i c a l e f f e c t s o f n u c l e a r t r a n s f o r m a t i o n s " , A S p e c i a l P e r i o d i c a l R e p o r t, R a d i o c h e m i s t r y _1, C h e m . S o c . ( 1 9 7 2 ) . A D L O F F , J . P . , R a d i o c h i m . A c t a 15 ( 1 9 7 1 ) 1 35 . L A N G H O F F , H . , P hy s. R e v . A 3 ( 1 9 7 1 ) 1 . G E N E T , М . , F E R R A D I N I , C . , R a d i o c h i m . A c t a 11 ( 1 9 6 9 ) 19 a nd 2 5 ; G E N E T , М . , R a d i o c h i m . A c t a 12 (19 6 9 ) 193. H O W A R D , J . M . , S E Y K A R A , E . J . , W A L T N E R , A . W . , P hy s. R e v . A 4 ( 1 9 7 1 ) 1 7 4 0 . P O R TE R , F . T . , F R E E D M A N , M . S . , W A G N E R , F . , J r . , P hy s. R e v . C 3 ( 1 9 7 1 ) 2 2 4 6 .
[1 0 ] [1 1 ] [1 2 ]
L O W , J . , C A M P B E L L , J . L . , Phys. Rev. A 18 7 (1972) 525. I S O Z U M I , Y . , S H I M I Z U , S . , P hy s. R e v . C 4 ( 1 9 7 2 ) 5 2 2 . I S O Z U M I , Y . , M U K O Y A M A , T . , S H I M I Z U , S . , P hy s. R e v . L e t t . 2 9 ( 1 9 7 2 ) 2 9 8 .
[1 3 ]
W E X L E R , S . , A N D E R S O N , G . R . , J. C h e m . P hy s. 3 3 ( 1 9 6 0 ) 8 5 0 .
251
IA E A -P L-6 1 5/15
[1 4 ]
S H I O K A W A , T . , 1 Z A W A , G . , N A G A S H I M A , K . , H I R A G A , M . , Y O S H I H A R A , K . , Ma ss S p e c t r o s c o p y
[1 5 ]
21 (1973) 179. CA R LS O N , T . A . , W H IT E , R . , " C h e m i c a l Effects o f N u c le a T r a n s fo rm a tio n s - 1 9 6 4 (P ro c. S y m p .
[1 6 ]
V ie n n a , 196 4)^1 , IA E A , V ie n n a (19 65 ). S H IO K A W A , T . , T A K I T A , Y . , H IR A G A, M . ,
[1 7 ]
(1971) 3 13. C H U P K A , W . A . , L I N D H O L M , E . , A r k i v . P hy s. 2 5 ( 1 9 6 3 ) 3 4 9 .
[1 8 ] [1 9 ]
Y A G I, M . , K O N D O , K . , R a d io c h e m . R a dio an al. L e tt. 5 (1970) 75. Y A G I , M . , K O N D O , K . , K O B A Y A S H I , T . , B u ll. C h e m . Soc. (Japan) 4 4 (1971) 5 80.
[2 0 ] [2 1 ] [2 2 ] [2 3 ] [2 4 ]
Y A G I, M . , KO N D O , K . , KO B AY A SH I, Y A G I, M . , K O N D O , K . , KOBAYASH I, Y A G I, M . , K O N D O , K . , KO B AY A SH I,
Y O S H IH A R A , K . , R a diochem . R adioanal.
T . , R a dio ch e m . R adioanal. T . , i b id . , p . 281. T . , R a dio ch e m . R adioanal.
L e t t . 1_
L e tt. 7 (1971) 2 75. L ett. 1 4 (1 9 7 3 ) 123.
[2 5 ] [2 6 ]
C A C A C E , F . , S T O C K U N , G . , J. A m . C h e m . Soc. 94 (1972) 251 8 . N I C H O L A S , J . B . , R A C K , E . P . , J. C h e m . P hy s. 48 ( 1 9 6 8 ) 4 0 8 5 ; T A C H I K A W A , E . , Y A N A I , K . , R a d i o c h i m . A c t a 17 ( 1 9 7 2 ) 1 38 . LOB ER G, M . D . , W E L C H , J . , J. A m . C h e m . S o c . 9 5 ( 1 9 7 3 ) 1 0 7 5 . C A C A C E , F . , G U A R I N O , A . , S P E R A N Z A , M . , J. A m . C h e m . S o c . 9 3 ( 1 9 7 1 ) 1 0 8 8 .
[2 7 ]
C A C A C E , F . , P ER EZ , G . , J. C h e m . S o c . B ( 1 9 7 1 ) 2 0 8 6 .
[2 8 ] [2 9 ]
C A C A C E , F . , C IP O LL IN I, R . , C IR A N N I, G ., p . 2089. B A B E R N IC S , L . , C A C A C E , F . , i b i d . , p . 2 3 1 3 .
[3 0 ]
H E I T Z , C . , E RN ST BERGER, R . , K U H R Y , J . G . , R a d i o c h e m .
[3 1 ] [3 2 ]
T O M I N A G A , T . , F U J I W A R A , K . , B u l l . C h e m . S o c . ( J a pa n ) 4 3 ( 1 9 7 0 ) 2 2 7 9 . O M O R I, T . , S H IO K A W A , T . , R a d io c h e m . R a dio an al. L e tt. 3 (1970) 39.
R a d i o a n a l . L e t t . 12 ( 1 9 7 2 ) 3 4 5 .
[3 3 ]
IA E A , B io l o g ic a l E ffe cts o f T r a n s m u t a t io n and D e c a y o f I n c o r p o r a te d R adioisotopes (P ro c . P an e l,
[3 4 ]
V ie n n a , 1967), IA E A , V ie n n a (196 8 ). D a F O U S E C A , A . J . R . , S H A W , D . , S H A W , P . F . D . , R a d i o c h i m . A c t a 17 ( 1 9 7 2 ) 8 1 .
[3 5 ] [3 6 ] [3 7 ]
M O H A N , H . , IY E R , R . M . , R a d io c h e m . R a d io a n a l. L e tt. 7 (1971) 3 81 . M O H A N , H . , I Y E R , R. M . , i b i d . 8 ( 1 9 7 1 ) 1 . M O H A N , H . , IY E R , R . M . , R a d ia t. E ffects 16 (1972) 17.
[3 8 ] [3 9 ] [4 0 ]
E D W A R D S , R. R . , C O R Y E L L , C . , A E C U - 5 0 ( 1 9 4 8 ) 5 2 . G L E N T W O R T H , P . , B E T T S , R . H . , C a n J . C h e m . 39 ( 1 9 6 1 ) 1 0 4 9 . G L E N T W O R T H , P . , W IS EA LL, B . , i n C h e m i c a l Effects o f N u c le a r T ra n s fo rm a tio n s - 1 9 6 4 (P ro c. S ym p .
[4 1 ]
G L E N T W O R T H , P . , W R I G H T , C . L . , J. I n o r g . N u c l . C h e m . 3 1 ( 1 9 6 9 ) 1 2 6 3 .
[4 2 ] [4 3 ] [4 4 ] [4 5 ]
O M O RI, O M O R I, O M O RI, O M O RI,
[4 6 ] [4 7 ]
H E I T Z , C . , I n o r g . N u c l C h e m . L e t t . 7 (1 9 7 1 ) 4 7 . LEBEDEV, V . A . , B AB ESH KIN , A . M . , N E S M E Y A N O V , A . N . , L A M Y K I N , E . V . , V e s tn ik M o s k o v .
[4 8 ] [4 9 ]
U n i v . (5) (1969) 4 5 . B A B E S H K I N , A . M . , L A M Y K I N , E . V . , L E B E D E V , V . A . , N E S M E Y A N O V , A . N . , i b i d . (1 ) ( 1 9 7 0 ) 1 1 7 . L E B E D E V , V . A . , B E B E S H K IN , A . M . , N E S M E Y A N O V , A . N . , F A T I E V A , N . L . , R a d i o c h e m . R a d i o a n a l . L e t t. 5 (1970) 83.
V ie n n a , 1964) 2 , IA E A , V ie n n a (1965) 4 83 . T ., T ., T ., T .,
K U D O , H . , S H I O K A W A , T . , B u l l . C h e m . S oc . ( J a pa n ) 3 8 ( 1 9 6 5 ) 1 3 4 0 . S H IO K A W A , T . , i b i d . , p. 1892. S H I O K A W A , T . , B u l l . C h e m . S o c . (J a p a n ) 4 2 ( 1 9 6 9 ) 6 96 . K I D O , T . f S H I O K A W A , T . , B u l l . C h e m . S o c . ( Ja pan ) 43 ( 1 9 7 0 ) 2 0 7 6 .
[ 5 0 ] U L L R I C H , J . F . , V I N C E N T , D . H . , J. P hy s. C h e m . S o l i d s 3 0 ( 1 9 6 9 ) 1 1 8 9 . [ 5 1 ] - J O N E S , C . H . W . , W A R R E N , J . L . , J. C h e m . P hy s. 5 3 ( 1 9 7 0 ) 1 7 4 0 . [ 5 2 ] H I L L M A N , М . , WE IS S , A . J . , R a d i o c h i m . A c t a 15 ( 1 9 7 1 ) 7 9. [5 3 ] [5 4 ]
Z A IT S E V , V . M . , G U SE LN IK O V , V . S . , M A K H O M A L K IN A , S . М . , R a d io k h im iy a 1 4 (1 9 7 2 ) 783. F R I E D T , J . M . , A S C H E N , L . , R a d i o c h i m . A c t a 1 2 ^ (1 9 6 9) 2 0 8 .
[5 5 ] [5 6 ]
FR IE D T, J . M . , CRUSET, A . , A S C H , L . , AD LO FF, J . P . , R a d io c h e m . R adio a n al. L e tt. ¡3(1970) 81. L L A B A D O V , Y . , R a d io c h im . A c ta 1 4 (1 9 7 0 ) 100.
[5 7 ]
S A T O , T . , K O N D O , K . , S H IO K A W A , T . , B u ll. C h e m . Soc. Japan, 4 4 ( 1 9 7 1 )
[5 8 ]
S A N O , H . , Môssbauer S pectroscopy, K ondansha, T o k y o (197 2 ).
[5 9 ] [6 0 ]
O M O R I, T . , W U , S . C . , S H IO K A W A , T . , R a d io c h e m . R a d io a n a l. L e t t. 3 (1970) 405. O M O R I , T . , W U , S . C . , S H I O K A W A , T . , B u l l . C h e m . S o c . ( J a pa n ) 4 4 ( 1 9 7 1 ) 1 0 1 4 .
[6 1 ] [6 2 ]
B AR O , G . B . , A T E N , A . H . W . , i n C h e m i c a l E f f e c t s o f N u c l e a r T r a n s f o r m a t i o n - 1 9 6 0 ( P r o c . S y m p . P r a g u e , 1 96 0 ) 2 , I A E A , V i e n n a ( 1 9 6 1 ) 2 3 3 . H A L P E R N , A . , S A W L E W I C Z , K . , N u k l e o n i k a 13 ( 1 9 6 8 ) 9 2 1 .
[6 3 ]
G E N E T , М . , FERRADINI, C h . , R a d io c h im . A c ta И (1969) 19.
1746.
252
S H IO K A W A
[6 4 ]
G EN E T , М . , FERRADINI, C h . , i b i d . , p . 25.
[6 5 ]
G E N E T , М . , i b i d . , 12 (1 9 6 9 ) 1 9 3 .
[6 6 ]
A N D E R S O N , T . , K N U T S E N , A . B . , J. I n o r g . N u c l . C h e m . 2 3 ( 1 9 6 1 ) 1 9 1 .
[6 7 ] [6 8 ]
A N D E R S E N , T . , C H R I S T E N S E N , F . , O L E S E N , К . , T r a n s . F a r a d a y S o c . 62 ( 1 9 6 6 ) 2 4 8 . F A C E T T 1 , J . F . , R a d io c h im . A c ta 4 (1 9 6 5 ) 164.
[6 9 ]
LEBEDEV, R . A . , B A B E SH K IN , A . M . , N E S M E JA N O V , A n . N . , POPOV, E . A . , Vestn. M o sk. U n iv . S e r .II . K h i m i a П ( 1 9 7 0 ) 6 25 .
[7 0 ] [7 1 ]
F E R R A D I N I , C . , C A R L I E R , R . , G E N E T , М . , P U C H E A U T , J . , R a d i o c h i m . A c t a 12 (1 9 6 9 ) 1. M Ü N Z E , R . , K e r n e n e r g i e 4 (1 9 6 1 ) 8 0 8 .
[7 2 ]
N E F E D O V , V . D . , P ETRO V, L . N . , AVROR1N, V . V . , R a d io k h im iy a 14 ( 1972) 4 36.
[7 3 ] [7 4 ] [7 5 ]
M U L L E R , H . , J. I n o r g . C h e m . 131 ( 1 9 6 9 ) 1 5 7 9 ; A t o m k e r n e n e r g i e ^ 6 ( 1 9 7 0 ) 2 3 6 a n d 3 2 3 . M ÜLLER, H . , M A R T IN , S ., In o rg . N u c l. C h e m . L e tt. 5Д1969) 761. M U L L E R , H . , C R A M E R , D . , R a d i o c h i m . A c t a iA ( 1 9 7 0 ) 7 8 .
['7 6 ] [7 7 ]
JO N E S , C . H . W . , R a d i o c h i m . A c t a 1 4 ( 1 9 7 0 ) 1. A R N 1 K A R , H . J . , R A O , B . S . M . , J. I n d i a n C h e m . S o c . 4 8 ( 1 9 7 1 ) 3 2 3 .
[7 8 ]
L A Z Z A R I N I , E . , F A N T O L A - L A Z Z A R I N I , A . L . , J. I n o r g . N u c l . C h e m . N u c l e a r e ( M i l a n ) 17 ( 1 9 7 0 ) 4 4 4 ; J. I n o r g . N u c l . C h e m . 3 3 ( 1 9 7 1 ) 6 3 1 .
31(1969)
1947;
E n e r g ia
[7 9 ]
L A Z Z A R I N I , E . , E n e r g i a N u c l e a r e ( M i l a n ) 16 ( 1 9 6 9 ) 6 54 .
[8 0 ] [8 1 ]
S A S A K I , T . , T A K A H A H S I , S . , S H I O K A W A , T . , R a d i o c h e m . R a d i o a n a l . L e t t . 1 ( 1 9 6 9 ) 3 1. S A S A K I , T . , S H I O K A W A , T . , B u l l . C h e m . S o c . ( J a pa n ) 4 3 ( 1 9 7 0 ) 2 8 3 5 .
D IS C U S S IO N J .P . A D L O F F : I had in mind the preparation of a paper on the use of inorganic synthesis in beta decay, related to the rev iew paper by Don W ile s in Advances in O rganom etallic C hem istry (see J .P . Adloff, IA E A -P L -6 1 5 / 5 , these P ro c e e d in g s). This is a v ery attractive subject because som e new and unknown compounds have been p repared fo r the firs t time using betadecay p ro c e s s e s . The p rim a ry atomic and m olecular effects of beta decay a re rath er m ild — a low re c o il energy plus the sh ak e-o ff excitation which is not expected to produce severe radiation dam age. M olecu lar synthesis through beta decay has already been of interest in hot atom chem istry, but m ainly in the field of organom etallic compounds. My goal has been to draw attention to the possibility of using beta decay with the aim of producing new compounds o r new, unusual oxidation states in inorganic chem istry. The best dem onstration of this p ro c e ss has been given by the initial p r e paration of p erbrom ate. P e rb ro m a te w as fo r a long time an unknown species, and w as firs t synthesized by the beta decay of selenium in a selenate compound. If you look at the book by Dasent on the Chem istry of N on-E xistent Compounds you w ill find many suggestions of m olecules whose synthesis could be attempted either through beta o r alpha decay. The second exam ple I suggest in my paper concerns n o b le -g a s compounds form ed v ia beta decay. Such hypothetical pathways have been suggested s e v e ra l tim es, even fo r helium and neon compounds. The suggestions are m ore re a listic when the form ation of compounds of krypton and xenon is considered. F irs t, we have convenient isotopes fo r radiochem ical investiga tions, and second we have also v e ry suitable isotopes fo r M ossbau er investigations. I have included a discussion o f the synthesis of krypton compounds by the decay of brom ine isotopes in brom ates and in p erbrom ates. Up to this point, however, no experim ents have been attempted. F ro m M ossbau er experim ents one already has indications that the till now unknown krypton oxide could be form ed by beta decay in selenium sou rces. F o r xenon compounds, the situation is still m ore favourable because there are
1AEA-PL-615/15
253
s e v e ra l radioactive chains starting with iodine isotopes leading to iso m eric states in xenon nuclides. A lre a d y se v e ra l investigations have been c a rrie d on by Nefedov and his c o -w o rk e rs fo r the radiosynthesis o f many xenon compounds which have never been prepared by direct chem ical m eans. One point of sp ecial interest is that such beta-decay-in duced synthesis could occur under natural conditions. 1 would like to stre ss this point, fo r this is a topic which is com pletely m issin g in this panel, and also in m ost hot atom sym posia — nam ely, the ro le of hot atom reactions in natural p ro c e sse s, fo r instance, in the atm osphere, o r for the reactions of hot species form ed by cosm ic ra y s, or in geological system s. If I re m e m b e r w ell, in one o f the two Agency sym posia, there w as one discussion of such p ro b lem s. This could be an important application of hot atom chem istry. G . H A R B O T T L E : The synthesis of unknown compounds by beta decay is an application that has intrigued me fo r a long time, although I have yet to do one experim ent. A few y e a rs ago, I gave a long discussion o f the p ossible synthesis o f radon compounds from radium decay. It could lead to a whole range o f totally unknown compounds — I'm su rp rise d that someone hasn't tried it yet. Since xenon compounds are rath er easy to make, including even xenonium organic m olecules, it seem s to me that radon compounds should be even e a s ie r to m ake. Radon fluoride can be synthesized under conditions which are considerably m ild er than those for the preparation of xenon fluoride. I think that there is a v e ry interesting field of study here, and it has the great advantage that it can be done by someone who doesn' t have a n uclear re a c to r available. A ll you need is some radium . W ith re g a rd to your v e ry interesting point about hot atom reactions in the atm osphere, I think Rowland published. . . F .S . R O W L A N D : It w as ou r pap er at the Vienna sym posium that A d loff w as re fe rrin g to, I think. A . P . W O L F : W olfgang also w rote a p ap er on the chem istry of 14C produced by cosm ic radiation. G. H A R B O T T L E : I want to follow this up and ask if anyone has ever w orked out how it is that carbon atoms form ed in the atm osphere — o rig in a lly fro m the 1 4 N(n, p )14C n uclear reaction — becom e incorporated as 1 4 C 0 2 in plants. They probably react o rigin ally to fo rm 1 4 C O . How does the 14CO get converted into 1 4 C 0 2? F .S . R O W L A N D : T h ere is a tentative answ er to this sequence of reactions. Experim ents done by W o lfg a n g 's group with n C, and also with 14C when corrected for radiation chem ical oxidation o f 1 4 CO, show that the product form ed by the reaction of b a re carbon atoms with m olecu lar oxygen is c le a rly carbon monoxide. The question then becom es: in the atm osphere — how does 14C O becom e 1 4 C 0 2? This is obviously v e ry closely related to the p roblem of what happens to 12C O in the atm osphere, and this problem has been puzzling atm ospheric scientists for quite some tim e. The current explanation fo r the atm ospheric conversion of carbon monoxide to carbon dioxide is that carbon monoxide is attacked by OH ra d ic a ls and oxidized to C 0 2. The current explanation fo r the rem o val of C H 4 fro m the atm osphere is also that it is attacked by OH, and that there should th erefore be some p a ra lle l between the re m o v a l of CO and the rem oval of C H 4. In the tro p o sphere, this hypothesis looks fa ir ly good, but in the stratosphere it begins to bre a k down. I think that it is still a reason able hypothesis, but that
254
SH IO K A W A
m ore experim ents could be done. I know that s e v e ra l of the atm ospheric scientists would like to have the 14CO experim ent of W olfgang done over again — that is, the search fo r natural atm ospheric 1 4 C O . N . G E T O F F : W e have done^some p re lim in a ry experim ents with vacuum ultraviolet light at 1236 A and some at 1470 A in the gas phase — not yet published. When oxygen is present, ozone is form ed, and C O can then be oxidized to C 0 2. On the one hand, the triplet state of ozone can do the oxidation; on the other hand, oxygen atoms are form ed which are v e ry reactive tow ards C O . The rate constants have not yet been elucidated, but it seem s to me that not only OH ra d ic a ls w ill participate but also ozone. The ozone m echanism w ill involve its form ation and photolysis in the upper atm osphere under the influence of vacuum ultraviolet light. F .S . R O W L A N D : The vacuum ultraviolet isn 't v ery important fo r the oxidation of C O in the atm osphere because m ost of the vacuum ultraviolet light — at 1236 and 1470A — has alread y been rem oved from the sunlight at 1 0 0 km altitude. N . G E T O F F : Not 100 km but 10oto 12 km. F .S . R O W L A N D : No, the 1470 A light is alm ost all gone at v e ry high altitudes. О N . G E T O F F : 2000 A is good enough. W e 'v e also done the experim ents at 1849 A with the lo w -p re s s u re m ercu ry lam p. F .S . R O W L A N D : 2000 A , y es, but not 1470 Â . Some of the 2000 Á light gets down to 15 km o r so. G. HARBO*TTLE: I want to pursue this business of the 14CO in the atm osphere. A s I r e c a ll when W olfgang did this experim ent, did he not find that the 14CO specific activity in 14C O was rather high com pared with 1 4 C 0 2? W a s this not the result? I wonder if one couldn't turn this to good use — since the carbon monoxide coming from automobile fuel should be dead carbon, if one collected carbon monoxide and m easured its specific activity, one would have a read y-m ad e way of a ssessin g the effective level of exhaust carbon monoxide anywhere com pared with natural 1 4 CO. F .S . R O W L A N D : You have to set up an experim ent to count lo w -le v e l 14C in C O . G. H A R B O T T L E : A s it gets diluted by automobile exhaust. The automobile exhaust would have no 14C in it. T h ere are natural p ro c e sse s that generate CO, p resu m ably at a steady state, and this would be labelled at the n orm al specific activity le v e l o f 1 4 C . By m easuring how much this has been depleted, one could then say what fraction of that 14CO is coming from automobile exhaust and what fraction is natural. I would think this would be a v e ry lively problem at the present tim e. A . G . M AD D O CK : I seem to r e c a ll seeing the figure that se v e ra l hundreds o f tons of nitric acid fa ll on the earth every y e a r from thunder storm s. This is an oxidizing mechanism , I feel sure. F .S . R O W L A N D : It doesn't affect carbon monoxide appreciably. This is an a re a of study which is under intense investigation now because of the p ossible contributions of N O x to the stratosphere fro m the exhausts of supersonic tran sports. The rate constants for alm ost a ll of those compounds n itric acid is included — have been com pared and 50 o r 60 rate constants are involved. In those system s, it is still OH plus CO that seem s to be the dominant reaction fo r oxidizing CO. S. A M IE L : Going back to H arb o ttle's suggestion about the 14C specific activity of natural carbon monoxide. F riedm an and others have m easured
IAEA-PL-615/15
255
the 1 3 C/12C ratio in the m ass spectrom eter. That seem s to me much e a sie r than going into 14C counting. Isn't it a solved problem ? G. H A R B O T T L E : Y e s, I think it w as solved through the 1 3 C / 12C m easurem ents. H ow ever, it never hurts to check things by an independent method. S. A M IE L : But as a routine, the m ass spectrom etry is probably much e a s ie r. N . G E T O F F : Singlet oxygen, as we a ll know, can also react with CO to give C 0 2 . And singlet oxygen can be produced even in the 2500 to 2200 A range in the fa r ultraviolet. F .S . R O W L A N D : T h at's also been evaluated, and there is relatively v e ry little singlet oxygen present. The rate constants fo r that reaction have also been evaluated, but the rates a re not la rg e enough to be important in CO oxidation. N . G E T O F F : M aybe partly participating in the oxidation of C O . A . G . M A D D O C K : I'm switching to a different topic. One should b e a r in mind that the oxidizing activity is only associated with beta-m inus decay in principle — in beta-p lu s decay, in principle we have a reduction p ro c e ss. I don't believe that any exam ple of that has ever been demonstrated; at least none that I am aw are of. A second point: I want to make another plea fo r iso m eric transition studies to point out what unique system s they are from the point of view of studying post-tran sform ation effects. They present one of the few system s fo r which you can look at the effects o f som e v a ria b le , and then allow another generation of daughter activity to grow in and look at the effect of another v a ria b le — on exactly the sam e sam ple. You have a complete answ er to the difficulties o f reproducibility often encountered in this type of experim ent. F .S . R O W L A N D : I have a follow -up comment to the question o f A d lo ff about hot atom p ro c e sse s in geolo gical system s. I have the p ro o f o f an a rticle h ere which is being published in Science by D r . Ralph B eck er and two o f his c o -w o rk e rs , with the title, "Hot hydrogen atoms: Initiators of reactions in in terstellar chem istry and evolution. " H is paper is related to a chem ical paper published by Sagan, an astronom er at C o rn ell, in which they photolysed hydrogen sulphide in the usual prebiotic atm osphere of methane, w ater and amm onia, and then isolated amino acids from it. Sagan attributes in his experim ent the presence of hot hydrogen atoms as important fo r the form ation of the amino acids. In B e c k e r's view , the hydrogen atoms are hot and abstract hydrogen, leaving behind ra d ic a ls which a re involved in the form ation o f amino acids. T h ere is some comment going on in the scientific community about hot atom chem istry in natural p ro c e sse s — by people not represented at this panel. How much o f that book on non-existent compounds is radiochem ically based? P a r t of it, or none of it? J .P . A D L O F F : It is not radiochem ically based; it's a textbook on chem istry. A v e ry interesting, v e ry scientific book — an excellent book. H. SAITO: Y e s, it's a v e ry gen eral chem istry book. K. R O SSLER : Some striking things occur during re c o il by electron capture. T h ere is only one system in which the bonds are not broken, or experim entally seem not to be broken, and this is the hexahaloghenate system studied by M u lle r and mentioned in your talk. This is also one o f the v e ry r a r e cases in which ligand re c o il has been studied. F o r this
256
S H IO K AW A
low energy — 10 eV — w e did a computer sim ulation study which showed c le a rly that the re c o il range is not la rg e enough to brin g about a separation. O ur calculations have w orked w e ll with a spontaneous recom bination radius of 8 A , and the distribution of p rim a ry vecto ral ranges for 10-eV reco il puts m ost of them as le s s than 8 A . F ro m the view of our calculation, the experim ental observation is quite natural — a bond is broken, but immediate recom bination occurs because the atoms cannot go v e ry fa r into the lattice. The behaviour is quite different when brom ine is the central atom, o r with tellu riu m . Then the dam age is m ore severe, and recom bination is not so easy as in the case when the atom must only make a jump back into its own o rig in a l ligand position. L . L IN D N E R : I would like to ask D r . Shiokawa about the la rg e influence of im purities on beta decay that he mentioned. Have any quantitative relationships been established with m easured c ry stal defects? The c o r r e lation with im purities which you showed w as a rath er striking relationship. T . SH IO K AW A: W e have not m easured c ry stal defects, and have no answ er to your question. G. H A R B O T T L E : I have no specific knowledge, but can comment gen erally . If the chem ical consequences of iso m eric transition in a solid involve the depletion of electrons — the rapid depletion of electrons from the vicinity of the initial К shell vacancy — then one would expect that cry stal defects within a reasonable radius would make them selves felt because they could function either as electron-trapping o r hole-trapping centres. H oles could be stabilized — there w ill be plenty of holes in the vicinity of one of these internal conversion or К shell A u g e r vacancy defects. W hether these holes w ill sim ply go out and annihilate with electrons in the cry stal w ill depend upon whether they are trapped — whether the concen tration of im purities is high enough within a reasonable radius of the location of the initial event. In that sense, the study of im purity doping for trapping holes might be an extrem ely interesting experim ent — the influence of this p ro c e ss on the chem ical consequences of internal conversion. I do not, however, know of any experim ents of this type which have actually been done. A . G . M AD D O CK : A s fa r as iso m eric transition is concerned, it has been suggested that exactly the re v e rs e is true. Y o u 'll find that Halpern in one paper d rew the conclusion that the behaviour in one isom eric transition system w as v e ry insensitive to the presence of site defects. On the other hand, I wouldn't say that his conclusion is altogether in keeping with some of the m ore recent evidence. If his conclusion w ere alw ays true it would be hard to see why the retention following isom eric transition in telluric acid using tellurium iso m ers is a function o f the d o se -ra te of ionizing radiation supplied while the daughter and parent are equilibrated. This does look like a profitable a re a fo r experim ental development, and o ffers advantages in com parison with any n eutron -irradiated sam ple. T . SH IO K AW A: T h is is not a direct answ er to the e a r lie r question, but I did r e fe r to a study of the beta decay of 127Sn and 125Sn to antimony. The amount found in the pentavalent state of antimony in hexachloroantimonate compounds depends upon the parent state, and upon the purification procedure. A retention of 10% w as found in the pure crystal, and 90% in a highly impure one. G. H A R B O T T L E : W ith chem ical effects o f nuclear transform ations in v e ry pure c ry sta ls, extrem ely pure c ry sta ls, the presence of minute amounts
IA E A -P L-6 1 5/15
257
of trace im purities can have a great effect. The p u re r the cry stal, the g re a te r the effect. W ith a compound such as tellu ric acid, which is grow n or precipitated fro m solution, it is certainly not a good c ry stal to begin with in the sense o f having a v ery good lattice. W e did some experim ents on the implantation of thallium re c o ils fro m a source plated with an.activated thoron deposit — and the 3-m in thallium w as dissolved into thallous and thallic fo rm s in solution. W e did these implantations into H arsh aw KC1 and N a C l plates in a vacuum, and found that the resu lts depended v e ry strongly on minute amounts of w ater vapour present. Some w ater p ersisted even though pumped at tem peratures w e ll over 100°C, and the thallous/thallic ratio could still be changed enorm ously just by heating the c ry stals still further and pumping some m o re. It is certainly true that minute amounts of electron traps or hole traps are effective in determ ining hot atom resu lts if your c ry stal is extrem ely pure. If the c ry stal isn 't pure, then you are only looking at a hodge-podge anyhow. G. S T O C K L IN : I w ish to ask a biological question to the experts on beta-d ecay . 32P can be incorporated into the D N A chain, and when it decays it produces strand break s because the re c o il energy is high. H ow ever, to a certain extent, with a probability as high as 0 . 2 it produces breakage of the opposite strand. L . F E IN E N D E G E N : No, only about 0.05. G. S T O C K L IN : I thought in some cases it w as higher. W hy does this occur? The re c o il range of the 32S is certainly much too sm all to reach the other strand. Why is there such a high probability fo r the other strand also breaking? Do the beta-decay specialists have an explanation fo r this? J .P . A D L O F F : The 32P beta transition has an extrem ely high energy. G. S T O C K LIN : Y e s, but if you go through the calculation o f the mean re c o il energy and the range, you find that the range is much sm a lle r than the distance from one strand to the other strand. It cannot be that the strand bre a k is caused by the recoilin g sulphur atom. A . G . M A D D O C K : I don't think one should be su rp rised by the strand break age. T h ere are a variety of energy tra n sfe r p ro c e sse s in solids that have enorm ously long ran ges, but even in liquid system s, tra n sfe r over a few m olecules distance is not a rem ark ab le event. It does seem p ossible to me that one could find events of the kind you d escrib e. A fte r all, if this w ere not true, one would have difficulty in having liquid scin itllators. L . L IN D N E R : In addition, the electrons are affected, esp ecially in the case of the hard betas of 3 2 P . In this case in just about every instance there is a sudden perturbation, as com pared with some o f the much w eaker betas fo r which the emitted beta departs rath er slow ly. L . F E IN E N D E G E N : Stocklin's question is v e ry pertinent, and I 'll discu ss it later on. I think that I should just comment here that the re c o il effect is obviously not so important in making the D N A break. A s D r. Apelgot has shown, and as has been confirm ed recently by K risch , it is m ainly the chem ical transition which is involved, and not the re c o il phenomena. T . SH IO K AW A: I believe that the most important role in Stocklin's case is the electronic excitation of the beta decay, rather than the energy of the decay itself. N . G E T O F F : W e have been studying the form ation of electrons in organic and inorganic m olecules — in tram olecular energy tra n sfe r leading to the form ation of electron s. When excited states of these m olecules are form ed, the energy can move se v e ra l m olecules distant — the electrons are
258
S H IO K A W A
emitted fro m OH, imino, and other groups. It is w e ll known that sulphur is v e ry reactive tow ards electrons with rate constants that are v e ry high — perhaps the 32S w ill scavenge som e electrons. P e rh a p s this is part of the p ro c e ss. L . L IN D N E R : It would be an interesting experim ent to com pare 32P with 33P in the D N A experim ent. In the high re c o il energy 32P case, it is re a lly v e ry sudden - non-adiabatic. In the 3 3 P case, you might have a fa ir contribution fro m adiabatic p ro c e s s e s . H as this com parison been made? L . F E IN E N D E G E N : This has been done by K risch . L . L IN D N E R : W a s there any effect? L . F E IN E N D E G E N : In E . coli there w as zero p e r cent effect. In phage, about 50% of the effect in sin g le -stra n d b reak s — not the double-strand b reak s — w as attribted to the re c o il. In E . c o li, for which lethal effects a re p ractically synonymous with double-strand b reak s, there w as zero effect. N . G E T O F F : L e t me mention one o f the system s we are studying. W e a re sp ecifically exciting in the benzene ring of a com plex arom atic system , and the energy m oves to the oxygen, to the phosphorus, and finally leads to phosphorescence and ejection of electrons. These electrons then lead to ra d ic a ls in aqueous solution which can hydrolyse and react to give other products. The main idea is that in tram olecular energy tran sfer fin ally lead s to the ejection o f electrons. K. R Ó SSLER : This m orning there w as a question ra ise d that has not yet been answ ered. This w as the discrepancy between the statement of D r . Feinendegen that the A u g e r electrons have a range of at least 250 A , and higher, and D r . A d lo ff who im m ediately said that in the solid state it w a s quite different, and the range w as only a few A n gstrom s. I think that this difference might be v ery important, whether the m olecules are in solution or in a solid. In the solid the electrons can be trapped im m ediately after they are form ed. L . L IN D N E R : I don't think that there is any p articu lar reason to believe that these two condensed phases are so much different to give that much difference in range. A . G . M AD D O CK : In all these system s we are talking about, the F o rs te r m echanism is operative. If we get an electronic excitation in one place, fo r which a ll the energy dissipation p ro c e sse s are slow, and there is another atom fo r which there is an overlap of the absorption spectrum , then tran sfer can take place. The tra n sfe r is a com paratively fast p ro c e ss, and if the new location is a chem ically vulnerable part o f the m olecule, the resu lt can be chem ical decom position. T h e re is an enorm ous literatu re when it leads to fluorescence; there is a much m ore restricted literatu re, but still quite num erous, when it leads to chem ical effects. It doesn't m atter within rath er wide lim its what m olecu lar groupings lie in between, and this sort of in ter action can g e n erally extend o v er ran ges as long as 50 - 70 A . L . F E IN E N D E G E N : In connection with the beta decay of 32P in D N A , there is evidence fro m variou s la b o ra to rie s that it takes fro m 33 to 35 eV in D N A to produce one sin gle-stran d break at 40°C o r at room tem perature, and about 115 eV to produce a sin gle-stran d break in frozen D N A . This m eans that there must be tra n sfe r of energy — because there are many places w here the p rim a ry event can occur, but the break in the strand occu rs at the p h o sp h o ru s-ester linkage.
IA E A -P L-61 5/15
259
S. A M IE L : I should like to return to the beta re c o il business mentioned e a r lie r . One experim ent which I believe has a bearin g on this discussion is th a to fM a c F a rla n e with monatomic la y e rs of beta-decay species, done with about a dozen o f them. What he observed w as that he found a v e ry n a rro w spectrum of re c o il e n ergies. He has a beta detector a c ro s s from the m onolayer of the beta species, with a 5 - o r 10-keV high voltage and electrostatic particle guides between. Instead of finding a wide dispersion of beta re c o il en ergies, he found a n arro w spectrum — the re c o il energy is taken up by the system itself, and not by the recoilin g atom. It must be something of a M ô s s b a u e r-lik e or re c o il-fr e e em ission of the fragm ents, and probably has some bearin g on the kind of dam age that can be done by the re coilin g atom after beta-decay. D .J . M A L C O L M E -L A W E S : I want to em phasize the comments e a r lie r about the tra n sfe r o f energy over v e ry la rg e distances. W hen you irrad iate gadolinium acetylacetonates, the fluorescence observed is in fact europium fluorescence, and this seem s to happen no m atter how much you pu rify the gadolinium . This observation indicates that the europium can pick up the excitation energy even when it is p resent only at the le v e l of parts per m illion. This resu lt seem s to be relatively insensitive to phase, and liquid-ph ase r a r e -e a r t h la s e r s can be constructed which use this p ro c e ss. F .S . R O W L A N D : Let me return to A d lo ff's question about hot atom effects in natural p ro c e sse s. I have with me an article in the International H erald Tribune o f 16 M ay, 1974 — that is, today's paper — which is con cerned with the effects o f 210P o in cigarettes as the source of lung cancer. T his p ro c e ss involves the chem ical effects of n uclear transform ations in term s o f w here the polonium ends up — on what size of aero so l p article. A . P . W O L F : N o. I know this argum ent w e ll. F .S . R O W L A N D : Y ou 've read M a r t e ll's paper? A . P . W O L F : Y e s — as a m atter of fact, I'v e written a couple of comments on it. The argument about 2 1 0 P o , and it 's one that the cigarette industry is taking v e ry seriou sly, is that 2 1 0 P b , the p re c u rso r, is collected by the le a f of the tobacco plant. T here are little h airs on the tobacco leaf, and every tobacco le a f has about a thousand h a irs p e r square centim etre, and these h a irs have a sticky substance on the end. These h airs concentrate 2 1 0 P b, with about a factor of 100 m ore 210P b in these little sticky h airs than anywhere e ls e . The argument is that when the tobacco is cured and smoked, p art of the 210P b is deposited in the lung in an insoluble form . Then, the insoluble 210P b gro w s into 210B i which gro w s into 2 1 0 P o , and it is then the alpha fro m 210Po which causes radiolytic dam age to the lung. The presum ption underlying all this is that the 210P b is not gradu ally washed out of the lung, but that it stays there. If you look at the ratios o f h a lf-liv e s , it is a v ery cute radiochem ical problem , but it doesn't have anything to do with hot atom chem istry. F .S . R O W L A N D : D oes not whether the 210P b is being picked up by the h a irs have to do with the size p a rtic le s on which it is present in the atmosphere? A .P . W O L F : No. F .S . R O W L A N D : You are saying that the h airs pick up everything in a ll sizes? A . P . W O L F : Right. S. A M IE L : Is this behaviour typical of the tobacco leaf, and not to others?
2 6 0
S H IO K A W A
A . P . W O L F : Y e s, the tobacco le a f is a v ery prim itive plant. It is the only one with all these little fe e le rs on the surface, thousands of them. A . G . M AD D O CK : H as anyone checked whether atomic lead is not liab le to oxidation and rendered soluble in the process? T h ere is this old busin ess that you've got to allow fo r the heat o f sublimation and atomization of lead, which a ll w ork s to your advantage in the oxidation p ro c e s s . I wouldn't be su rp rised if lead isn 't liable to be oxidized quite quickly in aqueous solution. A . P . W O L F : Oh, y es. T h at's what M a rte ll says. It's an oxidized fo rm of lead which is insoluble. F .S . R O W L A N D : What form? A . P . W O L F : He doesn't know what form .
IAEA-PL-615/16
INORGANIC SYNTHESES VIA BETA DECAY J.P. A D L O F F Laboratoire de chimie nucléaire, Centre de recherches nucléaires de Strasbourg, Strasbourg, France
Abstract IN O R G A N IC SYNTHESES V IA B E T A DE C AY . M o l e c u l a r s yn t h e s e s v i a b e t a d e c a y h a v e f o u n d w i d e i n t e r e s t i n h o t a t o m c h e m i s t r y , p a r t i c u l a r l y t o p r e p a r e u n k n o w n c o m p o u n d s . T w o e x a m p l e s i l l u s t r a t i v e o f t h i s m e t h o d f o r i n o r g a n i c sy n t h e s e s are p r e s e n te d . T h e se i n v o l v e b e t a d e c a y as an o x i d i z i n g p ro ce ss a n d t h e s y n t h e s i s o f n o b l e gas c o m p o u n d s .
The p rim a ry atomic and m olecular effects of pure /З-d e c a y are rather mild: a low re c o il energy amounting to a few electron volts, and a shake-off excitation which is not expected to produce severe damage. Quite sim ila r are the consequences of /З- 7 transitions in so fa r as internal conversion p ro c e sse s have a low probability. In many cases, th erefore, it can be foreseen that most of the daughter atoms rem ain at the place occupied by the ra d io active parent atoms before the decay. The final state of the daughters depends rather on chem ical and therm odynam ical properties and on the structure of the parent compound than on the effects of the transmutation. M olecu lar syntheses via 0 -d e c a y have found wide interest in hot atom chem istry, p articu larly with a view to preparin g unknown compounds. Owing to the extrem ely sm all quantity of products which can be form ed, the daughter nucleus must be radioactive, or in an iso m e ric state, or at least in a sh o rt-liv ed excited state if it is to be detected by M ossbau er em ission spectroscopy or perturbed angular correlation m easurem ents. So fa r, attention has been given m ainly to the 0 - de cay-induced syntheses of organic d erivatives of the elements. A gen eral review on the subject has been written by Nefedov et al. [ 1], and m ore recently by W ile s [ 2] with special em phasis on organom etallic compounds. O ur aim is to show, with two illu strative exam ples, the potentialities of /З-d e c a y fo r inorganic syntheses. Only "w o rk a b le " 0 -e m itte rs, i.e. with h a lf-liv e s la r g e r than about 1 0 min, are of interest.
1.
0 -D E C A Y AS A N O X ID IZ IN G PR O CESS
Starting with a radioactive parent in a given oxidation state, the daughter form ed in a 0 -d e c a y b e a rs a positive charge higher by one unit. Thus, high oxidation states, some of which m ay be unstable or unusual, can be observed. The use of 0 -d e c a y as a pathway to such states has been c le a rly demon strated by the firs t synthesis of perbrom ate [ 3]. In the decay of 83Se in selenate, brom ine in the oxidation state +7 is form ed according to 83SeOÏ = -
83
B rO ¡ + 0-
261
262
ADLO FF
A few other elem ents of the firs t long period of the periodic chart show a reluctance to fo rm compounds in which the element exhibits its highest oxidation state [4 ]. Thus, the only known pentahalide of A s is A sF g. But the neighbour germ anium fo rm s tetrahalides and also hexahalocom plexes with fluorine and chlorine. The 77Ge (11.3 h) 77A s (38.7 h) chain is v e ry convenient fo r the radiosynthesis of arsen ic derivatives and results on 7 7 GeI 4 are alre a d y available [ 5]. Although se v e ra l other decay schem es m ay be of interest, the radiochem ical analyses do not alw ays appear appropriate fo r the detection of high oxidation states. A particular oxidation state m ay be therm odynam ically unstable in aqueous solutions. T h erefore, the use of in -situ techniques (M o ssb au er em ission spectroscopy and p e r turbed angular correlation m easurem ents) is strongly suggested. With these methods, unusual valence states form ed in beta em ission have been detected, fo r instance in r a r e -e a r t h compounds [ 6 ], and in the 181Hf -* 181T a decay [ 7 ].
2.
N O B L E GAS C O M PO U N D S SYNTH ESES V IA 0 -D E C A Y [
8
]
The decay of tritium to helium and of radiohalogens to the heavier ra re g ases has prom pted many possible experim ents fo r obtaining noble gas compounds. It has been suggested that 3H decay in a solid KHF 2 lattice could fo rm the unknown helium difluoride HeF 2 ; other suitable chem ical environments could be T F or m ixed fluoride salts [ 9]. The amount pro duced by 20 Ci of tritium would reach 10Mmol in four or five months, a quantity sufficient fo r a spectroscopic determination. 20F being too short lived, the radiosynthesis of neon compounds re sts on the (3+-em itter 2 2 Na. Although never prepared, the oxide NeO might have an extrem ely slow decom position rate [1 0 ]. The preparation of NeO from the elements is out of question but the compound should grow fo rm a lly i n 2 2 N a 2 0 . Sim ilar hypo thetical pathways to argon compounds start with the /3- de cay of chlorine radioisotopes. Unfortunately a ll these daughter nuclei are neither radioactive nor M ossbau er candidates. F a r m ore re a listic are the radiosyntheses of krypton and xenon d e riv a tives. In addition to the in creased stability of their compounds, the heavy r a r e g ases o ffer convenient isotopes fo r radiochem ical and M ossbauer spectroscopic investigations. 2.1. Krypton compounds 83B r (2.3 h) decays to an iso m e ric state of 83K r (1.9 h). Attempts to detect 8 3 K r m0 3 in 8 3 В г Од have so fa r rem ained unsuccessful [1 1 ], probably because of in correct experim ental conditions, and further w ork on this decay scheme is n ecessary. F ro m M ôssbau er source experim ents (on the 9.1-keV transition of 8 3 K r m) it has been concluded that no K r0 3 species is produced as a resu lt of p -d e c a y in 8 3 B r 0 3K at 90°K [ 12]. Actually, the M ossbau er transition is preceded by a com pletely converted transition at the 41-keV level. H ow ever, despite this unfavourable step in the decay to the M ossbau er level, indications fo r chem ical bonds of krypton have become apparent fro m source experim ents with 83Se compounds [1 3 ]. The /3'decay of the latter nuclide has alread y been d escribed in the previous paragraph.
1АЕА-РЬ615/16
263
A ll source lines of selenium derivatives, with the exception of ZnSe, have la rg e line-w idths suggesting various degrees of chem ical bonding in the m atrix. The la rg e st line is observed with the 8 3 SeC> 2 source. The broadening has been a scrib ed to unresolved hyperfine interactions. F u rth erm ore, the noble gas atom has a la rg e re c o il-fr e e fraction which could reflect a chem ical bonding of the K r atom to its n earest oxygen neighbours. T hese encouraging observations should promote further w ork on the radiosyntheses of krypton compounds, fo r exam ple with 8 3 B r -la b e lle d B r F 3 and B rF 5.
2.2.
Xenon compounds T h ree radioisotopes of iodine decay to iso m e ric states of xenon nuclides:
131т ..P'iffi—p. i 3l x e m 1
I3 3 j
8
d
1 2
3-4%^1ззХеш h
2 0 .8
135j
30% r 6.7 h~
136
Xen
d
2.3 d
15.3 min
131Xe
►133Xe
-135Xe ■
5.27 d
9.2 h
70%
131I is re a d ily available in la rg e quantities, so that the low branching to the iso m e r state 1 3 1 X e m is not a hindrance. Num erous syntheses of xenon compounds form ed in the 0 -d ecay of 131I in inorganic and organic d erivatives of iodine have been investigated [ 1 ]. Most of the compounds form ed in organic iodom olecules (iodobenzene, diphenyliodonium salts, etc.) have never been prep ared chem ically. X e 0 3 is read ily form ed with 40 to 70% yields in iodates [1 4 - 1 5 ] and periodates [ 1 6 - 17] both in crystals and in acidic solutions. In I 2 O 5 [1 8 ] the yield is about 20%. A s expected, no bound xenon is observed in alkali iodides. S im ilarly, part of Xe is chem ically retained in IF5 [ 19]. The occurrence of these syntheses under natural conditions has been discussed [ 20]. Decay of fission iodine or 128I and 126I produced by reactions of fission -n eutron s on 127I m ay possibly fo rm X e0 3 in uranium ores. A s a m atter of fact, X e 0 3 has been found in n eu tron -irrad iated I2 O 5 . S everal xenon compounds have been observed by M Ossbauer em ission spectroscopy on variou s 1 2 9 I - and 1 3 1 I - labe lied iodine derivatives. B esid es the w ell-know n X e 0 3 and XeF 4 [ 21 ], they also include halides which have never been p rep ared [ 22 - 23]: XeCl^, X e B r 2 and XeBrg". The latter com pounds exist at least as long as the M ôssbau er le v e l (1 ns) and, although ephem eral, are of considerable interest. A ll astatine isotopes are 0" stable, so that the radiosyntheses of radon coumpounds r e lie s on the a - decay of radium.
264
ADLOFF
REFERENCES [1 ]
N E F E D O V , V . D . , T O R O P O V A , M . A . , S I N O T O V A , E . N . , Russ. C h e m . Re v. 3 8 ( 1 9 6 9 ) 883.
[2 ] [3 ]
W I L E S , D . R . , T h e r a d i o c h e m i s t r y o f o r g a n o m e t a l l i c c o m p o u n d s . A d v . O r g a ñ o m e t a l l . C h e m . 11 ( 1 9 7 3 ) 2 07 . A P P E L M A N , E . H . , J . A m . C h e m . S o c . 90 ( 1 9 6 8 ) 1 9 0 0 .
[4 ] [5 ]
D A S E N T . W .E ., N o n - E x is t e n t C o m p o u n d s , M a r c e l D e k k e r , N e w Y o r k (1965). H A L P E R N , A . , S A W L E W I C Z , K . , N u k l e o n i k a 23 ( 1 9 6 8 ) 9 21 .
[6 ] [7 Î
K H U R G I N , B ., OFER, S . , R A K A V Y , М . , P h y s . T e t t . 3 3 A ( 1 9 7 0 ) 4 . V A R G A S , J . I . , M T P I n t . R e v. o f S c i e n c e , R a d i o c h e m i s t r y , I n o r g a n i c C h e m i s t r y , S er ie s 1 8 , B u t t e r w o r t h , L o n d o n ( 1 9 7 2 ) .
[8 ]
A D L O F F , J . P . , R a d i o c h i m . A c t a 6 ( 1 9 6 6 ) 1.
[9 ] [1 0 ] [1 1 ]
P IM E N T E L , G .C ., S PR A TLEY , R .D ., M ILLER , A . R . , S cie n ce 143 (196 4 ) 674. N O Y E S , R . M . , J. A m . C h e m . S oc . 85 ( 1 9 6 3 ) 2 2 0 2 . M O U R IN , A . N . , NEFEDOV, V .D ., K IR IN , I.S ., G R A C E V .S .A ., GUSEV, Y .K ., C H A P K IN , G .N ., Z h . O b s c h c h . K h i m . 35 ( 1 9 6 5 ) 2 1 3 7 .
[1 2 ]
P A S T E R N A K , М . , S O N N I N O , T . , Phys. Re v. 1 6 4 ( 1 9 6 7 ) 384 .
[1 3 ]
H A Z O N Y , Y . , HERBER, R . H . , J. I n o r g . N u c l . " c h e m . 33 ( 1 9 7 1 ) 961 .
[1 4 ]
K I R I N , I . S . , G U S E V , Y . K . , D o k l . A k a d . N a u k . SSR 1 6 7 ( 1 9 6 6 ) 1 0 9 0 .
[1 5 ]
G U S E V , Y . K . , K I R I N , I . S . , I S U P O V , V . K . , R a d i o k h i m y a 9 ( 1 9 6 7 ) 7 36 .
[1 6 ]
M U R IN , A . N . , NEFEDOV, V .D ., K IR IN , I.S ., G R A C E V .S .A ., GUSEV, Y .K ., S AJKO V, Y .P ., R a d i o k h i m y a 7 ( 1 9 6 5 ) 631 .
[1 7 ]
G U S E V , Y . K . T l S U P O V , V . K . , K I R I N . I . S . , H i g h E ne rg y C h e m , 1 ( 1 9 6 7 ) 5 3 1 .
[1 8 ] [1 9 ]
K I R I N , I . S . , M U R I N , A . N . , N E F E D O V , V . D . , G U S E V , Y . K . , S E L I K H O V , G . G . , R a d i o k h i m y a 8 ( 1 9 6 6 ) 1 04 . M U R IN , A . N . , N E F E D O V , V . D . , K IR IN , I. S . , LEO N O V, V . V . , Z A I T S E V , V . M . , A K U L O V , G.P., R a d i o k h i m y a 7 ( 1 9 6 5 ) 629 .
[2 0 ]
G U S E V , Y . K . T l S U P O V , V . K . , K I R I N , I . S . , D I K H O N O V A , A . E . , G e o k h i m y a 7 ( 1 9 6 6 ) 1 04 .
[2 1 ]
P E R L O W . G . J . , PER LO W, M . R . , C h e m i c a l E ff e c t s o f N u c l e a r T r a n s f o r m a t i o n s — 1 9 6 4 ( P r o c . S y m p . V ie n n a , 1 964) 2 , IA E A , V ie n n a , (19 65 ) 443.
[2 2 ] [2 3 ]
P ER L O W , G . J . , —P E R L O W , M . R . , J. C h e m . Phys. 4 8 ( 1 9 6 8 ) 955 . PER LO W, G .J . , Y O S H I D A , H . , J. C h e m . Phys. 4 9 ( 1 9 6 8 ) 1 4 7 4 .
IA E A -P L-61 5/17
FISSION RECOIL CHEMISTRY S. A M IE L Department o f Nuclear Chemistry, Soreq Nuclear Research Centre, Yavne Z.B. A L F A S S I Department o f Nuclear Sciences, Ben Gurion University o f the Negev, Be’ er Sheva, Israel
Abstract FIS S IO N R E C O IL C H E M IS T R Y . A r e v i e w is g i v e n o f t h e a v a i la b l e i n f o r m a t i o n o n c h e m i c a l r e a c t i o n s o f f i s s i o n r e c o i l i n gaseous a n d i n c o n d en se d phases.
The m ajor difference between fission and other nuclear transform ations, such as /З-decay, iso m e ric transitions and (n, y) reactions, is the multitude of elem ents and isotopes produced in se v e ra l modes characteristic of fission, the enormous kinetic energy and the range of the fissio n re c o il atoms. These features are associated with a la rg e quantity of energy deposited in the medium, resulting in m olecular damage (bond ruptures, ionizations and excitations) which induces num erous chem ical p ro c e sse s. This review touches upon chem ical p ro c e sse s associated only with the incorporation in the final m olecular species of the fission fragm ent o r its decay product form ed in the im m ediate m atrix of the fissioning system . M ost of the reactions of fission re c o ils have been studied with the aim of either producing volatile compounds, fo r the sake of rapid separation of fission -p rod u ced nuclides [1 - 3 ], o r fo r the production of various labelled compounds [ 4 - 5 ] . A s fa r as the m echanism s of the reactions a re concerned, the m ajor difficulties in a system atic study a re due to the com plexity of the pro p erties of the recoilin g fragm ents, viz. their kinetic energy, excitation and charge state at the point of the chem ical interaction. A s a consequence, most mechanistic studies a re m ore easily perform ed in gaseous system s where m oderation and scavenging of slow ions and rad icals can be controlled to an appreciable extent.
1.
GAS P H A SE R E A C T IO N S
1.1. Halogens The gas phase reactions of fission re c o ils have been studied mainly fo r the halogens, brom ine and iodine. The reactants for such reactions
265
266
A M IE L and A L F A SS I
w ere m ainly methane and methyl iodide. M ost of the studies w ere undertaken fo r the sake of fast separation of the halogen nuclides fo r the m easurem ent of their independent yields and nuclear properties. D enschlag and Gordus [ 3] studied the products of halogen fission re c o ils with methane by gas chrom ato graphy and found methyl iodide and methyl brom ide to be the main products, together with s m a lle r amounts of vinyl iodide and ethyl iodide. An interesting finding w as the difference in reactivity ("se le c tiv ity ") between the p rim a rie s and the secondaries (nuclides directly produced in fissio n and nuclides form ed by (3-decay fro m p re c u rs o rs , resp ectively). The selectivity was proved and m easured by the follow ing methods: (a) C om parison of the yields of different isotopes with their known o r calculated independent yields; (b) the effect of addition of an inert gas m oderator; and (c) com parison of the yields im m ediately after irrad iation and after a delay period allow ing m ore secondaries to be form ed. The p rim a ry halogens react m ore efficiently than the secondaries both with methane and with methyl iodide. D enschlag e ta l. [ 6 ] studied the reaction of fission -p rod u ced iodine isotopes with gaseous methane and found that the reaction is m ainly with the p rim a rie s with only a sm all contribution fro m the secondary iodines. Silbert and Tom linson [ 7] studied the form ation of volatile products from the reaction of fis s io n produced brom ine isotopes with methane and found that the p rim a rie s react about thirty tim es m ore efficiently than the secondaries. A m ie l and co  w o rk e rs [ 8 , 9] studied the reactions of fission -produ ced brom ines and iodines with methyl iodide and found in both cases the reaction of the p rim a rie s was m ore efficient. One of the dominant explanations fo r this selectivity w as the difference in the kinetic energy between the p rim a rie s (in the m ega-electro n volt region) and the secondaries (a few to hundreds of electron volts). It seem s that the difference in the kinetic energy cannot be the m a jo r explanation since the energy of the secondaries resulting from decay of the v e ry sh o rt-liv ed p re c u rs o rs is not much low er, if at all, than the energy of the (n, y) produced iodine, w here quite high yields of the reaction with methane and methyl iodide [ 1 0 , 1 1 ] w e re found in the absence of scaven gers. P o ssib le causes fo r the different efficien cies are: (a) The different charge states [ 6 ]; (b) lo ss of the p re c u rs o r to the w all [ 7 ]; and (c) the p re c u rs o r is not a fre e atom but fo rm s a compound with a hydrogen atom and the 3 -decay does not have enough energy to break this bond, thus leading to the form ation of HI and H B r. Kikuchi and Church [1 2 ] studied the effect of the addition of an inert gas m oderator on the reaction of fission -produced iodines with methyl iodide. They p re fe rre d to study the yield of CH 2II and not of C H jI since the latter is also form ed by rad io ly sis. The com parison of their resu lts fo r 1 3 1 I, 133I and 135I with the known cumulative and independent yields of these isotopes led them to the follow ing conclusions: (a) The yield of the reaction of the secondaries is independent of the inert gas m oderator; and (b) the yield of the reaction of the p rim a rie s is lin early dependent on the m olar fraction of the inert gas m oderator. They suggested using the dependence of the yield on the m olar fraction of an inert gas m oderator fo r m easuring the independent fractional fissio n yields of other elem ents. A lfa s s i and A m ie l [ 13] studied the lin e a r dependence fro m the point of view of the kinetic theory of Estrup and W olfgang [ 14], and concluded that fo r lin ear dependence it is required that a[ea = cemo(] (where a is the respective average logarithm ic energy decrem ents with the reactant and m oderator) and hence recom m ended the use of CH 4 instead of CH 3I as a reactant.
1АЕА-РЬ615/17
1. 2.
267
Elem ents other than halogens
Only a few experim ents, many of them unsuccessful, w e re perform ed to study the gas phase reactions of fissio n fragm ents other than the halogens. Rowland and c o -w o rk e rs [ 15] suggested that since sulphur atoms react v ery efficiently with carbon monoxide to fo rm COS, selenium and tellurium would a lso react with carbon monoxide and fo rm COSe and C O T e. The reaction S + C O -* COS proceeds by therm al atoms and not by energetic atoms, as was prove by the effect of the addition of inert gas m oderators [ 15]. Baldw in e ta l. [ 16] studied the reaction of fissio n fragm ents with carbon monoxide and obtained quite a s m a ll yield of COSe and did not find C O T e at a ll. Strickert et al. 117] repeated this study and v e rifie d the absence of C O T e in a static reaction system . To check the possibility that C O Te w as form ed but decom posed before the analysis w as completed, they studied the reaction of fissio n re c o ils with carbon monoxide in a flow system and proved the form ation of a volatile tellurium compound. Owing to its rapid decomposition, the exact identity of this compound could not be studied, but it is reasonable to assum e that it is C O T e. One cannot conclude whether CO Te is an unstable compound o r that, because this m olecule is form ed in an excited state which is unstable, decomposition takes place, while the C O T e at ground state is stable. Baldwin et al. [1 6 ] also studied the reaction of fission fragm ents with gaseous carbon dioxide but did not find any COSe o r C O Te, in contrast to studies'with recoilin g sulphur atoms [1 5 ]. Since these experim ents w ere in a static system , it is p ossible that COSe and C O Te w ere form ed but decomposed because of high internal energy, and it may be possible to v e rify the possible form ation of these m olecules in a flow system . Z v a r a and c o -w o rk e rs [ 18-20] studied the reactions of fission fragm ents with vapours of metal ch lorides at high tem peratures (usually above 150 - 200°C). F o r exam ple, they studied [1 9 ] the reaction of fis s io n produced 97Z r with m ixtures of Z r C l 4, C I 2 , 0 2 etc. They found that the form ation of 9 7 Z r C l 4 was dependent only on the presence of Z r C l 4 , while the presence or absence of C l 2 and oxygen did not in terfere. They concluded that 9 7 Z r C l 4 is form ed by exchange of 97Z r ions with Z r C l 4 m olecules. Both p rim a ry 97Z r fission fragm ents and the secondaries resulting fro m the 13decay of 97Y take p art in this exchange. In other studies they followed the effect of different m etal chlorides, such as Z r , Nb, Ta, Mo, Se, T i and Sn [2 0 ]. The form ation of 9 7 Z r C l 4 does not depend on the nature of the gas used and it is assum ed that the m etal chloride vapours act on the reco il atom s as chlorinating agents, form in g m olecules of higher chlorides, e. g. 9 7 Z r C l 4 and 1°1.1°2moC15.
2.
C O N D E N SE D P H A SE R E A C T IO N S
In the condensed phase the reactions a re le s s controlled, the yields a re higher and the reactions le s s selective since the atoms a re m ore closely packed and there is the possibility of form in g a compound in m any-body collisions o r by su ccessive collisions. Thus in the condensed phase, the chances of a w id er range of fissio n products to fo rm compounds is much higher. The halogens react m ore efficiently in the condensed phase than in the gas phase: 20-30% of the fission -p rod u ced iodine reacts with liquid [ 6 , 21, 22] and about 40-60% with solid media [21, 23], com pared with yields
268
A M IE L and ALFA SS I
of about 10% with gaseous methane [ 6 ] . Neidhart et al. [2 4 ] reported on the stopping of 235U fission products in solid chlorides and the re le a se of chlorides and oxychlorides of s e v e ra l elem ents. In the reaction of fission re c o ils with liquid n-pen.tane som e indication of organic tellurium compounds was found [ 2 5]. Another kind of reaction of fission fragm ents, the displacem ent of the central m etal atom in a com plex m olecule, such as m etal carbonyls [2 6 ], ferro cen e [2 7 -3 0 ], m etal acetylacetonates [3 1 ], etc. was introduced by B aum gaertner and Reichold [2 6 ]. When fission re c o ils are stopped in chrom ium hexacarbonyl, the chrom ium is replaced by molybdenum with a yield of 60% [2 6 ]; this w as used fo r rapid separation by sublim ation fo r the detection of the 42-s isotope. M ore studies w ere made by Baum gaertner and his c o -w o rk e rs on the replacem ent of iron in ferrocen e by fission produced ruthenium [4 , 5, 2 7 -3 0 ]. F o rty -fiv e to sixty per cent of fis s io n produced 103Ru was found as c a r r ie r -f r e e 1 0 3 Ru-ruthenocene [ 1 0 3 R u (C 5 H 5 ) 2 ]. These high yields indicate that the labellin g was m ainly by secondary 1 0 3 Ru, form ed by )3-decay. The yield of 1 0 3 Ru-ruthenocene w as found to be dependent on the ratio uranium /ferrocene in the range 0. 013-0. 325, the yields being 60 and 45%, respectively. F u rth er studies [29, 30] confirm ed that the only origin fo r the form ation of ruthenocene are the secondaries form ed by |3-decay and the only role of the p rim a ry fission fragm ents is to tra n sfe r the fragm ents deep into the solid m atrix. This w as confirm ed a lso by the study [2 9 ] of the effect of s im ila r compounds with different central m etal ions (Ru, F e, O s), where a dependence on the chem ical bond strength was found; hence the reaction occurs in energy ranges close to, or sm a lle r than, the chem ical bond energies. Other reactions of fission products fa ll into the sam e categories already mentioned. W e would like to note, in addition, the reaction with tetraphenyl tin to give diphenyl telluride and triphenyl antimony [ 32, 33] and the reaction with solid p eriodic acid where elem ental iodine was found [ 34]. W e did not consider the reactions of fission re c o ils with m etallic and other solid envelopes and liquid m etals as used in re acto rs. The reaction of fission fragm ents with liquid sodium is an interesting subject which might be v e ry important fo r fa s t-b re e d e r reacto rs which have started to receive attention in the last few y ears [35, 36].
REFERENCES [1 ] [2 ]
A M I E L , S . , i n N u c l e a r C h e m i s t r y ( Y A F F E , L . , E d . ) 2 , A c a d e m i c Press, N e w Y o r k ( 1 9 6 8 ) 2 5 1 . H E R R M A N N , G . , D E N S C H L A G , H . O . . A n n u . Rev. N u c l . S c i. ¿ 9 ( 1 9 6 9 ) 1.
[3 ] [4 ]
D E N S C H L A G , H . O . , GORDUS, A . A . , Z . A n a l. C h e m . 226 (19 67 ) 62. B A U M G A E R T N E R , F . . C h e m i c a l E ffects o f N u c l . T ra n s fo rm a tio n s - 1964 (P ro c . S y m p . V ie n n a , 1964) 2,
[5 ] [6 ]
IA E A , V ie n n a (1965) 507. B OR N, H . J . , A E C A c c e s s i o n N o . 1 1 1 6 5 , R e p t . E U R - 2 2 0 9 . D E N S C H L A G , H . O . , H E N Z E L , N . . H E R R M AN N , G . , R a d io c h im . A c t a 1 (19 6 3) 172; D E N S C H L A G , И . О . , A E C A c c e s s io n N o . 1 46 3 3 0, Rept. N P -1 5 3 7 0 .
[7 ] [8 ] [9 ]
S IL B E R T , M . D . , T O M L I N S O N . R . H . , R a d i o c h i m . A c t a 5 ( 1 9 6 6 ) 2 1 7 . PA1SS, Y . , A M I E L , S . , J. A m . C h e m . S o c . 8 6 ( 1 9 6 4 ) 2 3 3 2 ; R a d i o c h i m . A c t a 4 ( 1 9 6 5 ) 1 5 7 . B R A U N , C . , A M I E L , S . , i n I A - 1 1 6 8 ( 1 9 6 7 ) 7 1 ; a n d I A - 1 1 9 0 ( 1 9 6 8 ) 1 0 4 ; B R A U N , С . ,M . Sc. piesented to W e iz m a n n I n s titu te , Re ho vo t, Is ra e l (19 7 1 ).
[ 1 0 ] R A C K , E . P . , G O R D U S , A . A . , J. C h e m . P hy s. 3 4 ( 1 9 6 1 ) 1 8 5 5 ; a n d 3 6 ( 1 9 6 2 ) 2 8 7 .
T hesis
269
IA E A -P L-61 5/17
[1 1 ] [1 2 ]
G ELD , Z . , M . S c. T hesis, W e iz m a n n I n s t itu te , R ehovot, Is ra e l (1 9 7 2 ); G ELD , Z . , A LF A S S I, Z . B . , A M I E L , S . . i n I A - 1 23 8 ( 1 9 7 1 ) 9 2 - 9 3 . K IK U C H I, М . , C H U R C H , L . B . , R a d io c h im . A c ta 19 (1973) 54.
[1 3 ]
A L F A S S I , Z . B . , A M I E L , S . , J. I n o r g . N u c l . C h e m . , i n press.
[1 4 ]
E S T R U P , P . J . , W O L F G A N G , R . , J. A m . C h e m . S o c . 82 ( 1 9 6 0 ) 2 6 6 5 ; P hy s. 3 9 ( 1 9 6 3 ) 2 9 8 3 .
W O L F G A N G , R . , J. C h e m .
[1 5 ]
LEE, E . K . C . , T A N G , Y . N . , R O W L A N D , F . S . , J. P hy s. C h e m . 68 ( 1 9 6 4 ) 3 1 8 .
[1 6 ] [1 7 ]
B A L D W I N , R . S . , P R O T O , D . O . , C H U R C H , L . B . , R a d i o c h i m . A c t a 16 ( 1 9 7 1 ) 9 4 . S TR IC K E R T , R . G . , A M IE L , S . , W A H L , A . C . , In o rg . N u c l. C h e m . L ett. 1 £ ( 1 9 7 4 ) 129.
[1 8 ]
Z V A R A , I . , Z V A R O V A , T . S . , C A L E T K A , R . , C H U B U R K O V , Y u . T . , S H A L A E V S K IJ, M . R . , R a d io k h im iy a 9 (19 6 7 ) 231 and S o v ie t R a d io c h e m . 9 ( 1 9 6 7 ) 226 .
[1 9 ]
Z V A R A , I . , T A R A S O V , L . K . , K R Z H 1VA N EK , М . , H U N G -K U E I SU, Z V A R O V A , T . S . , D o k l. A k a d . Nauk SSR 148 ( 1 9 6 3 ) 5 5 5 ; a n d S o v i e t P hy s. D o k l a d y 8 ( 1 9 6 3 ) 6 3 .
[2 0 ]
Z V A R A , I . , Z V A R O V A , T . S . , K R I V A N E K , М . , C H U B U R K O V , Y u . T . , R a d i o k h i m i y a 8 ( 1 9 6 6 ) 77 ; S o v i e t R a dio ch e m . (1966) 72.
[2 1 ]
D E N S C H L A G , H . O . , Z U R HEIDE, F . , H E N Z E L , N . . H E R R M A N N , G . , HUBSCHER, D . , K R A T Z , K . L . , c ite d i n Ref. [ 2 ] .
[2 2 ]
T S O U K A T O S , M . P . , T h e r m a l N e u t r o n I r r a d i a t i o n o f U r a n i u m - 2 3 5 , B r o m i n e , I o d i n e i n t h e P re s e n c e o f O r g a n ic L i q u id , P h . D . Thesis, U n iv e rs ity o f M i c h i g a n , A n n A rb o r (1 9 6 7 ).
[2 3 ]
C H E N , T . , Y E H , S . , N u c l . S c i. 4 (19 6 4 ) 65. (E d . In st. N u e . S c i. N a t ' l T sin g H ua U n iv e rs ity , T a iw a n ) .
[2 4 ]
N E ID H A R T , B . , B À C H M A N N , K . , LAUER, R . , SAHNER, H . P . , S T O J A N IK , B . , 7 th I n t . H o t A t o m C h e m is try S y m p . , J ü lic h , F .R . G e r m a n y , 1973.
[2 5 ]
Z U R H E ID E , F . , D i p lo m a r b e i t ( 1966).
[2 6 ]
B A U M G A E R T N E R , F . , R E I C H O L D , P . , Z . N a t u r f o r s c h . 1 6a ( 1 9 6 1 ) 9 4 5 .
[2 7 ] [2 8 ] [2 9 ]
K IE N L E , P . , W E C K E R M A N N , B . , B A U M G A E R T N E R , F . , Z A H N , U . , N a tu rw is se ns ch afte n 4 9 ( 1 9 6 2 ) 2 9 5 . B A U M G A E R T N E R , F . ,R E I C H O L D , P . , Z . N a t u r f o r s c h . 1 6 a ( i 9 6 1 ) 3 7 4 . B A U M G A E R T N E R , F . ,S C H O E N , A . , R a d i o c h i m . A c t a 3 ( 1 9 6 4 ) 1 4 1 .
[3 0 ]
COHEN, I . , G IL A T H , U n iv e rs ity , 1970.
[3 1 ] [3 2 ] [3 3 ] [3 4 ]
M E I N H O L D , H . , R E I C H O L D , P . , R a d i o c h i m . A c t a 11 ( 1 9 6 9 ) 1 7 5 . B L A C H O T , J . , C A R R A Z , L . C . , R a d io c h im . A c t a 11 (19 6 9 ) 4 5 . B L A C H O T , J . , V AR G AS , J . I . , C e ntre d 'é tu d e s n uc léa ires G re n o b le Rept. I N T / C H N / 6 7 - 0 1 (19 6 7 ). G REENDALE, A . E . , LO V E, D . L . , D E L U C C H I, A . A . , A n a l. C h im . A c ta 34 (19 6 6 ) 32.
J . , in I A - 1 1 9 0 (1 9 6 9 ) 1 02, I . N i t z a n ( C o h e n ),M . S c . T hesis,
Hebrew
[3 5 ]
C A S T L E M A N , A . W . , J r ., T A N G , I . N . , M a c K A Y , R . A . , A E C A cce ssio n N o . 4 4 8 1 , Rept. B N L -1 0 7 2 7 .
[3 6 ]
C LIFFO R D , J . C . , W IL L IA M S , J . M . , M cG U IR E , J . C . , in A l k a l i M e t a l C o o la n ts ( P ro c . S y m p . V ie n n a , 1966), IA E A , V ie n n a (1 9 6 7 ) 7 59.
IA E A -P L-61 5/18
APPLIED HOT ATOM CHEMISTRY, LABELLING OF COMPOUNDS, AND ISOTOPE PRODUCTION A.P. W O LF Chemistry Department, Brookhaven National Laboratory, Upton, Long Island, N .Y ., United States o f America
Abstract A P P L IE D H O T A T O M C H E M IS T R Y , L A B E L L I N G O F C O M P O U N D S , A N D IS O T O P E P R O D U C T IO N . T h e g e n e r a l a sp ec ts o f a p p l i e d p r o b l e m s w h i c h c a n be o f c o n c e r n t o t h e h o t a t o m c h e m i s t a re p r e s e n te d . T h e p r e p a r a t i o n o f l a b e l l e d r a d i o p h a r m a c e u t i c a l s is dis cu ss e d w i t h s o m e e m p h a s i s g i v e n t o t h e p r o d u c t i o n o f r a d i o n u c l i d e s as w e l l as t h e c h e m i s t r y o f l a b e l l i n g . O t h e r a p p l i c a t i o n s , i n c l u d i n g p r e p a r a t i o n o f o r g a n o m e t a l l i c s a n d c o m p l e x e s o f a ll t y p e s a n d a lso a n a l y t i c a l c o n t r o l a p p l i c a t i o n s , a re disc us se d.
The direct application of re s e a rc h in hot atom chem istry to the field of nuclear medicine in the development and preparation of. radiopharm aceutipals has no p a ra lle l in any other a re a of hot atom chem istry. In attesting to this fact one need only consider the num ber of young scientists who did their re s e a rc h in hot atom chem istry, who a re now employed in nuclear medicine related endeavours and who still use hot atom chem istry techniques and knowledge in the daily execution of their work. The em phasis on hot atom chem istry in the literatu re of these other disciplines is another indication. Another perspective of this strong influence can be seen in the fact that groups in these other disciplines (nuclear m edicine and radiopharm aceutical re se a rc h and application) d eliberately seek people trained in hot atom chem istry in o rd er to apply that discipline in these new a re a s. Thus, young scientists a re being sought fo r their skill as hot atom chem ists rath er than as chem ists gen erally qualified fo r a particular job. The title of this short paper is indeed broad and would req u ire a long m onograph to do it justice. My aim is to focus attention on som e of the m ore gen eral aspects and problem s that the hot atom chemist can concern h im self with. T h ere is now a fa ir amount of inform ation in the literatu re to allow the chem ist to see the broad perspective of the in terdisciplinary aspects of radiopharm aceutical re s e a rc h and production. The m ost recent compendium that can be used as a starting point are the P roceedin gs of a Symposium on Radiopharm aceuticals and L a b e lle d Compounds, held by the IA E A and W HO in Copenhagen 26-30 M arch 19731. Many of the pertinent re fe re n c e s can be found in the article by W olf, Christm an, F o w le r and Lam brech t, p .345, Vol.I. An additional bibliography of about 800 referen ces is available fro m the author of this comment. 1 I A E A , R a d i o p h a r m a c e u t i c a l s a n d L a b e l l e d C o m p o u n d s ( P r o c . S y m p . C o p e n h a g e n , 1 9 1 3 ) 1 a n d 2, I A E A . V ie n n a (1 9 7 3 ).
271
272
W O LF
Hot atom chem istry had its initial and m ost direct impact in developing radiopharm aceuticals labelled with carbon-11, nitrogen-13, oxygen-15, flu o r in e -18 and io d in e -123. The re se a rc h and development needed to prepare a p articu lar radiopharm aceutical starts with production of the nuclide bym eans of an appropriate nuclear reaction. The most prevalent important "re a g e n ts" are protons, deuterons, h e liu m -3 and h e liu m -4. Photons for photonuclear reactions have also been used but only to a very lim ited degree. The sam e is true fo r fast neutrons. The nuclide can be prepared as starting m aterial (using standard radiochem ical techniques) in the form of some sim ple product p re c u rs o r (usually using hot atom techniques) or occasionally in the form of the final product (using radiochem ical or hot atom techniques). Once the nuclide has been obtained it can be used in ordinary synthetic or biosynthetic p ro c e sse s in o rd e r to incorporate it into suitable compounds. A nalytical control of these compounds is then the last step before delivery to the u se r (either fo r bio lo gical re se a rc h , preclin ical evaluation or clinical evaluation). T here is perhaps one notable exception h ere, namely the use of excitation labellin g which deals with nuclide incorporation after parent nuclide production. It would be my contention that hot atom chem ists, be they p rim arily radiochem ists, organic chem ists or physical chem ists, would be best suited to c a rry out the realization of radiopharm aceutical preparation from target bom bardm ent to the delivery of compound. T h ere are, however, some p e rip h e ra l aspects of the field to which hot atom chem ists can make important contributions. I would include here the radiolytic self-destru ction of the product (which draw s heavily on the field of radiation chem istry) and the effect of the results of the nuclear decay pro cess on the compound in situ (organism , anim al or man: B io lo gical effects of nuclear transform ations). A review of the early literatu re of this latter field shows a total lack of involvement by hot atom chem ists with a resulting confusion in interpreting som e resu lts. This is now changing. A spects of nuclear medicine such as absorption edge scanning and the use of potentiators for therapy in in -situ nuclear decay therapy, which could also profit from overlap with hot atom chem istry, w ill not be considered here.
N U C L ID E P R O D U C T IO N T h ere are a number of aspects of nuclide production: choice of target nuclide, availability of excitation functions, design of target (gas, liquid or solid), special problem s in dissipation of heat and controlling unwanted ra d io ly sis in the target, health physics considerations in facilitating fle x i bility and cost of preparation, development of on-line methods, emphasis on nuclides of short h a lf-life with appropriate decay schem es. One of the most notable features in starting w ork in this are a is the lack of accurate, published, excitation functions. The lack of data in this field is perhaps understandable since radiochem ists and nuclear chemists in the past had no re a l need to consider their efforts from the point of view that someone who, in fact, might need these data for production work. T a rg e t design presents many problem s. Since yield is of great practical im portance there has been a shift tow ards using gas targets wherever possible. This shift has been a direct resu lt of re se a rc h in hot atom
IAE A -P L-61 5/18
273
chem istry. To cite just one exam ple, the use of gas targets has replaced the use of boron oxide as a source of c a rb o n -11. Each topic noted in the firs t paragraph can be sim ila rly documented.
P R E C U R S O R P R E P A R A T IO N It is here that hot atom chem istry is perhaps most directly applicable. The important problem s include: the development of on-line methods fo r p re c u rs o r preparation utilizing hot atom chem istry and radiation chem istry to re a liz e product form ation; production of the nuclide in the " c a r r i e r - f r e e " state; choice of target if the element is to be its own c a r r ie r for the nuclide produced, e.g. suitability of the target for direct use in further reaction; and choice of target for direct use of the nuclide. H ere is a rich field for re se a rc h and application of hot atom chem istry. One might focus on the production of a " c a r r i e r - f r e e " nuclide and indeed ask how one determ ines that it is truly " c a r r i e r - f r e e " . The preparation of a " c a r r i e r - f r e e " nuclide is facilitated if the target and subsequent processin g do not put it into contact with other nuclides of the same element. The boundaries of " z e r o " contamination become critic a l when one considers the nuclides currently being m ost actively studied: carbon, nitrogen, oxygen, the halogens. The importance of producing the highest specific activity p o ssible is inherent in the nature of the use of a radiopharm aceutical. Loading dose effects a re amply documented in the literatu re. The product should be a true tra c e r and not have concomitant drug action. An aspect of p re c u rso r preparation (o r final product preparation) which is again p articu larly germ ane to problem s in hot atom chem istry is involved in the chem istry of n itro g e n -13 and oxygen -15 labelled radiopharm aceuticals. In contrast to tritium and carbon-11, the inform ation on the hot reactions of n itro g e n -13 and o xy gen -15 is sp arse indeed. Much of what is available is concerned only with phenomenological approaches to studying the isotope. B asic inform ation is c le a rly needed. A further aspect has to do with the choice of nuclide such that a parent useful for excitation labellin g can be prepared. This w ill be considered further under labellin g methods.
L A B E L L IN G M ETHODS Conventional labellin g methods have been amply documented and hardly need comment here. Exploitation of non-synthetic methods such as re c o il methods, radiolytic methods, labellin g by exposure to atomic species, accelerated ion methods and excitation labellin g methods are again the province of the hot atom chem ist. Much needs to be done here and the problem s are best exem plified by considering excitation labelling methods. P erh ap s one of the most useful methods of labelling radiopharm aceuticals, in a n on-specific way, is excitation labelling. The importance of this method stems from two facts. The most obvious is that it is frequently successful. L e s s obvious is the fact that many potentially useful compounds cannot be conveniently labelled by conventional m eans and thus the necessity to p repare such a compound for testing must re ly on le s s conventional methods. In excitation labellin g the labellin g nuclide is either positively or negatively
274
WOLF
charged or it is neutral. It may be electronically excited and it may have som e sm all resid u al kinetic energy but it is not its kinetic energy which is respon sible for its efficacy. The charge state and excitation are responsible fo r its "la b e llin g p o w e rs". The requirem ents fo r this nuclide are sim ple. It should label the substrate in a single product determ ining bim olecular reaction. It is usually generated by bom barding a nuclide, X, to produce a parent, Y , which decays with a convenient h a lf-life to give the labelling daughter, Z . It is important that Y be produced in a radionuclide that is in a pure form and the relevant nuclear reaction should have a la rg e c ro s s section for particles which can be obtained in high beam currents. The m ajor p ro c e sse s which resu lt in charged species from the parent Y as we know, are negatron decay, positron decay, electron capture, and internal conversion. The h a lf-life of Y should be minutes to at m ost a few hours. The prop erties of the labellin g nuclide Z should include t ^ Z =10 s to a few days, Y -* Z should resu lt in a high proportion of the charged state, the Y -* Z decay should resu lt in negligible re c o il energy for Z and, finally, Z should have a sim ple decay scheme. An exam ple of what can be accom plished in excitation labellin g involves the decay of 123Xe in C l 2 gas resulting in a 100% yield of 1 2 3 IC1. Another example of labelling is the decay o f 123Xe on indocyanine green cry stals resulting in a non -specifically labelled product in 20% yield, of potential use as a liv e r function agent. Fortunately already som e excellent basic w ork has been done in this a re a by hot atom chem ists, but much m ore needs to be done before a method as sim ple as this one can be routinely re a liz e d for la r g e -s c a le production. An are a of great im portance in labelled compounds has to do with inorganic compounds, organom etallics and com plexes of all types. The surface of this vast a re a has hardly been scratched. The application of inorganic hot atom chem istry is obvious and necessary. F o r exam ple, the very high yields one som etim es observes in inorganic salts can be made use of for direct application in synthesis. The selectivity of com plexes in reco il reactions and the application of annealing may w ell afford new routes to useful compounds.
A N A L Y T IC A L C O N TR O L W hile this may seem fa r rem oved from hot atom chem istry, it must again be pointed out that the p articu lar interests and training of a hot atom chemist put him in an advantageous position in appreciating the problem s. R adiochem ical and radionuclidic purity are of p rim ary interest. The peculiar problem s relating to radiochem ical and radionuclidic purity are the daily concern of the hot atom chemist. T ra c e contaminants and the questions they r a is e have been addressed in alm ost every responsible re s e a rc h paper that has appeared in the literatu re of hot atom chem istry. H ere again the hot atom chemist can bring his special talents to bear, in addition to utilizing his expertise in radioassay.
C O N C LU SIO N S In considering labellin g of a radiopharm aceutical and the application of hot atom chem istry one should not lose sight of the fact that the potential for
IAEA-PL-615/18
275
la r g e -s c a le production of the nuclide should be high (the scale is defined by the m edical community need; thus, " la r g e " is a subjective term ) and the chem ical and radiochem ical yield of the radiopharm aceutical should be optimized. Much interesting basic re se a rc h in the biological and m edical fields has been done using labelled compounds with no or very lim ited clin ical application. H ow ever, the underlying driving force in m edical re se a rc h is to be able to apply any new and useful finding to the treatment of disease in the gen eral population. This is as true of radiopharm aceuticals as it is of any other drug or technique the m edical p rofession ultimatelyuses. W hile these comments may be obvious, their substance has been ignored in many of the efforts involving direct use of cyclotron produced nuclides. The purpose of this report was not to outline in detail each aspect of re se a rc h and application in radiopharm aceutical production, but rath er to touch b riefly on just som e of the problem s that a ris e in the period between "b e a m -o n -ta rg e t" and "d e liv e r y -t o -t h e -u s e r " . It is hoped that some appre ciation of the ro le of the hot atom chemist has becom e apparent in this p rim a rily heuristic exercise. The intention was to stimulate discussion rath er than to provide an extensive monograph. What is m o re essential, however, is to note the im portance, or even the necessity of a great deal m o re effort in basic re se a rc h in hot atom chem istry as a whole in o rd e r to develop fully the benefits this branch of chem ical science can provide for an applied field.
D IS C U S S IO N A .P . W O L F : A great deal m ore basic re se a rc h needs to be done in conjunction with applied work. One doesn't w ork without the other. Hot atom chem istry has hardly come to the point that it can provide a ll the an sw ers for people who are going to work with radioactive pharm aceuticals. I think that one must have p a ra lle l development of basic re se a rc h and applied u ses in the field of radiopharm aceuticals. I must say that, having started out entirely in basic re se a rc h and now having m ore than half my group working in the applied area, everyone finds it a satisfactory practice, and just as challenging as concentrating on the solution of basic problem s alone. But I think the reason that the laboratory people are excited is that they have one foot in basic re se a rc h and one foot in applied work. I think that they would be desperately unhappy if they w ere forced to do only applied work because they a re able to see every day in their applied work the tremendous need for m ore basic inform ation. If they have the option of following some of the basic aspects at the sam e time, it makes fo r a very exciting laboratory experience. G. H A R B O T T L E : I think D r. W olf has opened up our horizons con sid erably regard in g what constitutes hot atom chem istry, and I think that this is important. It was probably not the intention of the o rgan izers of this panel to re stric t our consideration to the very narrow chem ical effects of nuclear transform ations, as ord in arily understood, but to include all the p erip h eral field s, many of which W olf has mentioned in his talk â&#x20AC;&#x201D; fields which a re going to be important for those working in radiopharm aceuticals. He has certainly m ade a good case for the broad, interdisciplinary nature of hot atom chem istry.
276
W O LF
G. S T O C K L IN : I am very glad that D r. W olf has mentioned not only isotope production and labellin g by (a) re c o il, and (b) excitation, but also the bio lo gical effects of nuclear transform ations. The decay of radioactive atoms in a biom olecule can be used as a tool for m ic ro su rg e ry in m acro biom olecules, not only to get inform ation on the reactions of biom olecules, but also with the possibility of therapeutic aspects. An elaborate review of this field was published by K risch in 1969. D .M . R IC H M A N : P e rso n has also published a review of this field. G. S T Ü C K L IN : I have a question about decay-induced labelling for the inorganic chem ists. A s W olf mentioned, the 123Xe decays 72% by electron capture to i 2 3 l n+ , and 28% through beta-plus to form 1 2 3 l '. These a re then the p rim ary reactive species, even though we a re not sure which ionic state they w ill eventually react from in the condensed state. In the gas phase they are certainly there as ions; this is evidence that in the condensed phase they undergo atomic reactions. But I want to make a p ractical point. The gas exposure technique has been w ell known since the tritium decay experim ents of W ilzbach, even though somewhat different there since it also involves radiation damage. The recent experim ents in W o lf's and W e lc h 's laborato ries, and in ours, have used liquid-m olten solid system s. Unfortunately, most of the biom olecules are solids, and the iodine yields from the decay of xenon are very low, generally only 1 to 3%. Now, there is a possibility of getting around this problem , and it involves some inorganic chem istry. W e have exposed К Ю 3 with 123Xe for six hours — the m aximum for growth of the 123I daughter — and have then used this K I0 3, which, so to speak, "contains" the 123I daughter ions, and have dissolved this in m ineral acids such as 0.1N H C l containing the substrate biom olecule. A fte r the solvation pro cess has taken place, you obtain te rrific yields, 60-80%, and c a r r i e r - f r e e to the extent that one cannot detect m acroscopic related compounds by rather sensitive physical detection methods. This is a method of high practical use, because it is extrem ely rapid, and it is also a m ild p ro c e ss. Now, my question for the inorganic chem ists is, "H ow do you think this pro cess w o rk s? " Is it possible that this se rv e s as m atrix isolation for a reactive iodine species, or what? F .S. R O W L A N D : Room tem perature? G. S T O C K LIN : Y es. G. H A R B O T T L E : One would guess that the xenon decay probably leaves you with an iodide ion or an iodine atom, and the presence of the oxidizing agent generates I 2 in high specific activity, and you are then iodinating your target. You don't think this is true? G. S T Ü C K L IN : No. G. H A R B O T T L E : M olecular iodine would not iodinate the substrate? G. S T O C K L IN : What would iodinate the compound is either I+ or hypoiodite, or any type of so -c a lle d "p o sitive" iodine. G. H A R B O T T L E : It is very possible with the Ю 3 that you may be boosting your tra c e r iodine up to the I+ state. G. S T O C K L IN : This whole pro cess is a c la s s ic a l exam ple in which inorganic hot atom chem istry is very useful. A .G . M AD D O CK : This reaction is known kinetically to go through the I+ state, and ... A .P . W O L F : Which specific reaction are you talking about? A .G . M AD D O CK : The iodide-iodate reaction.
IA E A -P L-6 1 5/18
277
G. S T O C K LIN : Y e s, the c la s s ic a l iodinating reaction — iodide/iodate/ iodine proceeds by hypoiodite in equilibrium through disproportionation, which we probably circumvent by this method. A .G . M AD D O CK : Y e s, I think that what happens is that your substrate is mopping up the I+ state as it is form ed. G. H A R B O T T L E : T h ere is also the point that c a r r ie r -f r e e iodine alw ays behaves peculiarly. I rem em ber that m ore than 20 y ears ago at H a rw e ll there was a great amount of interest in c a r r ie r -f r e e iodine and the possible iodination of protein m olecules with it. H ow ever, they couldn't get the c a r r i e r - f r e e iodine out of the end of the apparatus because it was iodinating dust particles. A .P . W O L F : I think that the distinction needs to be made here between hot atom chem istry and excitation labelling — there are so many different aspects here; that's what makes it so interesting. If you allow the xenon to decay with C l 2, this is excitation labelling. But in the case StScklin is talking about, he allow s it to decay on to К Ю 3, and this la b e ls some p re c u rso r fo r the subsequent iodination p rocess. When this is dissolved in HC1, I think you have the c la s s ic a l method fo r making ICI. But you have to rem em b er here that this is in the c a r r ie r -f r e e state, so that it's hard to say what the iodinating agent is. It is probably I + — solvated I+. G. S T O C K LIN : I agree that the experim ents I have reported are not excitation labellin g in the usual sense. It has nothing to do with direct excitation labelling. W e have considered that the reaction possibly proceeds by IC I... A .P . W O L F : ICI? Shouldn't it be IC1¡ ? G. S T O C K LIN : We have looked with h ig h -p re ssu re liquid chrom ato graphy after dissolving the iodate, and it is not observed. So the reaction is probably not proceeding by IC12. The method also works in other m ineral acids such as acetic acid. The discovery of these high yields was itself rath er serendipitous — now we need to understand the m echanism of the labellin g p rocess. K. R O SSLER : In a special experim ent with desoxyuridine, with its rather isolated double bond, we have applied totally p a ra lle l procedures for iodine and fo r astatine. Obviously the astatine doesn't form the AtO " state — it goes directly to the At+ state. We now have the electrophilic astatonium ion, stable — we can prove by chrom atography that this ion exists. Now, when we apply it to the double bond, the reaction with astatine doesn't work. The m echanism appears to be sp ecifically additional to the double bond in the desoxyuridine case, and the experim ents work with iodine and not with astatine. So I just doubt that the attacking species in the iodine experim ents is I +. It must also be discussed whether the reaction is through IO ’ . With astatine, the labellin g is le s s than 1%, while with iodine, 60-70% labellin g is observed. A .G . M AD D O CK : I suggest that you gentlemen should read the Q u arterly R eview of the Chem ical Society of about 10 to 15 y e a rs ago for a discussion of the peculiarities of iodine at great dilution. The equilibrium constant for the se lf-d isso ciatio n in an aqueous medium is known at least to a precision of a factor of 3. The equilibrium constant for the interesting compound HIO is also known — on the one hand, it can dissociate to I+ and OH , but with an appropriate change of pH you can swing it over to IO " and H +. If you look at these data I think you can explain alm ost a ll the p eculiarities I have ever seen reported.
278
W OLF
L . L IN D N E R : How intimately was the xenon m ixed with the iodate in the Stocklin experiment? W as it in a condensed phase? G. S T Ü C K L IN : Y e s , we had solid iodate in an ampoule; xenon condensed on it; and the gas volume was com paratively sm all relative to that filled by the solid. L . L IN D N E R : I r e c a ll that in our work on the re c o il chem istry of iodine we have retriev ed most of the iodine as iodide. H ow ever, when the compound is annealed, the retention goes up to a very high value, indicating that there is some species present in a positive state. I don't think that this state is very much different from the one you have created by decay in your crystal. A fte r all, your re c o il energy is still considerable in this case. It is in the range of 30 eV from the beta decay p rocess. N . G E T O F F : Is 0 2 present during the procedure in which xenon is in contact with the К Ю 3? G. S T Ü C K L IN : Not in m acroscopic amounts. The system was pumped out fo r a long tim e, but certainly you cannot avoid traces of oxygen at the su rface. You alw ays have traces of oxygen at the surface of these solid cry stals. N. G E T O F F : P erh ap s peroxy ra d ic a ls are form ed from the xenon decay — so to say in the frozen state because of the solid. Then when you dissolve the system , these peroxy ra d ic a ls start to react and produce with organic m olecules fre e rad icals which are v ery reactive towards iodine. Am ong other things then, the reaction could be started by the presence of peroxy ra d ic a ls. G. H A R B O T T L E : I think we have had enough discussion of this specific technical point. I'd like to return to the basic thrust of W o lf's paper, which is on applied hot atom chem istry, and ra is e the issue of the "continuousproduction" nature of the pharm aceutical business. Short h a lf-liv e s imply v ery close proxim ity of the production centre to the location where they w ill be used. You need hospitals with a cyclotron close by, and this changes the whole situation. The intimate association, of the hot atom chemist directly in the production is a very interesting aspect of the use of these sh o rt-lived radiopharm aceuticals. I think W o lf's work amply dem onstrates why the short h a lf-liv e s are v ery important in the further development of these techniques. A .P . W O L F : I'd like to comment on this problem of short h a lf-liv e s as food fo r thought fo r hot atom chem ists. F o r m edical applications, short re a lly m eans short. W e are at present working on an isotope with a 1 7 .5 -second h a lf-life in m edical applications. When you are working with infants, if you w ere to give them the norm al radiopharm aceutical, the infant — either m ale or fem ale — would be exposed to a m edically damaging amount of radiation. This is particu larly true with fem ales, and is somewhat less true with m ales who seem to be le ss radiation sensitive throughout their liv e s. Consequently, the radioisotope must have a very low dose. When you do the calculations with this particular isotope, you find that the infant rec e iv e s a dose of 1 0 - 4 rad per m edical investigation, and that is below the le v e l any m edical people consider to be dangerous. The isotope we a re working on is 7 7 Sem , and w e've run into som e very strange things. A s a m odel fo r this system , we use th e 7 5 S e -75B r decay. This is a so -c a lle d generator system — you place 75B r on the column and allow it to decay to 7 5 Se, and then you strip the column. In using this model, we thought we had everything solved. Then we converted to 7 7 B r , and allowed
IAE A -P L-61 5/18
279
it to decay on the column, and stripped 7 7 Sem from the column. A sid e from the technical problem that the 77B r breaks through the column, we also find that the 7 7 Sem com es off the column in a different oxidation state from that of 75Se in the m odel system . O bviously, hot atom chem istry is doing its dirty w ork h ere, but w hat's going on? W e don't have the complete answ er yet, but one of them decays p rim a rily by electron capture, and the other p rim a rily by positron em ission. So this is an interesting hot atom problem , com pletely aside from the fact that w e 'r e trying to make an isotope for use in infants. G. H A R B O T T L E : I think we should congratulate D r. W olf and his group for having discovered what hot atom chem ists call "isotopic e ffects", which have only been known for about 25 y ears. S. A M IE L : Why do you use a radioactive isotope for this purpose? Couldn't you have used a stable one? A .P . W O L F : No. Not for instantaneous blood volume m easurem ents. Y ou can't use a stable isotope. G. H A R B O T T L E : Is this because you have to use such a minute amount of selenium? B ecause it is poisonous? A .P . W O L F : No. This is an in -v iv o m easurem ent, a dynamic m e a su re ment, and you want to m easu re not only blood volum e, but also blood flow. With a stable isotope, you have to withdraw blood and that is in itself a very dangerous procedure in the types of infants on which such m easurem ents must be made. You can only get an in -situ in -vivo m easurem ent with ra d io active isotopes — a dynamic m easurem ent of how fast the blood is flowing, and what its volume is, in different sections of the body. S. A M IE L : B ecause you don't want to take any sam ples out? A .P . W O L F : C orrect. F .S . R O W L A N D : Do you have to make an injection? A .P . W O L F : Y e s. G. S T O C K L IN : With a n C -la b e lle d compound applied as a radioph arm a ceutical for some diagnostic study, you know v ery w e ll that you have the problem not only of sterility but also of apyrogenicity. The test for apyrogenicity is a rabbit test — what do you do with a sh o rt-liv e d isotope? G. H A R B O T T L E : With a h a lf-life of 17 s. A .P . W O L F : One can get around this problem in different ways. F ir s t of all, the restriction s in the USA are g re a te r than in any other country in the w orld. T hat's both fortunate and unfortunate because of the amount of w ork w e've had to do to dem onstrate sterility and apyrogenicity, but what one does is to do ten runs and check those by standard techniques. It doesn't make any difference whether the isotope is still in there, or whether it has decayed — the solution is either sterile or p y ro g e n -fre e , or it isn't. And that certainly was true 1 0 s after you produced it if it is still sterile and p y ro g e n -fre e two days later. The rabbit test takes about two days, and that's slow . A new method has been developed called the lim ulus test, which u ses protein from the horseshoe crab, and that test takes about 15 min for determ ining whether the solution is p y ro gen -free. In producing these radiopharm aceuticals, we save a sm all sam ple of everything that is used in a human being. If there is an ad verse effect after we have gone through the p relim in ary proofs, we then check back to see whether the m aterial that left our laboratory was im pure. W e also check every n C radiopharm aceutical that we deliver while it is being delivered — using the lim ulus test. Just b efore the m a te ria l is injected, they get a go/no go statement fro m us. W e w ere origin ally v ery w o rried about this because
280
W O LF
we didn't know very much about biochem istry. But so fa r, we haven't had a single failu re in production. G. S T Ü C K L IN : This new test you mentioned — the lim ulus test — is this published? A .P . W O L F : No. It has not yet been accepted by the US Pharm acopoeia. J .L . SE R V IA N : Recently, we had a re se a rc h coordination meeting to discuss these rapid methods for the quality control of radioactive pharm a ceuticals, especially the one mentioned for the detection of sterility — using the substrate labelled with 1 4 C, and detecting the 1 4 C 0 2 to determine the activity of the m icro o rgan ism s in the pharm aceuticals. This is also a very fast method com pared with all the methods in a ll the pharm acopoeias. In that meeting, we have also discussed the lim ulus test mentioned by D r. W olf. I think that it is necessary to proceed in one of the two ways mentioned by D r. W olf. One way is to work in a very clean m anner in o rd e r to avoid the problem s of sterility or the presence of pyrogens in radiopharm aceuticals. The other possibility is to use these rapid methods fo r the determination of pyrogens and m icroorgan ism s using these rapid methods. The problem then is that these methods are not standard methods accepted by the pharm a copoeia. It is possible to use these methods in re se a rc h . The participants in our panel felt that the resu lts with these methods a re very good and that they w ill be accepted by the pharm acopoeias in the future. Still, we need much additional w ork in o rd er to have the acceptance. G. S T Ü C K L IN : Is this report available? J .L . SE R V IA N : The report w ill be published by the IA E A . L . L IN D N E R : I feel obliged to say something in a gen eral way, but D r. W olf has already said it very well. W e are now putting a re a lly sub stantial amount of work in this field in our institute. It is very apparent that a ll our previous experience as radiochem ists in general, and as hot atom chem ists in particu lar, are of utmost value in applications to this field. W olf has touched on the problem s of purity and quality control. W e had to point out to the pharm acists and the u se rs of 131I radiochem ical preparations that certain problem s of radiochem ical purity existed, which they had never observed, but which we ran a c ro ss because of our techniques. A .P . W O L F : Having red isco vered "isotope effects", I 'l l now red isco ver the A u ger coulomb explosion. Studies with tritiated u ra c il have shown that the lethality of the sam e level of tritium content has v aried by a factor of ten depending upon the position in which the tritium is substituted into the m olecule. This is obviously not an effect of the beta radiation itself, since that is the sam e fo r isotopes substituted-into different positions. I think that this is a c la s s ic a l example of what happens to an organism when the nuclide decays — the genetic effect. The problem is — what happens to the m ole cule that is left behind in a beta decay event. What happens to the decay species itself, in this case 3 He, is not very important; what is of consequence is what happens to the resid u al rad ical left after the decay. This is a very fundamental point and was made yesterday by D r. Shiokawa — the new point of view as to what happens in an A u ger event w ill tell us something about what w ill happen to these resid u al ra d ic a ls. We have p repared some n C -la b e lle d m olecules, thymines, fo r use in the human organism . W e have now completed some studies which show that such m olecules can isolate in tum ours of certain types, as one might expect, but the problem now is: what happens when the n C decays? If the m olecule is incorporated into D N A , and it is damage to D N A that causes genetic mutation, are we further
IAEA-PL-615/18
281
jeopardizin g the patient by giving him this isotope, not only by the radiation dam age from the decay of 1 :LC, but also from the damage to the D N A chain. T here is so little known about what happens to these m olecules in these decay events that it is a wide open field fo r research . N. G E T O F F : Many of these m olecules have been studied by pulse ra d io ly sis, from which you can get the rate constants fo r reaction. G. S T O C K L IN : This is a com pletely different system . In an aqueous solution, the important reactions are those of the ra d ic a ls or the ionm olecules with the water. With a long-chain m acroscopic biom olecule, it is the p rim a ry event that is important, and you cannot re a lly use any of the liquid phase ra d io ly sis studies to obtain inform ation about what happens to the biom olecule during the p ro cess of beta decay. The case that W olf mentioned with u ra c il is actually a c la s s ic a l organic chem istry situation â&#x20AC;&#x201D; he pointed this out h im self â&#x20AC;&#x201D; a carbonium ion is left behind, and there is a possibility fo r resonance stabilization in the 5-position which is not present elsew h ere. So every organic chemist w ill understand why the rad ical su rvives in one case and not in the other. G. H A R B O T T L E : In the past there has been a great deal of discussion about using radiopharm aceuticals, not only fo r diagnostic purposes, but also fo r therapeutic purposes. In this situation, you want to m axim ize the dose given to the patient in a particu lar specific site, and not to m inim ize it. I'd like to ask D r. W olf if there is any attention being paid to therapeutic studies with labelled m olecules. I rem em b er that at one time astatine was the favourite element of these people because it would deliver a la rg e dose of alpha radiation. A u ger effects in tum ours are also possible. A .P . W O L F : T here has not been a la rg e amount of work in this area yet, but I think that there w ill be in the future. The problem with using isotopes for therapy is the following: obviously the best therapeutic agent would be an alpha em itter. H ow ever, supposing there is a tumour in an organism , and the labelled compound is injected, and 2 0 % of it lo calizes in the tumour: that would be a v ery high figure fo r localization but it still leaves 80% som ewhere else in the organism . T h at's the first problem . Iodine therapy of the thyroid is the only good exam ple of radioisotope therapy, and that works because the thyroid is highly specific fo r iodine, and just scavenges up a la rg e fraction of it. You can also adm inister blocking agents which prevent the iodine from going to other places in the body. Under these conditions, you can shove a trem endous amount of radioiodine into the thyroid, and give it a blast of 500 to 1000 rad, and knock it out. With an alpha em itter, you would p re fe r that it be distributed uniform ly throughout the tumour. If it w ere only in the centre, it wouldn't do much good. But if it's distributed uniform ly, what happens to the surrounding tissue? There a re m echanical m eans fo r forcin g an isotope into an area, and that's one way of doing it. H ow ever, the big problem is how to concentrate the dose where you want it to go. G en erally, how ever, no one has re a lly come up with p rocedu res that give high localization fo r injected radioisotopes. One of the problem s we have been working on is the incorporation of radioastatine into ĂĄ compound which lo c a lize s in m elanom a, but so fa r we have't been able to get m ore than 10% localization. This isn 't satisfactory because it leaves 90% of the astatine distributed throughout the rest of the organism . W e 'r e now looking for other possible isotopes because there a re enormous d ifferences in selectivity for different types of isotopes. But the m ajor p roblem fo r therapy is: unless it lo c a lize s, y o u 're in trouble.
282
W O LF
G. H A R B O T T L E : What is re a lly important, of course, is not only the degree of localization but also the lifetim e in the body. Not only the lifetim e in the tumour, but also in the re st of the body. You could stand 80% non localization if the residence tim e in the body w ere short. A .P . W O L F : Unfortunately, the residence time is never short in these cases. S. A M IE L : You have mentioned that alpha em itters are very suitable fo r this isotope therapy. A .P . W O L F : O r soft X -r a y s , fo r example. S. A M IE L : What about coulomb explosions? If you want to attack on the s u b -c e llu la r line into a m olecular site only, you a re trying to damage certain c e lls. Then, the coulombic explosion would give damage to the particu lar site without affecting anything else. G. H A R B O T T L E : The trouble is that there is not much evidence for coulom bic explosions except in the gas phase. S. A M IE L : Selective damage to the m olecule. G. H A R B O T T L E : Y o u 'll get plenty of damage from an A u ger shower, that's a ll right. But it's not a coulom bic explosion. A .P . W O L F : The purpose of depositing energy in a localized site can be accom plished in many ways: a soft beta, som e kind of A u g e r event, soft X -r a y s . Even 131I which has a fa irly hard photon does it reasonably w ell. One of the things you alw ays have to balance — a favourite expression of the physician — is benefit v ersu s risk . If the patient is old and desperately ill, you can give them a rather high dose of 1 3 1 I. In a 2 0 -y ear old patient, you have to consider the possibility of damage to sensitive organs, such as the o v a rie s or gonads. G. H A R B O T T L E : You have dism issed the 5 0 -y e a r-o ld s in this room in rather cavalier fashion. K. R O SSLER : W e are obtaining som e resu lts from astatine distribution studies in m ice, and w e 'r e finding m ore than 50% of the sim ple inorganic astatine in the stomach. A .P . W O L F : How was it injected? Intraperiton eally? K. R O SSLER : No. They a re actually intravenous injections. A .P . W O L F : Then it won't get into the stomach. T h e re 's no way it can get into the stom ach it if its injected intravenously. K. R O SSLER : Sure it can. A .P . W O L F : No, it's not possible. Do you mean gut? K. R O SSLER : Magen. A .P . W O L F : Stomach, but... w ell, okay, I 'l l talk to you about it later. K. RO SSLER: The resu lts change totally when we have tumour m ice — leg tum ours. About 2 5 - 30% of the activity goes to the tumour, and the stomach activity is reduced to 1 0 %. L . L IN D N E R : W e have a visiting group of biochem ists in our institute working on the labellin g of proteins with astatine with the aim of achieving selective im m uno-suppression. F o r those who are interested the labelling of proteins works very w ell. F u rth erm o re, I must tell A l W olf that if you inject astatin e-labelled protein intravenously, quite a bit goes to the stomach. T h ere is no doubt whatsoever. A .P . W O L F : What do you m ean by quite a bit? L . L IN D N E R : 50%. W e only tried it once, and with healthy m ice. But this has nothing to do with hot atom chem istry.
IAE A -P L-61 5/18
283
G. S T O C K L IN : I would like to return to the hot atom chem istry itself. Although the astatine case is interesting, it is re a lly not hot atom chem istry — it is c la s s ic a l labellin g using c a r r i e r - f r e e radioactive species. A m ie l asked a very good question a few moments ago. It is known that isotopes that decay by electron capture do much m ore b iological damage than any other nuclide decaying in a biom olecule. In our laboratory, H alpern and I have shown — not using nuclear decay, but with m onoenergetic X -r a y s that correspond to the К edge of one heavy atom substituent — that we can m ore or le s s selectively create the A u ger effect in a la r g e r m olecule and see its consequences, for exam ple by doing ESR studies. Y ou re a lly can find a resonance effect if you go into the pro p er energy range with the X -r a y s . The resonance effect is a factor of three higher in brom odesoxyuridine than in thymidine, as shown by a quantitative determination of the ra d ic a ls p ro duced. W e a re now applying this A u ger resonance effect with m onoenergetic X -r a y s fo r tumour therapy in Jülich. H ow ever, the difficulty now is that it can only be used with tum ours that lie alm ost on the surface because the depth of penetration of the radiation is v ery sm all. F u rth erm o re, it is difficult to concentrate enough of the heavy element in the tumour. You alw ays have the problem of distribution of the target element, and the problem of the environment, but the external technique using m onoenergetic X -r a y s does have som e prom ise. S. A M IE L : Once we tried to see whether soft X -r a y s , corresponding to the L and К X -r a y s of iodine in iodine-loaded thyroid, would give any effect. H ow ever, we didn't do a detailed experim ent, and dropped it after som e quick tries. A .P . W O L F : The absorption edge technique has been used at Brookhaven National L a b o ra to rie s, and has been published by Hal Atkins, the physician in charge of the nuclear m edicine program m e. H ow ever, it is not used because there are better techniques. S. A M IE L : Exactly which technique? A .P . W O L F : Exactly the one you described — absorption edge. Loading the thyroid with stable iodine, and then taking a picture of the thyroid. You get re a lly elegant pictures. The National B ureau of Standards is working on m onoenergetic low energy X -r a y sources with this p articu lar purpose in mind — absorption edge scanning — so this is a technique that has been with us fo r a long time. The only reason that it isn 't used is that the other techniques a re sim p ler and just as effective. But, in prin ciple at least, it's an interesting technique. F .S. R O W L A N D : My question is gen erally ad dressed to W olf, but also to the others here who are actively involved in nuclear m edicine. The question that may come up to someone who is outside the field, and might in p articu lar occur to someone reading a rep ort from this panel, "W h e re does one start in finding out which isotopes and which labelled compounds are good? " Is it the radiation ch aracteristics that are important? Is it m edicinal? What are the factors that determ ine the particu lar isotopes, the particular compounds? G. H A R B O T T L E : What you are asking is — how do you make a start? What is the initial step? A .P . W O L F : In my review , I point out that this is the most difficult aspect of a ll of this work. T h ere a re , how ever, certain starting points. If you want to use an isotope fo r diagnosis, either dynamic or static, then you choose it to m inim ize the dose to the patient; you choose it so that the
284
W OLF
photon emitted is in a p articu lar energy range; because the presently a v a il able detectors operate m ost efficiently between about 100 and 500 keV; you choose it if you know something already about its localization in specific sites — you go through the literatu re and see if anyone has already worked on the localization pattern of this particu lar isotope. Then you can go into the drug literatu re, and look fo r the specificity of certain types of drugs. But, in spite of a ll of these ru le s, no one in the w orld has yet come up with a p rescription that says a p r io r i that compound A is the one to m ake, and that it w ill definitely work fo r a p articu lar problem . One exam ple is the com pound dopamine. Dopamine is involved in the physiological p ro cesses; it's known that the adrenal m edulla generates it and concentrates it -- so we synthesized it with radioactivity, and found indeed that the radioactive m aterial does concentrate in the adrenal m edulla. H ow ever, when you start talking about astatine and bismuth, you' re working in the dark. Knowing what to do is probably the most difficult problem in this whole field. N. G E T O F F : The situation is even m ore com plicated than described. T h ere a re re p a ir enzym es in the body which w ill start to re p a ir im m ediately after such ra d ic a ls are produced. M . N E W T O N : A b rie f comment on another com plicating factor. I don't think that the decay of tritiated thymine is as sim ple as you have suggested. I would suggest that the p rim ary carbonium ion is in the sigm a system, and not in the pi electron system . L ite ra lly , you are stripping off the helium ... A .P . W O L F : Y e s, that's right. M echanistically that's correct. M . N E W T O N : You can't talk naïvely or directly about conjugated carbonium ions, because the hole is not in a pi orbital but in a sigm a orbital. A .P . W O L F : Oh, but that's true of a carbonium ion in any conjugated system — arom atic. M . N E W T O N : No. Y ou can have something like a benzyl carbonium ion with the hole in the p i-sy stem . A .P . W O L F : But in the phenyl carbonium ion, it is in the sigm a-system .
1АЕА-РЬ615/19
BIOLOGICAL EFFECTS OF HOT ATOM REACTIONS L.E. FEINENDEGEN Institute o f Medicine, Nuclear Research Center Jülich GmbH, Jülich, Federal Republic o f Germany
Abstract B IO L O G IC A L EFFEC TS O F H O T A T O M RE AC TIO N S . N u c l e a r t r a n s m u t a t i o n s o f r a d i o a c t i v e i s o t o p e s i n c o r p o r a t e d i n t o b i o l o g i c a l s y s te m s m a y be v e r y e f f e c t i v e i n a l t e r i n g s t r u c t u r e a n d f u n c t i o n o f v i t a l l y i m p o r t a n t o r g a n i c c o m p o u n d s . I t is e s p e c ia l l y t h e D N A t h a t is c o n s i d e re d t h e c r i t i c a l m o l e c u l e f o r r a d i a t i o n e f f e c t s a n d t r a n s m u t a t i o n e ff e c t s . W h e r e a s si n g le - s tr a n d b re a k s o f D N A are e f f i c i e n t l y r e p a i r e d , d o u b l e - s t r a n d b r e a k s are p r o b a b l y l e t h a l eve nts . T o d e s c r i b e t r a n s m u t a t i o n e f f e c t s f r o m i n c o r p o r a t e d i s o t o p e s i t is n e c es sa ry t o d i f f e r e n t i a l l y r e c o g n i z e r a d i a t i o n e f f e c t s su c h as f r o m e m i t t e d b e t a - p a r t i c l e s i n t h e c o u r s e o f n u c l e a r d ec a y . F o r t h i s p u r p o s e m i c r o d o s i m e t r i c a p p r o a c h t o a n a l y s i n g d o s e - e f f e c t r e l a t i o n s h i p s is nec es sa ry . A r e v i e w is g i v e n o n t r a n s m u t a t i o n e f f e c t s i n D N A f r o m 3H , 32P, 33P, 14C, a n d 12SI. S p e c i a l e m p h a s i s is p l a c e d o n t h e b i o l o g i c a l c o n s e q u e n c e s o f t h e A u g e r e f f e c t p r o d u c e d b y t h e d e c a y o f 125I. A n a n a l y s i s o f t h e d a t a i n d i c a t e s t h a t t h e n u m b e r o r r a d i a t i o n - i n d u c e d D N A d o u b l e - s t r a n d b re a k s p r o d u c e d p e r d e c a y o f th e i n c o r p o r a t e d n u c l i d e s e x c e e d s t h e n u m b e r o f t r a n s m u t a t i o n - i n d u c e d D N A d o u b l e - s t r a n d b re a ks b y a f a c t o r o f m o r e t h a n 2 0 0 f o r 32P r a n d o m l y d i s t r i b u t e d i n tiss ue. T h i s f a c t o r is o f t h e o r d e r o f 4 0 f o r 14C r a n d o m l y d i s t r i b u t e d i n ti s s u e ; y e t i t is 3 f o r 14C l o c a l i z e d o n l y i n c e l l n u c l e u s i n c l u d i n g i ts D N A . F o r 3H e x c l u s i v e l y i n c o r p o r a t e d i n t o n u c l e a r D N A t h e f a c t o r is e x p e c t e d t o be a p p r o x i m a t e l y 8. I n t h e case o f 125I d e c a y , t h e t r a n s m u t a t i o n e f f e c t is c a l c u l a t e d t o be a t le as t t w i c e as t o x i c as t h e e f f e c t e x p e c t e d o n t h e basis o f a b s o r b e d r a d i a t i o n . Y e t e x p e r i m e n t a l e v i d e n c e i n d i c a t e s t h a t d e c a y o f 125I i n D N A m a y be a p p r o x i m a t e l y 10 — 1 0 0 t i m e s m o r e e f f e c t i v e i n i n a c t i v a t i n g cells a n d p h a g e t h a n is d e c a y o f 3 H i n D N A . T h i s d i s c r e p a n c y b e t w e e n e x p e r i m e n t a l e v i d e n c e a n d c a l c u l a t e d d at a i n d i c a t e d t h a t t h e “ a n a t o m y ” a n d r e p a i r o f t h e D N A d o u b l e - s t r a n d b r e a k s p r o d u c e d b y t h e A u g e r e f f e c t is d i f f e r e n t f r o m t h a t ca u se d b y i r r a d i a t i o n .
B e ca u se p o o r ly
b io lo g ic a l s u ite d
r e a c tio n s . o b s e rv e d to p e s .
W ith
s tu d y in g
Y e t,
th e s e
m a in ly
as
T h e y
fu n c tio n
s y s te m s
f o r
may
o f
be
to
do
to p e s
in c o rp o ra te d a s p e c ts
im m e n s e ly
of
in th e
e f f e c t iv e
im p o r ta n t
n u c le a r
th re e
o c c u r
re s u lt
v e ry
v i t a l l y
re s p e c t
a
a re
m e c h a n is m s
b io lo g ic a l d ecay in
o rg a n ic
b io lo g ic a l
of
b y
th e y
a re
n u c le a r
s y s te m s
in c o rp o ra te d
a lt e r in g
tra n s m u ta tio n s
in t o
c o m p le x ,
tr ig g e r e d
and
a re
is o
s t r u c t u r e
and
co m p ou n d s.
o f
r a d io a c tiv e
s y s te m s ,
is o
th e re
a re
m a in ly
b io lo g is t s
and
in
in v o lv e d :
1.
m e c h a n is m s
2 .
a p p lic a tio n
o f
r e a c tio n
as
a
to o l
f o r
m o le c u la r
m e d ic in e 3.
th e
h e a lth
m a x im a l th e
p h y s ic s
p e r m is s ib le
hum an
p ro b le m a m o u n ts
w ith o f
re g a rd c e r ta in
to
r e c o m m e n d in g
r a d io is o to p e s
to
b o d y .
285
286
F E IN E N D E G E N
The
m u ltitu d e
c lu d e s
a
o f
l i f e .
r a p id
m o le c u le s no
o rg a n ic
m o le c u la r
te n a n c e h a v e
o f
w ith
a c tio n s
n e c e s s a ry
u n iq u e ,
h a v e
a
f o r
In d e e d ,
in
c r i t i c a l ra n d o m
in
th e
in
c e l l ,
th e
e ffe c t s
(
s tra n d
(
ch a n g e s
a re
u s u a lly
w h e re a s
d o u b le - s tr a n d
C h ro m o s o m a l
The
p e rm a n e n t
b re a k s r is e m ay
a re
to
T e c h n ic s ch a n g e s w e ll
in
a re a n d
th e
as
a
o f
f o r
e a c h
b re a k s , a n d
a
v o lu m e
8
ra d 0 .1
fro m -
s im ila r o f
m ic ro n
2 70
0 .3
10
is
r e
i r r e fu n c tio n a l d e o x y
t h is
c o n s id e re d
g ro u p .
th e
a b s o rb e d
a t
d e te rm in e s
as
f o r
6
f i n a l l y
o r o f
o f
one
g
is
to
b a s e
a p p r o x im a te ly b re a k s , e v e n ts .
e q u iv a le n t
fu n c tio n a l
to
a lte r a t io n s S o m a tic
v a rio u s
a nd
ir r a d ia t io n .
n u c le a r
)•
).
g iv e
m u ta tio n s
tu m o u rs .
th e s e
e x p e c t
(4
d o u b le - s tr a n d
ty p e .
e f f e c t s ,
c e l l ,
r e p a ir e d (3
le a d
a nd
i n t e r b ase
li v i n g
e f f e c t s :
m a lig n a n t
a nd
and
m is r e p a ir
s o m a tic
m ay
m u ta tio n
th e
in v o lv e a nd
b re a k s b y
m ay
e v e n ts ,
d ia m e te r
o f
fro m
r e c o g n iz in g
)
DNA
a lt e r a t io n s
b io lo g ic a l
b io lo g ic a l
5 ,
o f
i n e f f i c i e n t l y
a lte r a t io n s
d o u b le - s tr a n d
_ иp
x
b a s e
a re
X - r a d ia t io n
n u m b e r
th e s e
a re
th e
in t o
is
DNA
a lt e r a t io n s
r e s u lt
c o n s e q u e n c e s (
is
th e
r e p a ir e d
g e n e tic
a v a ila b le
e v id e n c e
n u c le u s
m ay
le t h a l
a
fu n c tio n a l
e x a m in e d
th a t
to
h a s
m a te r ia l,
S in g le - s tr a n d
d e v e lo p m e n t
p u b lis h e d
th e s e
r a d ia tio n
m a in ly
b re a k s
p o t e n t ia lly o f
com pounds
p r a c t ic a lly
c la s s if ie d
DNA
W hen
e f f i c ie n t l y
im p re s s
m u ta tio n s
r e s u lt
).
s t r u c t u r a l a nd
be
o f
th e y
th e re
b io c h e m ic a l
to
g e n e tic
th e
b re a k s ,
2
a b e r r a tio n s
c o n s e q u e n c e s
a re
Dam age
to
m o le c u la r
lin k in g
o f
m a in
and
som e and
i n
).
d o u b le - s tr a n d
c ro s s
is
c e ll.
dam age 1
r a d ia tio n - in d u c e d
s in g le - a n d
i t
th e
th e is
i f
O th e r
a nd
to
re d u n d a n t
re p la c e d ,
s y s te m .
s y s te m .
s tr u c tu r e
im p o rta n c e
a re
s e q u e n ce
tu r n o v e r
th a t
r a d io b io lo g y
a
w h o le
l i v i n g
T h e re fo r e ,
r a p id ly
s p e c ia lly
(D N A ),
a ny
d if f e r e n t
c e lls .
a re
no
in
com pounds
o rd e re d
th e
o r
is
m o le c u le
b io lo g ic a l
The
I t
th e y
c e llu la r
a c id
v e ry
w it h in
f o r
s lo w th e
im p lic a t io n s . r ib o n u c le ic
o f
th e s e
o f
dam aged
in te r fe r e n c e
p la c e a b le
s p e c ie s M o st
tu r n o v e r
a re
com pounds
th e y On
s t r u c t u r a l h a v e
th e
b a s is
w it h in
a
c e ll
DNA
6
x
3
a b o u t S in c e 1 6 .9
o f - 6 5
b e e n
10
o f
c o n ta in in g — 1 2
g ,
s in g le - s tr a n d b a s e
1 ke V
ra d p e r
a lt e r a t io n s , p e r
n u c le a r
n u c le u s
and
287
1АЕА-РЬ615/19
c o rre s p o n d s is
w a s te d
b y
th e
t o t a l
to
in
f a c t
th a t
n u c le a r
p a ir s ,
a re
m ay
a d d e d
be
e ffe c t s
th e
7
to
th e
is
th e
th a t
th o s e in
in
o f
th e
a re
is
a f t e r
I f
DNA
o r
is
c le a r
T h is
is
2
% o f
o f
o n ly
6 0 0 ,
e ffe c t s
m o re
th a t
m uch
p a r t ia l l y
c o n s titu te s
o n ly
th e
L i t t l e
add
la b e le d
on
n a m e ly DNA,
e n e rg y e x p la in e d
a b o u t
2
12
a n d
io n iz a tio n s
to
th e
to
% o f
th e
io n
to
fro m
t h is in d ir e c t
u s u a lly
th e
l a t t e r
e q u a lly
d i f f i c u l t th e
a n d f a r
to
a
th a t
th e
fro m
to
c h e m ic a l
i l l
be
be
e x p re s s e d
to
m u ta tio n s
in
o f
to
o r
s t r u c t u r a l
DNA.
T h is
is
one
e f f e c t s ,
e ve n
i f
th e y
e m itte d
ta s k
to
b y
e f f e c t s :
th a t
It
b y
tr a n s m u ta tio n ,
th e
a nd
th e
b e c a u se is
s tr u c tu r e s
tr ig g e r e d
r e
fro m
r a d ia t io n e ffe c ts .
b io lo g ic a l
th e
s e p a r a te ly
a b s o rb e d
tra n s m u ta tio n s in
i l l
w
e v e n tu a lly
o v e r a ll
m e c h a n is m s
w
s im ila r
th e s e
th e
i t
e v e n ts be
r a d ia tio n
fro m
r a d io
m ay
"a n a to m y "
d i f f i c u l t
a p p r e c ia te
r e c o il,
i l l
a
DNA,
S uch
w h ic h
le a d in g
Y e t,
o u tw e ig h s
v a r io u s
w
a b o u t
y o u .
is
DNA
r a d ia t io n ,
w h e re
th e
a ffe c te d .
th e
fro m
s it e
tr a n s m u ta tio n s
dam age I t
e ffe c t s
m o le c u le - is
a nd
c o n tr ib u tio n s
e v e n t
n u c le a r
know n
p o s e d
tw o
o f
o f
o f
s p e c ific
p e rh a p s
n u c le a r
c o g n iz e
tr a n s m u ta tio n
th e
e x c lu s iv e ly
ch a n g e s
is b y
is o to p e .
th e
a t
ir r a d i a t i o n ,
tra n s m u tin g
t a t io n :
i t
DNA.
d is t r ib u t io n
o c c u r
a lt e r a t io n s ,
q u e s tio n s
im p o rta n c e
in
d ir e c t
n u m b e r
n e a r ly
p ro d u c e d
s m a ll,
ca se
ca u se
ra n d o m
s t r u c t u r a l
d e a th .
ch a n g e s
to
e v e n ts
fu n c tio n a l
c e ll
p a ir s ,
T h e re fo r e ,
sam e
b o u n d .
se e n
io n
ch a n g e s
).
n u c lid e
r e s u lt
6 0 0
t h is
m a ss.
tr a n s m u ta tio n
DNA
in
e x p e c te d
(
C o n tra ry
a b o u t
p ro d u c in g
th e
tra n s m u
c h a rg e
tr a n s fe r .
B io lo g ic a l a n d A t
th a t
and
tim e
p ro b le m s
t h is i t
e ffe c t s
re v ie w e d
p a n e l
a is o
s in c e
a
o n ly
new
d a ta
T h e y
b io lo g ic a l
s h a ll
be
tr a n s m u ta tio n s
c a lle d w e re
b y
s e rv e d
h a v e
to
O ne
r e g a r d in g
re v ie w
becom e
o f
h a v e
A g e n cy
m ay
s t im u la tio n
s h o r t ly
e ffe c t s
t h is
d ra w n
p h y s ic s .
c o n s id e ra b le
on
c a p tu r e .
n u c le a r
h e a lth
a d d e d . th e
o f
p a n e l
c o n c lu s io n s to
n o t
g a ve
1967
b y
b e e n
d is c u s s e d
O c to b e r
d a ta to
a v a ila b le new
u n t i l
new
c o n c e p ts
E m p h a s is a s s o c ia te d
w
i l l w ith
(
8
).
a p p lic a tio n
s ta te
d e v e lo p m e n t.
a n d
h e re .
tr a n s m u ta tio n
1967
m e c h a n is m s ,
r e t r o s p e c t iv e ly
a v a ila b le ,
re v ie w e d
in
th a t
th e n ,
b u t
In d e e d , w e re be
p la c e d
e le c tr o n
288
F E IN E N D E G E N
R a d io n u c lid e s
B e ca u se
o f
o f
th e
in t e r e s t
s t r i c t l y
r a d io n u c lid e s
th a t
o f
e x p e r im e n ta tio n ,
fro m a t
to
s u c h
c o u rs e
th e
th u s
e n v iro n m e n t f a r ,
l k - C ,
o r
T h e re
a re
w it h in
DNA o r
su ch
p o u n d s
b y
a ny
9
b y
" h o t
b y
)•
m eans
in
su ch
as
su ch
w h ic h
p ro d u c e d
in
p o t e n t ia lly
as
in t o
w e re
and
lo o k e d
3 2 -P ,
33~P ,
p ro d u c e d
l i t t l e
b io lo g ic a lly to o ls
DNA
w o rk
e x a m p le is
m e n tio n e d
u s e fu l
b ee n
1 2 5 -1 .
be
f o r
V e ry be
th a t 3 -H ,
m ay
co m p o u n d s,
ju s t
n u c le a r h a s
m e d ic a l
r a d io is o to p e s
ir r a d ia t io n . w i l l
b y
a tte n tio n
in c o rp o ra te d
c a p tu r e ,
o th e r
th e y
a re a re
DNA,
be
r e a c tio n s
o r
p a r t ic le
w h ic h
m ay
T h e se
e le c tr o n
a to m "
a nd
o f
b e t a - e m itt e r s ,
m o le c u le
e ffe c t s
r e a c tio n s
(
e it h e r
d e c a y
o th e r
ir r a d ia t io n
a to m
a re
th e y
th e
a b o u t
b io lo g ic a l
r o le
tr ig g e r e d
g iv e n th e
s p e c if ic
e v e n ts
a nd
in
th e
lo c a liz e d
tr a n s m u ta tio n
b y
know n
h e re .
H ot
r e le v a n t in
x -
b io lo g y
com and
m e d ic in e . 125i
,0
H
HN-H 3
0
hY ^ n ....... H - n A ^ N
зЛДп :N
/
u
H. 0 u J sN A m N'- 11
0
CHj
Ú
O
N'
°^0H
'
V h
^
o^ hTco
H-NH
0
0 hV h
n
-----r ^ N -H
/ /
XN‘ "N i jj-Н...О
H
0
l ^ uN -'
/
0
H FIG. 1.
Structure o f DNA.
289
IA E A -P L-61 5/19
The
DNA
to 5
is
e a c h o f
com posed
o th e r
th e
b y
th a t
a re
th e
tw o
th e
tw o
p u rin e s
lin k e d
can As
g ro u p th e in
to
o r
8
one
fig u r e
6
p o s itio n
3 2 -P
o r
is
a n a lo g u e
o f
1
1 2 5 -1
l i s t s
3 2 -P ,
o f
3 3 -P
to
1 4 -C ,
e ffe c t s
20
y e a rs
and
not
r e c e n t
re v ie w
T A B L E
I.
and b y
D E C A Y
33 p — ,4 c - *
m u ch
32s 33s MN
lin k e d
a c id
3He
o f
K ris c h
3 2 -P
w ith in
to
IA E A
b y
3 -H
in
5
a nd
6
th e
th e
m ay to
re p la c e
a ny
125Te
s p a c e .
th e
r in g
a ny
o f
n u c le o tid e
y o u ,
f o r
c o n c e rn in g
m e th y l
p o s itio n
a d e n in e
s p e c ific
be
a l.
o f
and th e
in
th e
w h ic h
DNA.
th e
d e c a y
x
0 .2 5
'
_________! _ 0 .1 5 6
i w a te r ( 1
'
1
been kn o w n fo r
I
re fe r
" B io lo g ic a l (
8
O F
)
jii)
S E V E R A L
a nd
in
th is
c o n te x t
E ffe c ts to
th e
0 .0 1 8 6
' !
2600 120
_
0 .0057
78
i I 1 1 ,
X
E ex c it. (eV) X
20
60
5. 1
1
1.7
60
7. 1
1
1. 8
45
1 .0
11
--------------
A u g e r e le c tro n s 0. 0005 - 0. 032
1
o f
m o re
1
1
m o re
R A D IO IS O T O P E S
E r e c o il (eV) m ax.
i
0 .09 3 0 .0 4 5
h e re .
on
1
1 I 0 .7 0
------------ \----------
have
(1 0 ).
1 ' x ra n ge in
(M eV )
1.71
s a id
p a n e l
C H A R A C T E R IS T IC S
1 1
DNA
R a d io is o to p e s "
e t
1
125I - >
m ay
p re c u rs o rs
33
-------------- 1Зн ^
and
th a t
5 -Io d o d e o x y u r id in e ,
s im ila r ly
to
m ilie u
e x t r a c e llu la r
o f
1 4 -C
and
1 2 5 -1 .
need
th e
D e ca y
Ep
32 p ^
n u c le ic
in
w ith
3
d e o x y c y tid in e
re p la c e d
r in g ,
know n
a nd
fro m
o f
m ax.
Isotope
and
c a rb o n s
e x t r a c e llu la r
th e
p o s itio n
lin k e d
c o rre s p o n d in g
r a d io n u c lid e
fro m
r in g .
a re
3 3 ~ P h o sp h o ru s
p ro c e e d in g s
T ra n s m u a tio n
8
e a s ily
w e ll
3 -H ,
T ra n s m u ta tio n
th e
be
A ny
be
in c o rp o ra te d
d a ta ,
a nd
to
and
th e
The
th e
r e la te d
m ay
g u a n in e
can
be
fro m
DNA
th y m in e 2
th y m id in e
som e
3 3 -P ,
th e th e
1 ).
th y m id in e ,
o th e r in t o
w h ic h
b e tw e e n
fig u r e
g u a n in e .
o r
th e
3 2 -P h o s p h o ru s
th a n
a n d
h y d ro g e n
in
m ay
b o n d s
in c o rp o ra te d
th e s e
o f
n u c le o tid e s
n u c le o s id e s
1 ,
r in g ,
a n d
Tab le
be
o f
in
c e l l ,
4
(se e
in c o rp o ra te d
p o s itio n
c a rb o n s ;
an
m ay
a d e n in e
be
c y to s in e th e
m o ie ty
p y r im id in e
r e a d ily show n
b a s ic a lly
p h o s p h o d ie s te r
d e o x y rib o s e
p re c u rs o rs
be
o f
<1
3. 6
1 1
1
<1
1 1 1 1
120
290
F E IN E N D E G E N
TABLE II. LETHALITY OF PHOSPHORUS DECAY IN BACTERIOPHAGES Phage and storage temperature
Individual experiments
T1 ( —196°c)
Percentage o f Difference " P lethalities a (recoil) due to recoil
Mean + S.E.
3-4 3-1
3-3 ±0-1
2-5 ±0-3
43
T1 ( + 4°c)
13-6 15-2 13-4
141 ±0-6
10-5 14-5
12-5 ±2-0
l-6 ± 2 -l
13
T4 ( —196°c)
10-2 8-5
9-4 ±0-8
4-7 3-6
4-1 ±0-6
5-3 ±1-0
56
T 4 ( + 4“c)
14-3 14-3
14-3 ± 0 1
10-0 96
9-8 ±0-2
4-5 ±0-2
31
fro m
A p e lg o t e n e rg y
f u l l y
3 2 -P
ca u se e t
in
m u ta tio n
a l.
o f
b y
w as
w as
n o t
r a t u r e
a t
w as
s tra n d
3 3 -P .
b re a k s
T h is
to
th e
p h o s p h o ro u s
tw o
o th e r 3 3 -P
a
fo r
w as
m o re
d u c e d
b y
b o th ,
b re a k s
in d u c e d
o f
d e c a y
th e
r e la t iv e
b re a k s
is
( 1 3 ).
b y
as
in
w e re
d e c a y
a nd
a re
th a n
as
in e f f ic ie n c y i l l u s t r a t e d a u th o rs
33~P
o f
to
is o la t e d
r e c o il
m o re
w ith in lo w
f i l l i n g on
% m o re
ta b le
2.
In
L
a nd
c e ll
3 2 -P .
is
b re a k s ,
In
a nd
h e re
on
th e
phage
e x p e rim e n ts ,
b re a k s
a re
i n e f f i c i e n t l y
d e c a y ,
a n d
th a t
due
c h e m ic a l
to
s in g le
c o n
e f f e c t iv e ; p h a g e .
te m p e
e f f e c t iv e
c o l i ,
d e a th
T h e se
o f
e f f i
th e
56
f o r
DNA
p r o b a b ilit y
th e
e q u a lly
to
r e c e n tly
th e
d e p e n d in g
le t h a l
tr a n s
o rd e r
th e r e p ro
s in g le - s tr a n d c o n se q u e n ce s
r e c o il.
3 2 -P
b y
in
p r e
b y
and
a b o u t
la r g e ly
due
a
13
th e
lo w
w e re
d o u b le —s t r a n d
a re
t o x ic
d a ta d e c a y
w a s ,
b e
c h e m ic a l
in v o k e d
w ith
b e tw e e n
d o u b le - s tr a n d
3 2 -P
r a t h e r
a ls o The
h a lf
3 2 -P
3 2 -P
fro m
b re a k s
i t s
th a t
in a c t iv a t io n
r e p a ir e d
m a in ly
is o to p e s
th a t
3 3 -P
i l l u s t r a t e d
r e s u lt
th a n
in d ic a te
b u t
o c c u rre d ,
is
T h e se
le t h a l
p ha g e
w ith
be
to
re -e x a m in e d
sh o w n ,
m u st
and
e q u a lly
h and, s in g le - s tr a n d
fo r e ,
3 2 -P
h e ld
w as
33_ P
w as
o b s e rv e d .
e f f i c ie n t l y
s id e re d
u se d
I t
s u lp h u r
h ig h e r ,
d e c a y
a re
be
be
w h e re a s o n ly
w h ic h
to
i n i t i a l l y
q u e s tio n
w ho
3 2 -P .
( 1 2 ):
to
w as
The
(1 1 )
to
e ffe c t s
show n
c ie n c y
th a n
1962
th e
0 . 05 ,
DNA
in
K ris c h
-
0 .0 1
in
e ffe c t s .
p h o s p h o ru s
e x p la in
E . c o li
d e c a y
o f
c o m p a ris o n
c o n firm e d
F o x
Individual experiments
5-8±0-3
d o m in a n t
The
Mean ±S.E.
5-5 6-5 5-5
R e c o il
o f
at(MP)
o.(” P)
th e
d e c a y
in
p ro d u c in g
e x p e rim e n ts
3 2 -P
la b e le d
o f
DNA
d o u b le -s tra n d
R o s e n th a l fro m
and
p n e u m o c o c c i
291
IA E A -P L-61 5/19
and
s to r e d
i t
a t
d e c a y s .
A f t e r
o c c u rre d
th e
s tra n d to
a
o f
DNA
b re a k s .
h ig h 3 2 -P
a re
h ig h e r
la rg e
o r
in
m o re
o rd e r
w as
o f
T h u s ,
1
an
tis s u e
f o r
n u m b e rs n u m b e r
th e p e r
c e lls ,
o f 5
3 2 -P
s in g le - a n d
% o f
th e
o f
o f
d e c a y s
h a d
d o u b le
d e c a y
le a d
s in g le - s tr a n d
d e c a y .
b re a k
m ay
la b e le d
fro m
a c c u m u la tio n o f
e f f ic ie n c y
e m in e n tly
phe n o m en a 3 2 -P
e ffe c t s
a re
th e
s in g le - s tr a n d
w ith
th e
a llo w
a p p r o x im a te ly
le a s t
r e p a ir
d e v e lo p e d
f o r
w h e re a s
n u c lid e
in fe c te d
3 2 -P
a t
to
in c r e a s in g
th a n
b re a k
t h is
n u c le i,
3 3 -P
С
e x a m in e d
e f f ic ie n c y m akes
b io lo g is t s .
o f
N o t
p ro d u c tio n
w h ic h
In
w as
d o u b le - s tr a n d
b re a k
The
-1 9 б °
s u c c e s s iv e ly
p ro d u c tio n s
u s e fu l be
s tu d ie d
p h a g e
w h e re
p e r
f o r
in
d e c a y
m o le c u la r
h o s t
b a c te r ia
in
r e la t iv e ly
( 1 4 ).
DNA
a b s o rb e d
t o o l
is
p la c e d
r a d ia tio n
fo llo w in g
d e c a y
p re d o m in a n t.
1 4 -C a rb o n
R e la t iv e ly t a t io n A p e lg o t
fe w
e ffe c t s e t
c o n f lic t in g . th y m id in e f o ld
1 4-C
(1 5 )
h a d
1 4-C
(se e
fig u r e
M cQ uade
in
FIG . 2.
e t
p u b lis h e d
b io lo g ic a l
o b s e rv e d
2 ).
1 4 -C
b e e n
in
d e c a y in g
c a r r y in g
in c re a s e
h a v e
fro m
a l.
in c o rp o ra te d th y m id in e
r e p o r ts
a l.
fre q u e n c y
E.
c o li
a
in
th e
s p e c if ic a lly O th e r
in
in
th e o f
e v id e n c e
(1 6 )
la b e le d
m e th y l
c o n c e rn in g
s y s te m s .
ch ro m o so m a l
le t h a l 2
a p p e a rs o n io n
g ro u p
a nd
tra n s m u
A lr e a d y
in
1962
e f f e c t
o f
p o s itio n
o f
a t
s ig h t
f i r s t
r o o t le t s
w ith
o b s e rv e d
a
tw o
o v e r
th a t
a b e r r a tio n s
E f f e c t o f 14C d e c a y o n s u r v i v a l o f E. c o l i (se e Re f. [ 1 5 ] ) .
292
F E IN E N D E G E N
T A B L E III. S P E C IF IC M U T A T IO N R A TE S W IT H 3H A N D 14C D E C A Y (see Ref. [1 7 ])
fo llo w in g th e
2
th e
in c o r p o r a tio n
p o s itio n
e x p e rim e n ts
o r
b y
la b e le d
P e rs o n
and
f iv e f o ld
in c re a s e
o f
w as
as
in s te a d
u s e d
la b e l
T h is
d a ta
is
Thus
f a r ,
e x a m in a tio n s
c u ltu r e
f a ile d
e f f e c t
fro m
e f f ic ie n c y
The
show n
a
c o u ld
th e
o th e r ,
c y to s in e :
c a u se d
and
be
re m in d s
in
d is c u s s e d
o f
w ith
g ro u p
in
one
p ro v e th e s e
w hen
w ith
1 4 -C
h a n d , in
in
m o re E.
re c e n t
c o li
in
th e
m e th y l
th e
o r a
m a m m a lia n
s p e c ific
e v id e n c e
know n
o v e r
tr a n s m u ta tio n
to
b e t a - p a r t ic le .
s p e c ific
th e
fro m a nd
1 4 -C
e ffe c t s
b re a k s
d e c a y s
m u ta tio n
p re d o m in a n tly a re
c e lls
o f
k i l l i n g
c o n Â
in d u c tio n 3~H
b a s e
d e c a y
ch a n g e s
p ro d u c e d .
T h is
on in
a re w
b e lo w .
c o n tin u e d
f o r
th e to
lo c a liz e d
p a s t
a n a ly s e
fe w th e
g ro u p .
in
d e v e lo p e d
h a n d
m o re
c e lls
T r itiu m
New
a
2 -l4 - C - th y m id in e
3~H
h ig h e r
r e s u lt s
one
p o s itio n
p o s itio n
w ith
o th e r
in d ic a te d
m u ta tio n ,
a s c rib e d
on
6
(1 7 )
D r o s o p h ila
s p e c ific
o f
la b e le d
th e
th y m id in e
b re a k s
5
On
3-
t o t a l l y
th e
th e
3_ H.
s p e c ific
ta b le
r a t h e r
in
h is
M o re o v e r,
ch ro m o so m e
DNA
th y m id in e
u n e q u iv o c a lly
1 4 -C .
d iv e r g e n t,
c e rn in g
be
to
in
o f
w ith
3~H
y e a rs .
tr a n s m u ta tio n E s p e c ia lly
m u ta g e n ic
e f f e c t
th e o f
e ffe c t s g ro u p 3- H
becam e o f
P e rs o n
lo c a liz e d
in
i l l
293
IAEA-PL-615/19
TABLE IV. TRITIUM MUTAGENESIS (see Ref. [19]) A rg in in e R evertan ts
p er
decay
o C o n tin u o u s
th e
5
d a ta
a re
14- 4 x
^ H -u ra c il-6
2' 4 x
Ю
^ H -th ym id in e - m eth y l
5- 2 x
Ю '9
in
ta b le 3- H
h ig h e r
m e th y l
in to
p o s itio n
p r ia t e ly
a n d c e lls
_
6
r e c t
e ffe c t s
tio n
(se e
A n o th e r t io n 2 to 8
and
8
be
ta b le
4 ).
p o s it io n a l a d e n in e
w ith
in
c u ltu r e s
in
to
th e
4°
th e
5
m u ta g e n ic
w as
Thus
in
a d e n in e
y ie ld
DNA
o f o r
and o f
6
E.
1 0 '9
18
a t
in
-1 9 6 ° o f
th e
th e 6
th a t
d e c a y s
s ig n i
p o s itio n l a t t e r th a t to
a
o f
tw o is
and
3~H
in
C.
A ls o
in
p y r im id in e
p la c e d
3- H
m u ta tio n s
r in g
2
p o s i
in
th e
10
in
e f f e c t
is o la t e d
th e
d e c a y s .
p n e u m o c o c c a l
r in g ,
i n d i
m u ta
( r e v e r s io n
g
p o s it io n a l
p y r im id in e
th e
d e c a y s p e r
fro m a p p ro
T h u s ,
s p e c if ic
3~H
th e
th e
o f
la b e le d
th e in
la b e le d
m u ta tio n s
a
o f
d e c a y
to
F o x .(1 3 )
p o
c o n tr ib u tio n w e re
in
i n
lin k e d
b re a k a g e
o r
la c k
p o s itio n .
w h e re a s
0 .1 7
b a se
be
0 .3 0
th e
is
t h is
d e c a y
in
th e
p o s itio n 6
o f
to
E.
c o l i .
Som e
p o s itio n
f o r
to
th e
).
th e
c o li
r e p o r te d
o n ly
6- 7 x
e f f e c t
c o n tr ib u te
g
in
th e
p o s itio n
10
s tra n d
p o s itio n
С
1 0 '9
a tte s t
e x a m in e d
r e s u lte d
p e r
to
in
th a n
m e a s u r a b ly
R o s e n th a l 6
a t
x
h is t id in e
s p e c ific
f u r t h e r
e ith e r
g u a n in e
re g a rd
d e c a y
The
The
e f f e c t
o f
im p o r ta n t. 3 -H
3_ H
2- 8
th e
fro m
in
a p p e a rs
w as
re q u ire m e n t) o f
3 - H T h is
fro m
(2 0 ).
p o s itio n
w ith
d a ta
d e c a y
n o t
p o s itio n
The
d e c a y
m o re
do
a r g in in e
A ls o to
o f
th y m id in e .
in
С
1 0 '9
c o n firm e d
to
th e
(
at 4
18- 4 x
p a r t ic u la r d e c a y
( 1 9 ).
3 _ H
tim e s
DNA
is
t h is
fro m
th y m id in e .
s to re d
i t
in
3 -H
c y tid in e
e ffe c t s
'9
r in g
in
Storage
Ю '9
w h e re
th a t
e f f e c t o f
o f
3
d e c a y
p r o te in s .
in d ir e c t
5
o f
s im ila r
g ro u p
fr o z e n
th a n
g ro u p
a re
tr a n s m u ta tio n
w as
c y to s in e
b y
c o rp o ra te d
5
th e
show n
s it io n s
m e th y l
o f
in d u c e d
f ic a n t ly th e
^ H -u ra c il-5
p o s itio n
ch a ng e
grow th
a p p e a rs DNA i t
294
F E IN E N D E G E N
T A B L E V . S T R A N D B R E A K S F R O M Зн D E C A Y IN IS O L A T E D D N A ( see Ref. [1 3 ])
C i/ m M E xp t.
breaks
p y rim id in e
sin g le
9. 34
T l
strand
per
decay
1 d o u b le 1
stran d
0 .2 8
< 0 . 02
T 2
15. 0
0 . 19
< 0 . 01
T 3
15. 0
0. 29
0 . 01
T 4
8. 9
0. 38
<0. 006 1
a n d
s to re d
a llo w
f o r
i t
in
a ve ra g e
o f
b re a k s 0 .3
d o u b le - s tr a n d v a tio n t h e ir
w as
3 -H
e v e r,
3 -H
d u c tio n
an
e f f ic ie n c y th e
w as
in
th e
h a s
a
s tra n d
b re a k a g e
w o rth y
th a t
h ig h e r
DNA
o f
w as
o b s e rv e d
3 -H
c o l i .
p o s it io n ,
t h is
l i t t l e
to
l e t h a l i t y ;
s tra n d
b re a k s
0 .0 9
is
3 "H
in
th e
sam e
k i l l i n g
s y s te m
a re c e ll
p ro d u c e d
b y
b u t
is
in
d e a th
le s s
o n ly
and
6
in
6
e f f ic ie n c y
o f
s a y ,
is
to
r e p a ir e d
la r g e ly
b e ta
in
o b s e r
as
d e c a y
p ro 5
p o
e f f e c t iv e I t
is
DNA
p e r
s in g le
on
n o te
s tra n d
c y to s in e
0 .0 1 5
and
in
p e r
th e
p o s itio n .
T h a t
0 .0 2
T h is
b re a k
o f
w e re
W h e n ,h o w
p ro d u c in g
p o s itio n
is
th e re
b re a k s
3 -H
to
s in g le - a n d
show ed
r in g .
1 /3
o rd e r
th a n
5-
a ls o
T hus
th e
e f f i c ie n t l y
th e
th e
s in g le - s tr a n d
d e c a y .
d e c a y
p o s itio n s .
w ho
in
th a t
ta b le
c y to s in e
e f f ic ie n c y 5
and
in
(2 1 )
th e
p e r
th e
o f
show ed
d ecay
show n
s ta te
s in g le - s tr a n d
o f
m u ta g e n ic ity
u n e q u a l
DNA
a l.
0 .3
5
o n ly
b o th
in
o f
p o s itio n
d e c a y
th e
f o r
e t
fr o z e n
a n a ly s is
p e r
a re
6
th a n
d e s p ite
fro m
E.
to
An
b re a k s
K ra s in
in
th e
is o la t e d
d a ta
w as
s it io n
b re a k s
The
b y
in
d e c a y s .
t h is
s in g le - s tr a n d
a m o u n te d
b re a k a g e
o f
in
b re a k s .
e x te n d e d
s y s te m
w hen
s o lu tio n
a c c u m u la tio n
d o u b le - s tr a n d an
d ilu t e
in
d e c a y s tra n d
c o n tr ib u te
a s c rib e d
to
th e
d o u b le
p a r t ic le s .
1 2 5 -Io d in e
A t
th e
D e ca y
p a n e l o f
A u g e r
1 2 5 -1
on
In c o r p o r a te d
p a r t ic u la r th e
m e e tin g
a tte n tio n e f f e c t
( 2 2 ) .
T h is
in
" B io lo g ic a l
Is o to p e s " be
DNA,
p a id
to
in
E ffe c ts 1 9 6 7 ,
th e
i t
o f w as
b io lo g ic a l
su ch
as
is
r a d io is o to p e
is
e a s ily
p ro d u c e d
b y
T ra n s m u ta tio n s u g g e s te d
th a t
c o n s e q u e n c e s th e
in c o rp o ra te d
a nd
o f
d e c a y
o f
in t o
D N A
as
a
295
IAEA-PL-615/19
tr a c e r
f o r
a p p e a re d m o re
5 “ Io d o d e o x y u r id in e .
b y
t o x ic
now, a l l th a n
c o u ld
e ffe c t s
c a lc u la te d
I
t r e a t
s h a ll
i t
h a s
n o t
th e
y e t
In c o r p o r a tio n
F o r
s tu d y in g
a g a in
DNA
th e
be
th e
A u g e r
in t o
is
w e ll
s u ite d .
in t o
DNA
as
w h ic h
d in e
is
m o re
th a n
is
Io d o d e o x y u r id in e .
r a te d
e f f i c ie n t l y
th e
DNA
a f t e r
d ig e s te d
DNA
to
c e ll (2 5 ).
i n e f f i c i e n t l y e f f i c i e n t l y e m ittin g
r e u t il i z a t i o n
b a s is
b io lo g y
su ch
m akes
can
la b e l
is
o f
1 2 5 -1
o f o f
m o re
fro m
be
o f
show n
d a ta in
A u g e r
is
e le c tr o n s .
e x te n s iv e ly
in
c e lls
c e lls
(2 4 )
c e ll
b e c a u se
DNA
and
is
and
c e ll
i t
is
a d v a n ta g e th e
Io d o d e o x y u r id in e
O nce
th e in c o rp o
tr a n s m itte d
lib e r a te d
p h a g o c y tiz e d
a ls o
is
in tr o d u c in g
u s e fu l
t o o l
f o r
THYMIDINE (TdR) F IG . 3.
a
H
lODODEOXYURIDINE (lUdR)
Structures o f t h y m i d i n e and io d o d e o x y u rid in e .
gam m a o f
s tu d y in g
О
HO
i n
in e f f ic ie n c y
О
H
fro m and
r e la t iv e ly
r e la t iv e ly
o f
r e la t iv e a
th y m i
s y s te m s
m ouse
is
o n ly
a re
a n a lo g u e
(2 3 ).
a nd
c e lls ,
s p e c if ic a lly
Io d o d e o x y u r id in e
DNA
The
6
l i v i n g
H o w e v e r,
n o rm a l
c e ll
re m n a n ts
w h e re
in t o
(2 6 ).
th e
th e
3.
v a r io u s
a p p r o x im a te ly
ca se s
th e
in
In
in
a n d
th y m id in e
fig u r e
o f
in
r. = 2.15 A
HO
h a s
DNA
r a d ia tio n th e
1 2 5 -1
e a s ily
th e
to
re m a in s
in c o rp o ra te d
in to
e f f e c t
1 2 5 -1
w hen
r e u t iliz e d
n u c lid e
th e
o f
e n e rg ie s
in c o rp o ra te d
d a u g h te r d e a th , In
am ou n t
d e c a y
DNA
a
a m o u n ts
Io d o d e o x y u r id in e
w ith
in
tr a n s m u ta tio n
fa c t o r
on
a b s o rb e d
e f f e c t
in c o rp o ra te d
d iffe r e n c e
th e
s u m m a riz e d .
1252l _
5 -Io d o d e o x y u r id in e
c o n s id e ra b le
th a t
e x p e c te d
fro m
b e e n
o f_
A
c o n fir m in g
296
F E IN E N D E G E N
c e ll
k in e t ic s
b re a k d o w n
(2 7 ,
r e la t iv e ly
lo n g
d e o x y u rid in e
n u c lid e
o rg a n is m s w o u ld
be
d e c a y
T h e
d ecay
t o t a l
l i t t l e
o f
60
a
d a y s .
s u ita b le
The
f o r
b e in g s .
lo w
o f
b y b y
o f
m e a s u rin g F o r
su ch
125 -1 b y Đ&#x161; -c a p tu re a re
100
p e r
K -c a p tu re , in t e r n a l
193
20
has
b ee n fu lly
d e c a y s
b y
L -c a p tu r e ,
c o n v e rs io n
p rim a ry
e le c tr o n
o f
and
1 2 5 -T e
h o le s
111
In
th a t
tu r n o v e r
in
m akes la r g e r
1 3 1 -1
p e r
o f
a b o u t
e n e rg ie s
o f
th e
p e r
lo s t
ke V
T h re e
d e c a y ,
b y
up
0 .5
ke V .
h a v e
e m is s io n
a b o u t
88
a p p r o x im a te ly
to o f
0 .1
s ix
X -ra y s The
570
o f
a nd
r e c o il
0 .5
e le c tr o n s
e m itte d 3 4 .6
e le c tr o n s
e n e rg ie s
o f
ke V .
a p p r o x im a te ly th e
a re
k e V
m a in ly
an
a re
a nd
th e
a v e ra g e
e m itte d
th e
due
v a ry
A p p ro x im a te ly
g a m m a -ra y s , to
w ith
a
a re
e le c tr o n s
w h ic h
ke V .
e n e rg y
h o le s
d e c a y s
e m is s io n
fro m
e le c tr o n
h o le s
n u c le a r
The
20
O n an
100
th e
a b o u t
3 1).
e le c tr o n
T h e re fo re ,
to
d is c r e t e ly
73
(30,
100
1 2 5 -T e .
le a d
is
Io d o -
to
e f f e c t .
a re
o f
(2 9 ).
d is c u s s e d
A u g e r
ta k e s
DNA
DNA i t s
r a d ia tio n
a p p r o x im a te ly
w h ic h
a v e ra g e
is
in v e s tig a t io n s
fo rm e d
o f
o f
1 2 5 -1
t o x i c i t y
gam m a
to
th e
o f
d is a d v a n ta g e
c h e m ic a l
r e la t iv e ly
hum an
r e u t il i z a t i o n
1 2 5 -1
th e re
c re a te d fo rm e d
o f
o f
a d v a n ta g e
c o n s id e ra b le
e m its
as
p ro b le m s
A n o th e r
b e t te r .
The
a v e ra g e
i t s
I
su ch
a nd
(2 3 ).
h a l f - l i f e
is
125-
a d d itio n , t h is
2 8)
p ro d u c ts
a t
42
keV
n u tr in o
d a u g h te r
n u c le u s
eV .
D o ÂŁ im e tr ic _ c o n s id e r a tio n s B e ca u se
o f
th e
s h o rt
e n e rg y
is
ra n d o m
d is t r ib u t io n
v a rio u s
a b s o rb e d
th e
c e ll
a s c rib e d
w ith
a
n u c le u s
lo w
e n e rg y
o f
The
o u ts id e
to
o f
th e
3 -H
b e ta
th e
A u g e r
h a s
o f
c lo s e DNA
to
0 .5
c h o se n
e n e rg y
in
(3 2 ).
The
ke V
p e r to
ra n g e
o f
In
o th e r
d e c a y be
s im ila r
s o ,
was
th e
an
th e
n u c le i
th a t ta k e n
is o to p e
d is c r e te
o f
d e c a y
in
RBE
o f
e le c tr o n s to
p a r t ic le
A s s u m in g
e m itte d
ta k e n
e le c tr o n s
to
c e ll
125-1
a re
re fe re n c e
o f
1 2 5 -1 . in
p e r
A u g e r w e re
a v e ra g e
th e
m o st
d o in g
w h ic h
a v e ra g e A u g e r
1 2 5 -1
ra d s
e le c tr o n s
e n e rg y
n u c le u s
d e c a y in g
w ith
d ose
ke V .
18 . 5
e le c tr o n s
th e
la b e le d
A u g e r
o f
e le c tr o n s .
A u g e r
c a lc u la te d 3
w as
o f
a v e ra g e
a b o u t
c e ll
an
an
was
f r a c t io n
s id e r a t io n . 3 -H
o f
th e
e n e rg y
t o t a l 1.
v e ry
d ia m e te r s ,
w as
ra n g e
p e r w ith
h a v e is
4
d e c a y
an
a RBE
a b s o rb e d
in t o
c o n Â
b e c a u se e n e rg ie s
th e o f
297
IAE A -P L-61 5/19
TABLE VI.
RADIATION ABSORPTION FROM 1Z5I AND 3H
N u m b e r
o f E n e r g y
e l e c t r o n s / d e c a y
1 2 5 j
3
e
3
e "
3
T o t a l
3 H
/ i c / g
1
e ~
1
pc/g
------------
i t
w as
------------- --------------►
/ - d e c a y
v o lu m e
tim e s
th e
th e
d e c a y
o f
th e
8
in
sam e
th e
The
th e in
o f
o f
th e
e x p e rim e n ts DNA
tu r n o v e r
e a s ily
k e V
1 .4
r a d / h
8
----------------------------- ►
k e V
x
r a d / h
n u c le u s
p
(8
r e m /h
54
5 . 6
k e V
----------------- ------------►
1 .1
1 3
0 )
r e m / h r
3 . 5
d en se b y
d ia m e te r,
3 -H .
b y
e t
125-1
t o x i c i t y
o f
dam age
d e f in it io n
was
c a r r ie d ch o se n
d e te rm in e d .
T hus
le a d s
d a ta
is
d e c a y
is
in
to
o f
th e
(3 1 )
to
s u m m a riz e d
to
n u c le i
in
o f
fro m
a p p r o x im a te ly
m a in ly
o f fro m
th a t
a p p r o x i 3- H
d e c a y
3- 5 -
A u g e r
th e
e le c tr o n s
th y r o id
131 - 1 .
k i l l i n g
o f
t h e o r e t ic l
o u t
w ith
as
a
i t
w as
w h o le
p a ra m e te r r e p o r te d
p r e d ic tio n s , m ic e , th a t b y
f i r s t
w h e re
c o u ld E r t l
e t
w h o le
be
b o d y
r e la t iv e ly
a l.
in
6.
ta b le
r e s p o n s ib le
in to
in c o rp o ra te d
a
v o lu m e
p r e d ic t
e ffe c t s
in c o r p o r a tio n b y
sam e
s h o rt-ra n g e be
w it h in
a b s o r p tio n
th a t
c e ll
th e
fro m
s e t
ra n d o m
j u s t i f i e d
fa c t o r
a l.
a t
w it h in
s u rp a s s a
io n iz a t io n
th e
i t
125-1
w ith
w e re
T h is
s h o u ld
F e ig e
d e c a y in g
a b s o rb e d
n u c le u s
E x p e r im e n ta l_ d a ta ,_ c e ll
F o llo w in g
1
1 8 . 9
1 2 5 -1
is
fro m
d ia m e te r
c o n s id e re d in c re a s e d
5
k e V
c a lc u la t io n
e ffe c t s
ty p e
c o m p a ris o n
k e V
1 7 .4
- 1
c e l l
m ic ro n
th a t
t h is
m ic ro n
r e la t iv e ly
w e re
8
o f
e n e rg y
r a d ia tio n
1 . 5
k e V
1 2 .1 4
th a t
in c o rp o ra te d
b a s is
m a te ly
f o r
c a lc u la te d
3 -5
th e
k e V
H - d e c a y
s p h e r ic a l
On
.5
4 0 . 3
0
r e m /
T h u s ,
0
3 4 . 2
e -
1
r e m
-
RBE
1969
f o r g la n d ,
298
F E IN E N D E G E N
F I G . 4.
(3 3 )
th a t
in
w h o le
m ic e
1 2 5 -Io d o d e o x y u r id in e fo llo w in g fa c t o r w as
d e c a y
r a n g in g
e x p e c te d
w o rk
w as is
in
DNA
tu r n o v e r o r
r e s u lt s
w h ic h
o f
o f
5
th e
o r
b y
on w as
a nd
c e ll
c e lls
m ic e ,
k i l l i n g
s u r v iv a l. c u rv e
to
fro m
w as
10
tim e s
I t
w as
in
H ughes w ith
w e re
c e ll
w h o le -b o d y
in d ic a te d
a b o u t
b y
3 - H
a
th a t
fa c t o r
d o s e .
A
o b s e rv e d
DNA
b y
3-5
o f
fo llo w - u p
o f
125-1
in
c a u s in g
la b e le d
w ith
a
as
th a t
e f f e c t iv e in
w ith
3 -H - th y m id in e
c o n firm e d
m o re
o f
la b e le d
e x c e e d e d
v a r io u s (3 5 )
c e ll
w o rk e d
in c r e a s in g
d e a th
in je c t e d
th e
a lm o s t m o re
o b v io u s
in c o rp o ra te d
w as
c o u n tin g
m o n ito r th a t
in je c t e d
c e ll
in H
th e s e w as
o f
w ith
ca u se d
fro m
id e n t ic a l.
a t o r
3 -H
3- H
a t
e x p e rim e n ts a
m
o f
t h is d e c a y ch a n g e s
3 -H -
u l t i - h i t
d if f e r e n t
le u k a e m ia
1 2 5 -Io d o Â
3 - H - th y m id in e .
p e r ito n e a l d a ily
T ra c e
la rg e b y
d e c a y
a nd
th e
1 3 1 -1
d e c a y le v e l
th a t ty p e
th e
o f o f
in
a m o u n ts
a m o u n ts
3 -H
th e
c a v ity
in t e r v a ls
1 3 1 -1 .
d e c a y
H o w e v e r,
th a n
b y
m ouse
w ith
th e
1 2 5 -1
d e a th
e f f e c t iv e
3-
in to
to g e th e r
e f f e c t
o r
assayed
o f
s y s te m s
w ith
a m o u n ts
1 3 1 - Io d o d e o x y u r id in e
1 2 5 - Io d o d e o x y u r id in e
d a ta
n o t
and
tim e s
DNA
w ith
a b s o rb e d
d e c a y
la b e le d w ith
th y m id in e , s e rv e d The
is
10
a nd
(3 4 )
c o n firm e d
H o fe r
la b e le d
m ic e
o f
1971
in
in
w h ic h
la b e le d
10 ,
-
1 2 5 -1
e ffe c t s
DNA
b a s is
th a n
w e re
n o n - la b e le d
th e s e
b e tw e e n
th e y
d e o x y u rid in e th e
to
in
o f
3 -H - Io d o d e o x y u r id in e .
la b o r a to r ie s .
A fte r
le d
a p p r o x im a te ly
th y m id in e
c e lls
d e c a y
3- H
on
DNA
T h e se
o f
s u m m a riz e d
in
D o s e - s u r v i v a l c u r v e s f o r 125I, 131I and 3H.
o f
3 -H -
( f i g . 4 ). d e c a y 1 2 5 â&#x20AC;&#x153; I 37
%
d o s e - e ffe c t
w ith
a
s h o u ld e r,
299
IAEA-PL-615/19 w h e re a s
th e r e
c u r v e .fr o m
I n - v i t r o f o r
a
w ith
o r
in c o rp o ra te d
c u ltu r e s
p e r io d
o f
w e re
o f
one
n e e d e d
% s u r v iv a l
W hen
th e
d a ta
d iffe r e n c e c lo s e
W hen
c e lls
o rg a n is m r a tu r e
o f
r a d ia t io n o f
m ouse c e ll
c y c le
w it h in
w hen
3 7° o r
th e y
C ,
th e a re
th e re
fro m
n u c le a r
r e p a ir
o b s e rv e d .
T h is
r e p a ir
in
o n ly
a n d
s e ts
e ffe c t s
a n d
c e ll
V 79
a nd
o f
a n d
in
1 2 5 -1
in
125-
I
w e re
20
la b e le d
tim e s
c e ll
le s s
k i l l i n g
a b s o rb e d
1 2 5 -1
o r
1 2 5 -1
to
a nd
d ose o f
3 _H
w e re
o rd e r
in
th e n
n e e d e d .
o rd e r fro m
to
w h ic h
a re
c e lls
t o t a l
to
T h e se
-1 9 6 °
c o ld
6.
o f
46
o n ly
39
d e c a y s
tim e s
d a ta
a re
is
la b e le d
С
o r
w as
t h is
as
a d d itio n a l tw o
m any
b y
f o r
th e
to
t h e ir
A t
c e lls The
d o s e -
th a t
37
th e
% s u r v iv a l,
d if f e r e n t
w e re
r e q u ir e d ,
n e c e s s a ry ; e f f e c t
a t
d e c a y s .
o b s e rv e d .
c o n s is te n t
o n e -
s to re d
o f
n o te w o rth y
3~H
c e ll
3- H -Io d o -
a nd
d e c a y s
o f
i n d i
le u k a e m ia
k i l l i n g
d e s p ite
p ro d u c e d
20
w e re
I t
a n
p u rp o s e
s to r a g e ,
p ro d u c e
s t r a in s c e ll
e x c lu d e
a c c u m u la tio n
s u r v iv a ls
d e c a y s th a n
to
f o r
ca se
c h a ra c te r iz e d
w e re
e f f e c t
m in im iz e d
r e p a ir ,
L5178Y
5 and
V79
w e re
In
n e t
th e
a re
fro m
e ffe c t s
th e
in
e ffe c t s
th e
and fig s
th e
m o re
in d ir e c t
in flu e n c e
n a m e ly
b e g in n in g
in
c e lls
e ffe c t s
p e r t in e n t
fr o z e n
c e ll
a
d ir e c t
t h is
a llo w
tw o
3 -H
te m p e
1 2 5 -Io d o d e o x y u r id in e
n e c e s s a ry
c e ll
l i v i n g
a
F o r
T h e se
th e
m any
a
a t
c u ltu r e
(3 7 )-
c e l l ,
to
o f
in
a ls o
a l l
th a w in g .
in
as
V 79
d e c a y s
d o s e - e ffe c t
c o n d itio n s
to
in d ir e c t
u s e d ,
w ith
show n
k e p t
w h ic h
d e c a y s
L 51 7 8 Y
tim e s th e
w e re
th e y
a re
s im ila r
th e
o f
o f
in te r fe r e n c e
h a m s te r
r a d io s e n s it iv it ie s :
6
d e c a y
p e rfo rm e d
s e e d e d ,
n u m b e r
in
te rm s
a d d itio n
c e l l ,
th a w e d ,
c u rv e s
a nd
th e
1 2 5 - Io d o d e o x y u r id in e a b o u t
p ro d u c e
p a r t ic u la r ly
a f t e r
r a p id ly
a b o u t
in
in t e r v a ls
e f f e c t
v e ry
th e
th e
a f t e r
e it h e r
te m p e ra tu re
w e re
is
lin e s
c y c le
tim e
to
p h y s io lo g ic a l
e x a m in e was
C h in e s e
d e o x y u rid in e
p ro p e r
in
r a d io s e n s it iv it ie s .
g e n e r a tio n
t h is
to
e x p e rim e n ts
th e
w ith
tr a n s m u ta tio n ,
is
a nd
d if f e r e n t
d e c a y s
p henom ena
fr o z e n
d if f e r e n t
in
(L 5 1 7 8 Y )
s y s te m
o p tim a lly
a re
In
o f
a l l
c e lls
e ith e r t h is
b e tw e e n
d o s e - r a te s .
r e c t
In
3 -H
lo w
s e t
le u k a e m ia
e x p re s s e d
fa c t o r
c o u rs e
f i n a l l y
a t
7-
g ro w
o r
s h o u ld e r
1 2 5 -1 .
th a n
w e re
th e
to
no
(З б ).
w as
a nd
l i t t l e
3 " H -Io d o d e o x y u r id in e .
d e c a y s 37
w as
in
d e c a y s w ith
y e t,
L - c e lls , th a n
th e
300
F E IN E N D E G E N
D O SE О
470
940
IN
RADS
1 41 0
1880
3H F IG . 5.
D ECAYS
th a t
tr a n s m u ta tio n
e f f e c t
th a t
r e p a ir
a n d to
th e
In
to
e x te n d
o rd e r
w e re
u n d e r in
n o rm a l
th e
fr o z e n
4 00 0
3 -H
k e p t
u n d e r
1 2 5 -1
c u ltu r e c e l l ,
d e c a y s
to
n o rm a l
d e c a y s
w e re
fro m
1 2 5 -1
fo llo w in g
hum an
a g a in
3 -H - Io d o d e o x y u r id in e a t
la b e le d
.
606
C ,
o r
c o n d itio n s r e q u ir e d
d e p re s s
1212
c u ltu r e
n e c e s s a ry
to
c u ltu r e
a t
37°
90
1 2 5 -1
a t
d e p re s s
ra d ia tio n
e ffe c t s w e re
s tu d ie d
1 2 5 - Io d o d e o x y u r id in e
w e re
to
o u tw e ig h
n é g lig e a b le ,
in d ir e c t
in
la b e lin g
th e y
c o n d itio n s
is
fa r
d e c a y .
w ith
s u r v iv a l
1 2 5 -1
d e c a y
3 -H
F o llo w in g
-1 9 6 °
i t
0
DNA
T - c e lls
c e lls
s to r e d
C ELL
re g a r d in g
(3 8 ).
a n d
2820
1 2 5 -1 ,
o f
fr o z e n
IN
N U C LEU S
o b s e rv a tio n s
d e c a y
w ith
2350
e ffe c ts
fo llo w in g
s it u a t io n
fo llo w in g The
CELL
D o s e - s u r v i v a l c u r v e f o r 3H i n D N A .
a s s u m p tio n
c o n tr a ry
TO T H E
C.
th e
W hen d e c a y s
37
c e lls
w e re
tr a n s fe r r e d
%■
37°
d e c a y s o r
W hen C,
s u r v iv a l
e ith e r
and
k e p t
a c c u m u la te d
a p p r o x im a te ly th e
a g a in to
o r
37
c e lls
w e re
o n ly
90
%■
T h is
301
IA E A -P L-6 1 5/19
DOSE
0
IN
RADS
127
is
show n
in
d e c a y
in
th e
c e lls
in
n o rm a l
1251
DECAYS
in flu e n c e
F ro m
to
th e s e
m a m m a lia n th a n
m ay
e f f ic ie n c y s e c o n d
o rd e r o f
d e c a y
q u ite
on
o f
0 .5
w ith
w as
w as
to
to
be
3_ H f o r
le t h a l
1 2 5 -1
a b s o rb e d b y
w o rk
3- H. 0 .2 2
d e c a y 1 2 5 -1
e v e n t,
1 2 5 -1 A
la b e le d is
d ose
c e lls
in
DNA
p h a g e ;
o f
e m itte d w ith
E .c o li
e f f ic ie n c y
d e c a y
in d ic a te d
a nd no
k i l l i n g
fro m
s t i l l
o r
DNA.
c e ll
k i l l i n g
(4 0 ).
as
in
d e c a y in
1 2 5 -1
l i t t l e
p e rfo rm e d
The f o r
o f
fr o z e n
d e c a y
e f f e c t iv e
o f
o r
in
a s c rib e
1 2 5 -1
th a t
m o re
1 2 5 -1
e f f ic ie n c y
id e n t ic a l
fo llo w in g
b a s is
f o r
fo u n d a
507
DNA
k i l l i n g
c o n firm e d
fo u n d
0 .0 0 5
p ro d u c e d
th e
o b v io u s
th e
f u r t h e r
e it h e r
m ic ro o rg a n is m th e
e ffe c t s
c o n s id e ra b ly
is
338
CELL
j u s t i f i e d
is
DNA
o f
is
is
la b e le d t h is
i t
i t
e x p e c te d
IN
p r a c t ic a lly
d a ta
be
NUCLEUS
169
D o s e - s u r v i v a l c u r v e f o r 125I i n D N A .
c e lls
T h is
is
is
CELL
0
7 -S in c e
c u ltu r e ,
in d ir e c t
r a d ia tio n . in
fig u r e
T - c e lls
THE
381
F I G . 6.
d a ta
TO
254
( 3 9 ) ,
h ig h e r i . e . in
in
i t
k i l l i n g
e v e ry ta b le
7
(3 9 )
302
P ercent
s u r v iv a l
F E IN E N D E G E N
О
500
1000
1500
Dose ( ra d s ) F IG . 7.
T A B L E VII.
T e m p e r a tu r e e f f e c t on d o s e - s u r v iv a l curves.
L E T H A L IT Y OF 125I D E C A Y A T -196°C (see Ref. [ 39]).
o( x
ю 'г)
K illin g M ethod of E xp erim en t num ber
p u rific a tion
in
W eig h ted
p er phage
-
t S. E .
S. D .
1
CsCl
43- 0 *
11-9
Sucrose
12* 5
5 3 -8-
3-4
3
Sucrose
21*9
46-
re a s o n DNA
on
m a m m a lia n d e c a y
to
7- 1 3
d a ta ,
f o r
th e
DNA
e n o rm o u s
s u r v iv a l
o f
p a r t ly
DNA
48-9
- 2-6
г * г.ь
s tra n d _ b re a k a g e
c e l l s , i s p ro d u c e
m ean
E xp e rim e n ts
2
E x p e rim e n ta l
The
125, t 1 atom s
phage
E ffic ie n c y
In d ividu al
b io lo g ic a l
la b e le d
p h a g e ,
e x p la in e d
d o u b le - s tr a n d
e ffe c tiv e n e s s
la b e le d b y
th e
b re a k s .
o f
c o li ■ a nd
h ig h
1 2 5 -1
d e c a y
la b e le d
e f f ic ie n c y
o f
t h is
303
1АЕА-РЬ615/19
S c h m id t
a nd
H o tz
is o la t e d
i t s
o rd e r
a llo w
o f
to
t h is
w as
a
to
d a ta ^ _
p a r t ic u la r
a ls o
o b v io u s
p ro d u c e
th e
W h e re a s
fo llo w in g
m u ta tio n
c o n s e q u e n c e s
fro m
o f
th e
th e
gam m a
d a ta
th e re
fro m
d e c a y
th a n
r e la t iv e ly
in d e e d io n s ;
s im ila r y e t,
as
fro m
m o re
to
S c h m id t
i t
p h a g e
42
s ta te
o f
e v e ry
th e
in
r a te
s e c o n d
1 2 5 -1
)c o n firm e d
e v e ry a re
e f f ic ie n c y
w ith
o f
h ig h
and
H o tz
to
t h is
3~H
p e r
le v e l
d e c a y
a re
is
1 2 5 -1
v e ry
d e c a y
d i f f i c u l t
th a t tim e s
H e n c e ,
th e
show ed
r a t io
a t
1 2 5 -1
(4 1 ) ,
1 2 5 -1
r e p o r te d
as
M u t a t i o n r a t e versus s u r v i v a l r a t e f o r 125I a n d 3H .
c o li
r e s u lts
50
%
d e c a y The
fro m
2
p r o
e f f e c t
d e c a y
be
show s E ..
m u ta tio n s
b re a k s to
8 in
th e
th e
m o re
s u c h
d o u b le - s tr a n d w as
to
a re d e c a y
P ig .
m u ta tio n s .
LE T r a d i a t i o n
a nd
1 2 5 -1 1 2 5 -1
s u r v iv a l
s im ila r
18
th a n
o f o f
d e c a y . o f
o b v io u s
e v e n ts
o f
F IG . 8.
th a t
tr a n s m u ta tio n
lo w
1 2 5 -d e c a y .
b re a k s
w h e re a s
fr o z e n
K r is c h (
r e s u lt s
a p p r o x im a te ly
s i n g l e —s t r a n d 1 ,
som e
th e
3“ H
le t h a l
th a t
b y
th e
a n a ly s is
e ffe c t s .
o f
y ie ld e d c lo s e
in
c o m p a ris o n
a re
An
in d ic a te d
T h e se
p ro d u c e d fro m
in
in d u c tio n
ir r a d ia t io n ,
le v e l
3 -H
in
1 2 5 - Io d o d e o x y u r id in e ,
i t
d e c a y s .
w o rk
r e la t iv e ly
m u ta tio n s
s u r v iv a l
d u c e s
th a t
r a d ia tio n
m u ta tio n s
n u m b e r
(4 3 ).
b y
o f
b re a k .
w ith
s to re d
re c e n t
in d ic a te d
s o le ly
E x p e rim e n ta l.
T -p h a g e a nd
p ro d u c tio n
M o re
d o u b le - s tr a n d
e x p la in
The
b re a k
a nd
i t
a c c u m u la tio n
e f f e c t iv e .
fin d in g
p ro d u c e d
la b e le d
d ilu t e d
f o r
d o u b le - s tr a n d
d e c a y
to
( 4 l)
DNA,
in a t :
is
a c c e le r a te d p ha g e a 1
DNA
r a t io to
4
:
1
3 04
F E IN E N D E G E N
a f t e r
DNA
o x y g e n , is
of
b o m b a rd m e n t
n itr o g e n ,
th e
o rd e r
H y p o th e s is _ o f
I t
is
c le a r th e
in to
a c c o u n t
20
:
a l l
e ffe c t s
s t r i c t
th e
DNA
m e c h a n is m s fro m
e v e n t.
The
o f
r e s u lt
in
in
g a s e o u s
d e c a y
T h e re as
a
th e
is
in
B io lo g ic a l
The
in
125-
e f f e c t be
I
m ay
m ay
R e c e n t
b y
in
o f
F o llo w in g te s te d . e n e rg y and
(4 5 ).
as
to
i t s
th e
A u g e r
e f f e c t
9
1 0 .0 1
s ta te
th e
th e
k e V
th e re
c o n c o m ita n tly
an
in v o lv e
w as
th e y
th e
w e re
e f f e c t
m o le c u la r o f
o f c h a rg e
w h ic h
w hen
co m p a re
to
su ch
o f
d im e n s io n s .
e ffe c t s
m o le c u le s
a p p ly in g
d e s tr u c tio n
o f
e s p e c ia lly
th e
b y
as
fro m
th e
o f
e ffe c t m a c ro Â
e ffic ie n c y
P o te n c y
a
1 2 5 -1
A u g e r
s e g m e n ts
b re a k s .
a p p lic a tio n
h a s
b e e n
to o l
re p o r te d
e n zym e ,
p o s s ib ilit y in
th e
w ith
e n e rg ie s
show s
to
c o n ta in in g
th e
ir r a d ia t io n ,
p o in t
o f
f o r
th e
w h a t
may
s u r g e r y " .
z in c
d ry
dam age
show n
s tu d y
th e
io n iz a tio n d a ta
m o le c u le ,
o f
ta k e
e f f e c t
d o u b l e â&#x20AC;&#x201D;s t r a n d
to
in
F ig . o f
r a t io
v i c i n i t y
w e ll
a p p lic a tio n
d e v e lo p m e n t
d ir e c t io n
The
b y
m o le c u le s
may
m u st
th e
im m e d ia te
d ia m e te r
A u g e r
L o c a liz e d
le a d
d is c r e te
a
lo c a liz e d
p ro d u c e
t h is
o rd e r
ir r a d ia t e d
c a rb o n ,
t h is
h y p o th e s is d e c a y
f a c t ,
m ay
o f
O ne
th e
th e
a c h ie v e d
to
in
p ro d u c in g
le v e ls
be
o f
in
c a r r ie r
(4 4 ).
fo re s e e
" m a c r o m o le c u la r
la b o r a to r y u se d
as
b io lo g y .
f u r t h e r
p e rh a p s
w o rk
su ch
i n it i a t e d
th e
th e
th e
1 2 5 -1
e f f e c t
d is r u p tio n
o f
b io lo g y .
d e c a y
te rm e d
to
fro m
to
to
beam
s t r i c t l y
e n c o u ra g e s
m o le c u le s o f
re a s o n
a p p lic a tio n
m o le c u la r
io n s ,
ir r a d ia t io n
s u p p o rts
th o s e
o f
Te
p h a se
m ic ro
m o le c u la r
a p p a r e n tly
d e c a y
th a n
e ffe c t s
v io le n t
a
fa r
ir r a d ia t io n ;
d a u g h te r
w ith
e v e ry
t o o l
o th e r
m e c h a n is m
show n
1 2 5 -1
th e
so
m o le c u le
e le c tr o n
fro m
to
h e a v y
e le c tr o n
1.
th e
tr a n s f e r
e x a m in e d
A f t e r
e v id e n c e
on
lo c a liz a t io n
d e c a y
a c c e le r a te d
b o ro n .
m e c h a n is m
and e x c it a t io n to
o f
th a t
th a t
w ith
and
z in c
fro m
b io lo g ic a l
is
a
in c re a s e
o f
c a rb o a n h y d ra s e ,
a to m .
The
I t
is
z in c
q u ite ris e
in
th e
enzym e
p h o to n s
a p p r o x im a te ly
a c t i v i t y
su d d e n
S t o c k lin â&#x20AC;&#x2122; s
in a c t iv a t in g
m o n o e n e rg e tic
ra n g in g
r e s u lt s .
o f
b y
o f
th e
c le a r enzym e
re le a s e .
The
a t 8
was v a rio u s
to
enzym e
th a t
a t
was
enzym e
1 0 .7 was th e
in a c t iv a t io n e f f e c t iv e
ke V .
30 5
1АЕА-РЬ615/19
%
н
31
27
inactivation
23
19
-Н 15
3 Z n - re lease 2
1
/ - i — *— i--------------------------------------------1----- 1------------- 1------------- 1------------- 1
О 756
8.14
8.74
9.37
10.01 10.69 11.38
Photon Energy [keV] F IG . 9.
e n e rg y th e re
c o rre s p o n d s is
a
p ro d u c in g d is r u p t A u g e r
h ig h th e
to
A u g e r
a t
th e
h ig h
c r o s s - s e c tio n
m ay
).
be
o f
th a t
and
th e
th e
in c o rp o ra te d
r e s u lt s
o f
X -ra y s
c o rre s p o n d s
to
ir r a d ia t io n p a r a lle l
r e s u lt s
u s in g
t h is
s tr u c tu r e s w h ic h
t h is
t h is
th e to
b y
le v e l
К
s h e ll
lo c a lly p la c in g
th e
is
m ade
th a t
h a s
s p e c ific
DNA. o f
in
p h o to n
th y m id in e e t
a l.
(4 7 )
b r o m o d e o x y u r id in e
К- a b s o r p tio n r e s u lte d
o f
H a lp e rn
e n e rg ie s .
in v e s tig a t io n
p la c in g
r e la t iv e ly
d is c r e te
a n a lo g u e
in to
b y a
e dge a
A t
su d d e n
w ith
th e
f o r
e n e rg y
b ro m in e , r is e
th y m id in e
o f
r a d i
f a ile d
e f f e c t .
e n c o u ra g e
fu r t h e r
p h e n o m e n o n ,fo r
s u c h
s u ita b le
A
fu n c tio n
a to m
d is c r e te
th e
A t
in
p o s s ib le
e f f e c t
ir r a d ia t io n
o f
p ro d u c tio n .
o f
A u g e r ta r g e t
an
m o n o e n e rg e tic
T h e se
a lt e r
a
s p e c if ic a lly
z in c .
p o s itio n s .
is
c a l
p ro d u c e
th u s
o f
q u ite
a t
th e
to
a
edge
p h o t o - e ffe c t
th u s
m o le c u la r
c e lls
f o r
is
th e
p h o to - e ffe c t
m o n o e n e rg e tic
le v e l
I t
a p p ly in g
l i v i n g
f o r
T hus, b ro m o d e o x y u rid in e
in v e s tig a te d w ith
e f f e c t .
p re c is e
in t o
e n e r g y (4 6
К- a b s o r p tio n
s tr u c tu r e
in to
a tte m p t DNA
th e
c r o s s - s e c tio n
m o le c u la r
e f f e c t
A n o th e r
a nd
I n a c ti v a t io n and z in c release in c a r b o n ic a nhydrase.
as
c e lls
ta r g e t
o r
e x p lo r a tio n
e x a m p le , t o fu n c tio n a l
n u c lid e s
m ay
be
in t o
th e
in a c tiv a te im p o rta n t
p o s s ib ilit y
s p e c ific
m o le c u le s
in c o rp o ra te d .
in to
F o llo w in g
”
306
TRANSM UTATION
AND
RADIATION
EFFECTS
FOR
F E IN E N D E G E N
te a
g Q g
2
л W
u «• O « 2
2
«
W u (tt _
eu a
o
o
*
g
J
Дo
> Í3
al
И Д <! <3 b K
2
ь< 4» (fl
ю H p
-<
« e 3
Q W H < K O
0 •o S * = 1 :
^ « û. £ . É¿ -s*?5 3 s 2 2 è 5
307
1АЕА-РЬ615/19
m o n o e n e rg e tic o f
a
f o r
ta r g e t e x a m p le fro m
o f
e f f e c t
a n d
th a t
in
in to
e f f e c t th e
X - ir r a d ia t io n
a to m
th e
th e
th e
D NA, DNA
e f f e c t
le v e l
o f
A u g e r
e ffe c t s
la b e le d
p h o to n s .
on
m ay
a t
o f
a b s o rb e d
d e p e n d s
th e
lo c a liz e d
th e be
c e lls
I t
is
d is c r e te
p r e c is e ly
th e
a nd
g e a re d
m ay
m ay
c le a r
e n e rg y
К- a b s o r p tio n
u se d
th e
th e
s p e c if ic it y
fo r
th e
edge
p la c e d
a m p lify
th a t
to
be
ir r a d ia t io n
ta r g e t
a to m
ch o s e n .
C o n c lu s io n
In fo r m a tio n
on
r e a c tio n s
is
s c a n ty .
o r
r a d io n u c lid e s
W h a t
is
e s p e c ia lly
In
ca se
p a r t ic le s Y e t,
i t
u n d e r o f
I
a re
is
25-
I
m a in ly
to
re a s o n
a tio n
f o r
th e
p a r t ic u la r
t o x i c i t y
c o n te x t
h e a lth
n u c lid e s
w h ic h
to
T a b le
8
o f
b y
th e
d a ta
in c o rp o ra te d
fro m
fig u r e
r a d ia tio n
tr a n s m u ta tio n
T h is
su m m a ry
s e r v a tio n s
m o le
e m itte d
b e ta -
e f f e c t
H o w e v e r,
is
o b se rve d .
tr a n s m u ta tio n in
th e
c o n s id e re d
to
be
th e
e m itte d
r a d ia tio n .
t o x i c i t y
a p p e a rs
to
p ro c e s s e s
a n d
be
e ffe c t s
ca se m ost
th e
lo c a liz e d
A u g e r
a l t e r
A u g e r I t
e f f e c t
a p p e a rs
a m o u n ts
to
and
is
im p o rta n t
j u s t i f i e d
hum ans
o f
to
a l l
in
r e su ch
m ay
be
in c o rp o ra te d
a tte m p t
to
d if f e r e n t ia t e
c lo s e i
s tr u c tu r e s .
r a d io n u c lid e s a
e ffe c t
im p o r ta n t
th e b io lo g ic a l e ffe c ts
fro m
К- c a p tu re
e ffe c t s
a t
th e
p h y s ic s .
tr a n s m u ta tio n
d e r iv e
th e
s p e c if ic
c o n d itio n s .
tr a n s f e r
im p o rta n t
s u m m a riz e s
1 4 -C ,
f o r
e ffe c ts
p e r m is s ib le
d e c a y
b io lo g ic a lly
m a te r ia l
s t r u c tu r e .
th e
m a x im u m
and
c o n s id e ra b le
The
e v a lu a te
tra n s m u ta tio n
b io lo g ic a lly
d if f e r e n t ia t e
c h a rg e
m o le c u la r
o f
3 3 _ P ,
tr a n s m u ta tio n
t h is to
b io lo g ic a l
DNA.
r e s p o n s ib le to
o u tw e ig h
le a d in g o f
th e
w it h in
th e
in to
e x p e rim e n ta l
decay, th e
a to m s
c o n ce rn s
3 2 -P ,
p o s s ib le
a n d
e f f e c t
in t o
3~H ,
p a r t ic u la r
s e v e re The
o f
kn o w n
h o t
in c o rp o ra te d
c u le s ,
th e
o f
fro m
in
an
r a d ia tio n
e ffe c t s
in t o
S uch
DNA.
g iv in g
th e
r e la t iv e
a b s o r p tio n
p e r
u n it
on
a n a ly s is
am ount
dam age
DNA
o f
th a t
p ro d u c e d a llo w s
to
r e le v a n t
is
c a u se d
b y
dam age b y
th e
e f f e c t .
is
a nd
f a r fa c ts
fro m w ith
c o m p le te . l i t t l e
I t
e m p h a s iz e s
in fo r m a tio n
on
e m p ir ic a l
o b
m e c h a n is m s
in -
308
F E IN E N D E G E N
v o lv e d . is a
In
th e
f a c t ,
th e
im p o rta n t
b e t t e r
q u e s tio n
q u e s tio n
u n d e rs ta n d in g
e v a lu a tio n
r e g a r d in g
p o t e n t ia l
o f
m e n ta tio n
a n d
o f
th e
a p p ly in g
r e g a r d in g
th a t
I
th e
w is h
m e c h a n is m s
a sk.
m e c h a n is m s
w i l l
q u e s tio n
h o t
th e
to
a to m
o f
h e a lth
r e a c tio n s
I
am
p e rm it
p h y s ic s
to
in v o lv e d
s u re a
th a t
r e a l i s t ic
and
th e
b io lo g ic a l
e x p e r i
m e d ic in e .
REFERENCES 1.
H.
D e r tin g e r ,
S p rin g e r 2 .
E .J .
H.
J u n g :
V e r la g ,
H a ll:
M o le k u la re
R a d io b io lo g y
H a g e rs to w n ,
M d .,
R .B . S e tlo w : The r e p a ir R a d .R e s . 1 9 6 6 , 5 2 5 -5 3 7
4.
H a nd b u ch S p rin g e r
5.
W. V e a tc h , S. O ka da : m a m m a lia n c e lls J o u r n a l,
10.
m o le c u la r
dam age
9 ,
2 7 9 -2 8 9 ,
a u f
b re a k s
S tr u k tu r
u nd
P ro g e s s
3 ,
Ю З
A c id
B io lo g ic a l e ffe c t s r a d io is o to p e s o f
an
IA E A
o f
R es.
tr a n s m u ta tio n
P a n e l,
V ie n n a ,
O c t.
r a d ia tio n
1 9 6 7 ,p u b l.
in
R .E .K r is c h , th e r o le o f
S.
A p e lg o t,
M .R . Z e lle : B io lo g ic a l e ffe c t s th e tr a n s m u ta tio n e ffe c t
R.
1 7 7 ,
R .E . K ris c h : C o m p a ris o n d e c a y in E. c o li and in
P .N . 3 -H
R o s e n th a l, a nd
3 2 -P
J . M o l. B io l.
on
1 8 ,
M .S . th e
197O ,
C o m p a ris o n
o f
on
n u c le ic
o f in c o rp o ra te d 1968. a re c u r r e n tly R a d ia tio n
r a d io a c tiv e
des s u ic id e s d ’ une
32
e t
o f th e le t h a l e ffe c t s b a c te r io p h a g e s 3 ,
F o x :
259- 266, E ffe c ts
p h y s ic a l
5 4 ,
c u ltu r e d
d e c a y :
1969
L a t a r je t :
I n t . J . R a d ia t. B io l .
3
d e r
E ffe c ts o f r a d io a c tiv e d e c a y w it h in th e DNA m o le c u le d is c u s s e d b y C o m m itte e 24 o f th e N a tio n a l C o u n c il on P r o te c tio n a n d M e a s u re m e n t (U S A )
165
13 .
DNA
F u n k tio n
a nd d e c a y
m a rq u é e p a r le s p h o s p h o re s r a d io a c tiv e s I n t . J . R a d ia t. B io l . 1 0 , , 1966 12.
o f
T e il
1973 io n iz in g
N u c le ic
DNA
1969
o f
in
to
S tr a h le n b io lo g ie ,
R a d ia tio n -in d u c e d
V o l. 9 ,N o .3 ,3 3 0 -3 4 6 ,
A d v .R a d ia t. B io l.3 , 11.
o f
R a d io lo g is t 1973
J .J . W ie s s : C h e m ic a l e f f e c t s a c id s a nd r e la te d com pounds
P ro c . 9 .
th e
R ow ,
U. H a g e n : S tra h le n w ir k u n g D e s o x y rib o n u k le in s a u re B io p h y s ik
8.
&
d e r m e d iz in is c h e n R a d io lo g ie , V e r la g , 1 9 7 2 , 1 2 7 -1 7 0
B io p h y s .
7.
f o r
H a rp e r
3.
6 .
S tr a h le n b io lo g ie ,
1969
4 4 1 -4 6 3
a nd
b a c té r ie
33
o f
3 2 -P
and
33~P
1970 o f
d is in t e g r a tio n
b io lo g ic a l
o f
p r o p e r tie s
in c o rp o ra te o f
DNA
309
IA E A -P L-61 5/19
14. В . W.
G lic k m a n ,
dam aged E . c o li M u t. 15.
p ha g e К
K .J . Л
in
A.
R o rs c h :
The
r e p a ir
r a d ia t io n - s e n s it iv e
o f
3 2 -P
s t r a in s
d e c a y -
o f
12
R e s.
1 4 ,
2 6 5 -2 7 0 ,
1972
S. A p e lg o t, R. L a t a r je t : M a rq u a g e d ’ un a c id d e o x y rib o n u c lé iq u e b a c te r ie n p a r le ra d io p h o s p h o re , le ra d io c a rb o n e e t le t r i t i u m , c o m p a ris o n
d es
e ffe t s
B io c h im .b io p h y s . 1 6 .H .A . a nd
17-
W a lk e r , v a rio u s
M cQ uade,
M.
th y m id in e
5 5 ,
4 0 ,
F r ie d k in :
1962
R a d ia tio n
e ffe c t s
o f
3 "H -th y m id in e
1 4 -C
E x p t1 .C e ll
R e s .2 1 ,
S.
S .L .
P e rs o n ,
lé ta u x
A c ta
1 1 8 -1 2 5 ,
P h illip s ,
I9 6 0
F .A .
C h a r a c te r iz a tio n o f g e n e tic b y th e r a d io a c tiv e d e c a y o f
A n d e rs e n ,
H.
N e w to n :
c o d in g ch a n g e s in in c o rp o ra te d 3- H -
b a c te r ia p ro d u c e d a nd 1 4 -C -la b e le d
com pounds CO N F-
7 O O 6I O
1,
—
C O O -3 4 1 9 -3
1 8. S. P e r s o n , M .H . S c la ir : ra te d t r i t i u m com pounds
19 .
R ad.
R e s.
3 3 ,
J .A .
S a n d s ,
6 6 -7 3 ,
W.
DNA d e g r a d a tio n b y d e c a y in E s c h e ric h ia c o li
in c o rp o
1968
S n ip e s ,
S .P e rs o n :
d e c a y s o r ig in a t in g fro m g ro w th and fro m c h e m o s ta tic g ro w th in a c id
fro m
M u ta g e n e s is
and th e
b y
t r i t i u m :
s to ra g e in t r i t i a t e d w a te r p re s e n c e o f t r i t i a t e d n u c le ic
p re c u rs o rs
I n t . J . R a d . B io l. ,1 9 7 2 ,V o l. 2 2 ,N o .2 ,1 9 7 -2 0 2 2 0 .
S .P e rs o n ,
W.
o rg a n is m s
g ro w n
P ro g re s s 2 1 . F .
a
in
R e p o rt
K ra s in ,
d e c a y :
S n ip e s :
S.
lo c a l
P e rs o n ,
and D e ca y
o f In c o rp o ra te d
e ffe c t s
in
m ic ro
o l.
and
DNA
s tra n d
b re a k s
fro m
t r i t i u m
c y to s in e - 6 ~ 3 _H
23 , N
o .4 ,4
17 - 4 2 0
a s s o c ia te d
m e d ic in e " ,
w ith
th e
B io lo g ic a l E ffe c ts
R a d io is o to p e s ,
P ro c .
u se
o f
la b e lle d
o f T ra n s m u ta tio n
of an IA E A
p a n e l, V ie n n a ,
1968.
L .E . F e in e n d e g e n , H .J . H e in ig e r , G .F r ie d r ic h , E .P . C r o n k it e : D iffe r e n c e s in r e u t il i z a t i o n o f th y m id in e in h e m o p o ie tic and ly m p h o p o ie tic T is s u e
tis s u e s
o f
th e
K in e t .6 ,5 7 3 ~ 5 8 5 ,
n o rm a l
m ouse
1973
L .E . F e in e n d e g e n , V .P . B o n d , W .L . H u g h e s: 1 2 5 -I-D U (5 - Io d o 2 ’ -D e o x y u r id in e ) in a u to ra d io g r a p h ic s tu d ie s o f c e ll p r o lif e r a t io : E x p t l. C e ll
25-
m u ta g e n ic
S n ip e s :
f o r
1973 , V
in b io lo g y
C e ll 2 4.
W.
e f f e c t
L . E .F e in e n d e g e n : " P ro b le m s
1 9 6 7 , p u b l.
a n d
g
m o le c u le s O c t.
23-
L e th a l 3
C O O -3 4 2 3 -1
I n t . J . R a d . B io l. , 2 2 .
h
R e s .4 3 ,1 0 7 -1 1 9 ,
1966
W .L . H u g h e s, S .L . Com m er f o r d , D . G it lin , R .C . K ru e g e r, B. S c h u ltz e V . S ha h , P. R e illy : D e o x y r ib o n u c le ic a c id m e ta b o lis m in v iv o : I . C e ll p r o lif e r a t io n a nd d e a th as a nd e lim in a tio n o f io d o d e o x y u rid in e F e d .
P ro c .
Chem .
M e d .2 3 ,
6 4 0 ,
1964
m e a s u re d
b y
in c o r p o r a tio n
310
F E IN E N D E G E N
2 6 .L .E . F e in e n d e g e n , V .P .B o n d , W .L . r e u t il i z a t i o n in r a t b o n e m a rro w
H u g h e s:
P r o c .S o c .E x p tl.M e d .,V o l.1 2 2 ,4 4 8 -4 5 5 ,
27 . K
.G . H o fe r: R a d ia tio n c e lls in m ic e
e ffe c t s
on
P h y s io lo g ic a l
th y m id in e
1966
d e a th
a nd
m ig r a tio n
o f
tu m o r
R a d .R e s .4 3 ,1 9 7 0 ,6 6 3 -6 7 8 2 8 .
W. P o r s c h e n , L .E . F e in e n d e g e n : In v iv o -B e s tim m u n g d e r v e r lu s tr a t e b e i E x p e rim e n ta ltu m o re n m it m a rk ie rte m J o d d e o x y u r id in S tr a h le n th e r a p ie
1 3 7 ,
7 1 8 -7 2 3 ,
1969
2 9 . L . C h e o n g , M .A . R ic h , M .L . E id i n o f f : In tr o d u c tio n -h a lo g e n a te d u r a c il m o ie ty in t o d e o x y rib o n u c le ic m a m m a lia n c e l l s in c u ltu r e
o f th e a c id o f
5
J .B io l.C h e m .2 3 5 , 3 0 .H .H . J Ă&#x153; L -
1 4 4 1 ,
E r t l : D o s im e tr ie E , I O
688- M
97
i
P ro c . o f 3 8 9 -4 0 2 , 3 2 .H .H .
th e
1971
T h ir d
E r t l ,
in c e ll P h y s .in
960
vo n
3 1 .Y .F e ig e , A . G a v ro n : On in DNA r e la t iv e to 3- H
in k o r p o r ie r te m
th e a n d
b io lo g ic a l 3 2 -P
S y m p o s iu m
L .E .
on
T r itiu m
u n d
e ffe c tiv e n e s s
M ic ro d o s im e try ,
F e in e n d e g e n ,
H .J .
H e in ig e r :
b io lo g y : p h y s ic a l p r o p e r tie s a nd M e d .B io l., V o l.1 5 ,N .3 ,4 4 7 ,1 9 7 0
J o d -1 2 5
o f
1 2 5 -1
S t r e s a / I t a ly ,
Io d in e - 1 2 5 ,
b io lo g ic a l
a
tr a c e r
a s p e c ts
0
3 3 -H .H . E r t l , L .E . F e in e n d e g e n : M ic r o d o s im e tr y re fe re n c e to th e A u g e r e f f e c t P ro c . o f 7 8 7 -7 9 8 ,
Z e ll-
th e
S econd
S y m p o s iu m
on
o f
Io d in e -1 2 5
M ic ro d o s im e try ,
w ith
S t r e s a / I t a ly ,
1969
34. L .E . F e in e n d e g e n , H .H . E r t l , V .P . B o n d : B io lo g ic a l t o x i c i t y a s s o c ia te d w ith th e A u g e r e ffe c t in " B io lo g ic a l A s p e c ts o f R a d ia tio n Q u a lit y " ,4 1 9 -4 3 0 , IA E A 3 5 -K .G .
V ie n n a , H o fe r,
S M -1 4 5 -2 2 , W .L .
1971
H u g h e s:
125- i o
R a d io to x ic it y
o f
in tr a n u c le a r
t r i t i u m ,
d in e , a nd 1 3 1 -io d in e R a d .R e s .,V o l.4 7 ,N .1 ,1 9 7 1
36.
R.
0
R o o ts ,
L .E .
F e in e n d e g e n ,
o f 3 - H -IU d R a n d m a m m a lia n c e lls P ro c .
o f
371 - 388,
th e
1 2 5 -IU d R
T h ir d
V .P .
a f t e r
S y m p o s iu m
o n
B o n d :
C o m p a ra tiv e
in c o r p o r a tio n
o f
M ic r o d o s im e tr y ,
r a d io t o x ic it y
c u ltu r e d S t r e s a / I t a ly
1971
37 â&#x20AC;˘ H .J . .B u r k i,
R.
o f m a m m a lia n DNA a t -1 9 6
R o o ts ,
c e lls ĐĄ
L .E .
a f t e r
F e in e n d e g e n ,
d is in t e g r a t io n
V .P . o f
B o n d :
H -3
o r
I n a c tiv a t io n 1 -1 2 5
in
c e ll
I n t . J . R a d . B io l, 1 9 7 3 ,V o l. 2 4 ,N o .4 ,3 6 3 -3 7 5
38.
L .E . F e in e n d e g e n , t o x i c i t y o f x - r a y in t o p e l l u l a r DNA
G. T is lj'a r - L e n t u lis , in d u c e d A u g e r e f f e c t
p re s e n te d
In te r n .
a t:
S e a ttle /W a s h .,
5 th
1974
C o n g re s s
o f
V .P .B o n d : The b io lo g ic a l o f b ro m in e in c o rp o ra te d R a d ia tio n
R e s e a rc h ,
311
1АЕА-РЬ615/19
3 9 .R .E .
K r is c h :
c a p tu re
in
L e th a l
e ffe c t s
E s c h e ric h ia
o f
c o li
a nd
io d in e -1 2 5 in
d e c a y
b y
b a c te r io p h a g e
e le c tr o n
T l
I n t . J . R a d .B io l,1 9 7 2 ,V o l. 2 1 ,N o .2 ,1 6 7 -1 8 9
40 . R. L a t a r je t , B . E k ert, b io c h im iq u e s s u r l - ’ ADN J .C h im .P h y s .5 8 ,1 0 4 6 , 4 1 .A . in
S c h m id t,
G.
c o lip h a g e
S.
A p e lg o t ,
N.
R e b e y r o t t e : E tudes r a d i o -
1961
H o tz :
The
T l-D N A
b y
o c c u rre n c e
io d in e -1 2 5
o f
d o u b le - s tr a n d
b re a k s
d e c a y
I n t . J . R a d . B i o l . ,1 9 7 3 , V o l . 2 4 ,N o . 3 ,3 0 7 -3 1 3 4 2 .R .E .
43.
K r is c h : G.
A h n s tro m ,
k i l l i n g A u g e r
a nd
d u r in g
io n s
M ilfo r d c h e m ic a l
E ffe c ts IA E A
d e o x y u rid in e s lo w -e n e rg y
o f
S. in
H u s s a in , E .
c o li
A .T .
N a ta ra ja n :
a s s o c ia te d
w ith
On
th e
th e
d e c a y
a nd
W h ite : " E x p lo s io n " c o n s e q u e n c e s
N u c le a r
V ie n n a
A . H a lp e rn , B . A u g e r p ro c e s s
th e
1 2 5 -J
:
1 9 6 4 ) 4 5 .
a c tio n
R.
in a to m s C h e m ic a l 1 ,
E h re n b e rg ,
1970, 247-250
C a rls o n ,
m o le c u la r
L .
c o m m u n ic a tio n
m u ta g e n ic
e f f e c t
M u t.R e s . 10, 4 4 .T .A .
p e rs o n a l
(1 9 6 5 ),
o f
o f
m u ltic h a rg e d
in n e r
T ra n s fo rm a tio n s
s h e ll
v a c a n c ie s
( P r o c . S y m p .V ie n n a ,
23
D ie h n , G. S to c k lin : C h e m ic a l e f f e c t s fo llo w in g in b io m o le c u le s : E S R -s tu d ie s on 5 -H a lo th e
in a c t iv a t io n
o f
c a rb o n ic
a n h y d ra s e
b y
x -ra y s
p re se n te d a t :
7th
In te rn .
Hot Atom C h em istry Symposium, J ü l i c h ,
1973 46. G.
T i s l j a r - L e n t u l i s ,
L .E .
F e in e n d e g e n ,
V .P .
S tr a h le n e ffe k te b e i E in b a u m it t e ls c h w e r e r V e rw e n d u n g w e ic h e r R ô n tg e n s tra h le n S tr a h le n th e r a p ie 4 7 . A d p R F
1 4 5 ,
. H a lp e rn , E . J . K e s A u g e r-E ffe k te s re s e n te d a t: "G e m a d io - u n d S tra h le n r e ib u r g ,
1972
B o n d :
K e rn e
B io lo g is c h e
in s
G ewebe
u n d
6 , 656- 662, 1 9 7 3
n u s an e in s c h e
t, G. S tS c k lin : o rg a n is c h g e b u n am e T a g u n g d e r m ie u n d d e r G es
C h e m is c h e F o lg e r e a k t io n e n d e n e n H a lo g e n a to m e n G D C h -F a c h g ru p p e K e rn -, . f ü r N u k le a r m e d iz in " ,
DISCUSSION G. H A R B O T T L E : My question is purely nuclear physics. What is the actual percentage of К capture relative to other orbital capture in the decay of 1 2 5 I? L . F E IN E N D E G E N : The firs t step to 1 2 5 Temoccurs 100% by electron capture, of which 80% is К capture, and 20% L capture. F ro m 125тет to 1 2 5 T e , there is 7% gam m a em ission and 93% internal conversion, of which 80% is К, 11% L , and 2% M. G. H A R B O T T L E : You find that the decay of 32P in D N A leads to a la rg e effect attributable to the elem ental change from P to S. Why can't som e of the effect observed with 125I decay sim ply be the resu lt of the change from I to Те? M aybe D N A doesn't like tellurium .
312
FEINENDEGEN
L . FEINENDEGEN-. T his is c ertainly an im p o rtan t question. T e llu riu m belongs to the sam e group with sulphur, and we've thought about whether it could cause this d ra stic , dram atic effect which I have indicated to you. We have observed the effect in sev eral c ell system s, and it has been con firm e d by H ofer and Hughes in Boston, and by A nstro m , also in E . coli, and by K risc h . It is inconceivable that the placing of S into the methyl group causes an increased effect of 40 in ba c te ria , 50 in som e phage, 100 in others, when com pared with tritiu m as an in ternal standard. T ritiu m decays to h eliu m ; the h e liu m goes off; the structure of the carbonium ion is altered; and a lo t of things m ay happen. So it is inconceivable to me at least that the decay of P into S would have so m uch greater effect than this. G. H A R B O T T L E : I agree with you. I don't think that could possibly be the answer. I think one m ust look into the nucle ar physics of this charging process. L e t's look a little m ore deeply into this. We know that you do not in v a ria b ly produce a high charge follow ing an o rb ita l electron vacancy. The work of C a rls o n and W hite showed that there is in fact a d is trib u tio n of charges — some very high; peaking in the m iddle; and a f a ir num ber of quite low charges. F ro m this spectrum of charge d is t r i bution, one would anticipate this phenomenon — there is a fluorescence yie ld each tim e you have a К shell vacancy, a ce rtain p ro b ab ility of em itting an X -ray ra th e r than em itting an A uger electron; the L -electron fa lls in, and again you have a ce rtain prob ab ility of em itting L X -rays ra th e r than electrons, etc., as we go out to the outer electrons. W hile these can be calculated approxim ately, it would probably be a better thing to use the actual charge d istribution s in an iodine vacancy, or in xenon which would be about the sam e. The question is this: if you are already finding 0.5 breaks per decay, then the actual num ber of breaks per highcharge process m ust be higher than that by some indeterm inate num ber, perhaps approaching 1.0. At least I would think one would not expect to have this extrem e le th a lity associated with the decay events which produce only Te+1 or T+2. I'm guessing at this point, but m y guess is based on your experim ents w ith 14C and tr itiu m . I suggest that the le th a lity per Auger event fo rm in g a high charge is very close to unity. It m ight be interesting to look at these iodine charge-production data since you have a com plicated case here which is not 100% К -capture. I think that you could probably construct a ra th e r accurate charge d is trib u tio n on the basis of what is known. D r. A m ie l has ju st w hispered to me that these num bers fo r charging are of course based on gas-phase experim ents. However, in the electron depletion w hich would follow the charging of an ion even in the solid state — o r here in what is m o re lik e a solution stage — I think that we can s till p roject the re la tiv e dam age in som e fashion. P erhaps not in accordance with the num be r of К vacancies, but in accord with the num ber of electrons ejected. Surely y o u 'll agree with me that a com plete fluorescence event with nothing being em itted except the fluorescence X -ray w ill not cause the damage in a c ell that would be observed if 15 electrons were ejected in the process. Although I'm not saying that an ion of +15 charge is actually produced — that's not the point — what I am saying is that you w ill produce a great deal m o re damage in that type of event which would have produced the +15 charge if it had occurred in the gas phase ra th e r than in the liq u id .
IAEA-PL-615/19
313
G. STO CKLIN : I think that these calcu lations have already been done. You have a ll the input data, and you have to m ake c e rta in assum ptions, and then calculate the A uger cascade. Y o u 'll come out with a c e rta in num ber. There have actually been charge spectrom e try m easurem ents made on the decay to te llu riu m , but I think that the im po rtan t point is that we cannot ne c e ssarily tra n sfe r these gas-phase re su lts into the condensed phase situation. However, you are alm ost c ertainly rig h t that the le th a lity for the m o st highly charged events m u st be very high. G. H A R B O T T L E : There is one other p o ssib ility . We have had d iscu ssio n here of other isotopes of iodine which also undergo Đ&#x161; capture. F o r instance, 123I. A re its prop e rties so s im ila r to those of 125I that it doesn't pay to do the experim ent? G. STO CKLIN : We are planning to do the experim ent with 123I. The 123I decay occurs 72% by electron capture, with the re s t by positro n decay. G. H A R B O T T L E : T here should be a d istin ct difference in the observed re su lts then. S. A M IE L : W hat is the shortest-lived isotope with which you can work in the iodine case? L. F E IN E N D E G E N : 1231 could ju st be done if you get high enough activity into the cell. L . L IN D N E R : W hat is the specific activity of your lab elled phage? L . F E IN E N D E G E N : W ith 125I we have about 2 C i/m m o le . G. ST O CK LIN : R eturning to H arbo ttle's question when he asked whether the c h e m ic a l change to te llu riu m m ight add to the effect: F ro m com parative studies with the iododesoxyuridine lab elled with 125I on the one hand, and irra d ia te d with m onoenergetic X-rays on the other hand, we find the sam e ra d ic a l y ie lds, the sam e c h em ical effects. In this case there is no in dicatio n of a ch e m ic a l effect fro m the te llu riu m le ft after 125I decay. In another case, Feinendegen has c le a rly indicated that it is not so m uch the c h em ical effect of the tra n s fo rm a tio n fro m P to S in the decay of 32P , as due to other effects, p a rtic u la rly to ra d ia tio n effects from electrons. L . L IN D N E R : In m y opinion there is no a p r io r i reason to believe that the c h e m ic a l bond is not ruptured. A .P . W O L F : It is ruptured. G. STO CKLIN : In the case of phosphorus the bond is certainly ruptured. You have such a high re c o il energy there. D .J . M A LC O L M E - L A W E S : The tra n sfe r w ill be to the outer electronic state of the ion, so that it cannot fa ll to pieces. L . LIN D N E R : In m y opinion, you can end up in re pu lsiv e states, and as a consequence of that, you obtain bond ru p tu re. This is probably the m a in cause. D .J. M A LC O L M E - L A W E S : Not if the sudden approx im ation is anything lik e valid. G. STO CKLIN : I don't understand what y o u 're d riv in g at. In the case of phosphorus you have about 60 eV fo r the m ean re co il energy. D .J . M A L C O L M E - L A W E S : Twenty. M ax im um re c o il of 70 eV. G. ST O CKLIN : W ith a m ean re co il energy of 20 eV this should be sufficient sim p ly to break the bond by m e chanical m eans. However, the topic we were re a lly re la tin g to was how the doublestrand sc is s io n occurs. The single-strand sc issio n where the phosphorus
314
FEINENDEGEN
is located is , I think, a c le a r case. The question is how the double-strand breakage occurs. A m I c o rrect? N. G E T O F F : I would lik e to m ake a c om p arison with som e re sults found in ra d ia tio n c h em istry . When DNA is lab elled with b rom in e or iodine — non-radioactive m a te ria l — and irra d ia te d with electrons or ga m m a rays, s im ila r processes have been observed. B rom in e atom s and iodine atom s have been assum ed to be very selective towards th e rm a lize d electrons in the solid phase, o r tow ards solvated electrons in solution or in the liq u id phase. In other w ords, the electrons re act sp ecifically with the b rom in e o r the iodine atom s, fo rm in g an interm ediate which is not the negative ion of b rom in e o r iodine, but som e other very reactive species. This reactive species then starts reactions of som e other kind in the im m ediate neighbourhood. P erhaps when you la b e l w ith radioactive b rom in e or iodine, s im ila r processes are occurring . L . FE IN E N D E G E N : W hen experim ents have been c a rrie d out with tritia te d iododesoxyuridine, the tr itiu m is in the 6 -position adjacent to the iodine. Any ra d io se n sitizin g effect of the halogen should also have been observed w ith the beta p a rticle s fro m the decay of the tr itiu m . S. A M IE L : T here is a newly discovered iso m e r of iodine, with an 85-min half- life, which decays 85% by iso m e ric tra n s itio n into the longer-lived 132I. P o ssib ly this isotope could also be used fo r the DNA experim ents. In this case, there would not be the problem of a change in the nature of the incorporated atom since the daughter is also iodine. Then perhaps you could isolate the effect of the iso m e ric tra n s itio n itself. The 85-m in132I m was found la s t year by Yaffe at M c G ill. I think that it is the longest-lived is o m e r of iodine decaying by iso m e ric tra n sitio n . A second re m a rk also occurs to m e. Can you use another ra d ic a l re p lacing iodine without affecting the m o le c u la r prop e rties? If this is possible, can you use BF4 instead of iodine? W ith BF4 you would have a ra d ic a l m o re lia b le to lo c a liz e d damage. L . F E IN E N D E G E N : You m ay be aware of the group in Tel A viv which is using 125I fo r the treatm ent of thyroid diseases. We have had several discussions with them regardin g whether th e ir observations can be explained by the re la tiv e ly high LE T effect, or by som e other explanation. This group also observes an increased effect in the thyroid gland after in c o r p o ra tio n of 125I. 125I has even been suggested as a good tool fo r treating h y p erth yroidism — a pathological superfunction of the thyroid gland. S. A M IE L : B ut are there c h em ical groups such as B F 4 which can replace iodine? L . F E IN E N D E G E N : Technetium has been used, which gets into a s im ila r pathway, but otherw ise I cannot say what we could do there. S. A M IE L : This is an area which lends its e lf to m o le cu lar surgery. A .P . W O L F : W hat is the m in im u m num ber of decay events in order fo r your b io lo g ic a l m easurem ent to pick up strand-breakage? L . F E IN E N D E G E N : That is a larg e num ber — several thousand decays ... A .P . W O L F : T hat's a s m a ll num ber. L . FE IN E N D E G E N : per c ell ... A .P . W O L F : T hat's a larg e num ber ! We did an experim ent about two y ears ago that eventually d id n't work because of the vagaries of c o m m e rc ia l com panies. We decided to m ake an experim ental approach to this p a rtic u la r p roblem which involved the
IAEA-PL-615/19
315
synthesis of a m olecule lab elled with both 14C and 32P . You go through the num bers w ith 75% 14C and e x tra o rd in a rily high specific activity 32P and then allow a ll this phosphorus to decay. We were looking fo r a sim ple c h e m ic a l m odel of the phosphorus decay in the DNA m o lecule to see if we could use this as a probe fo r understanding what happens c h e m ic a lly at this position. We thought that it would be a fir s t step tow ards perhaps extending the experim ent to say som ething about what was happening in the double-helix. When the phosphorus decays, it could be sim p le c h em istry causing the double-strand breakage, but we would have to know what the ch e m istry is , and the 14C tra c e r was going to be the key to this u n d e r standing. W e ll, we did the experim ent and, of course, there is an enorm ous amount of ra d io ly sis in the system . We went through the calcu lations and that looked okay, but what sunk was a com pany which sent us 5 C i of what was supposed to b e 32P C l3, and instead was a m ix tu re of 4 C i of 32P O C l3 and 1 C i of 32PC13 — which we d id n't re a liz e u n til we had already synthesized our compound. Then it was too late because our double-label was too low in activity to pick up the effects at that position. This is n 't the only double-label p o ssib ility fo r try ing to observe the chem ical effects on this p osition. C e rta in ly the tr itiu m decays m u st be a chem ical effect. In the phosphorus to sulphur case I w ouldn't be su rp rise d if it were not also a c h em ical effect. If you look at the m odel of the double helix and push out the re c o il sulphur atom — the re c o il energy is fa ir ly high — it's very easy to see how this thing would collapse and break on the other side. G. STOCKLIN: I don't think so. F .S. RO W LA N D : Why d id n't you do the experim ent again? A .P . W O L F : It's an expensive experim ent. G. STO CKLIN: This experim ent which you trie d is extrem ely interesting because we have in fo rm a tio n about the c h em ical effects of the t r a n s fo rm a tio n of 14C, and on tr itiu m , but not fo r phosphorus. However, I'm not sure about the com m ents you m ade on the double-helix. I think this can't be true . The range f o r oa 32S re c o il with 20 eV k inetic energy is very s m a ll — m ay be a few A n g stro m s... A .P . W O L F : T hat's not the point. G. STO CKLIN: W hat is the distance fro m one strand to the other? A .P . W O L F : A few A ngstrom s. G. STOCKLIN: No, I think it's m o re. A .P . W O L F : The strands are held together w ith hydrogen bonds. F ifte e n A ngstrom s. G. STO CKLIN: Sure. But that doesn't help. A .P . W O L F : There is a m odel over in our B iology D epartm ent, and we plucked out the phosphorus, atom. W hen we did that, the m odel bent, and c ertainly looked as though it would break. D on't laugh. T hat's the way m ost of the advances in physical organic ch e m istry have been m ade — by w orking with m o le c u la r m odels w ith accurate dim ensions. It is not a fo olish approach. M. NEW TON: E ven if you get to the hydrogen bond... A .P . W O L F : When we get back to Brookhaven, you and I w ill go over to the Biology D epartm ent and I ' l l p u ll the phosphorus atom out fo r you, and you can te ll me whether you think that m olecule is going to collapse. A .G . M ADDOCK: I would lik e to com m ent on the point m ade by D r. Feinendegen about the p ossib ility of К absorption edge studies in
316
FEINENDEGEN
enzym es. T here are som e very exciting p o ssib ilitie s there. As fa r as I re m e m b e r, the nitrogen fix ation process in azobacter is generally attributed to a m olybdenum -containing enzym e. However, to the best of my knowledge, this enzyme has never been isolated prop e rly , and there is even some doubt whether it re a lly is as described. It seems to m e that this would be possible to attack with К absorption edge ra d ia tio n , and that you would in fact have a selective method of deactivating a m olybdenum enzyme. I think that azobacter is probably fa ir ly tough fro m the point of view of standing up to the s m a ll doses of radiation. G. ST O CK LIN : T hat's an extrem ely interesting suggestion. We were looking fo r a long tim e for enzymes containing heavier atoms than zinc because only a very s m a ll frac tio n of the ra d ia tio n is absorbed by the zinc atom. M olybdenum would be m ore interesting. Now, what is the enzyme? and how do you get a supply of it? Can you te ll us m ore? A .G . M ADDOCK: I always speak about subjects I don't know anything about. I suppose you've noticed that. G. STOCKLIN: W hat did you find about this one? A .G . MADDOCK: This one is one of the nicest enzymes to work on because if you've got som e b io ch e m ic a l friends or b io lo g ists, they can very easily cook up nice p reparations of this stuff, and it produces a fine nitrogen-fixing system which you can handle fa ir ly easily. It strikes me that you could ju st look at the effects of com paratively s m a ll amounts of ra d ia tio n of the rig h t wavelength, and com pare it with s im ila r doses of a sligh tly shifted wavelength. L. F E IN E N D E G E N : There is only one experim ent done so fa r with zinc. S. A M IE L : I think som e m ore experim ents were done a few years ago trying to replace zinc by cad m ium — at Brookhaven, I believe. F .S . R O W LA N D : Does anyone else have some m o re experim ents to suggest fo r G e rhard [Mr Stocklin] to do? G. STO CKLIN : If it w orks, A lfie [Mr Maddock], y o u 'll get an acknow ledgm ent, and that's all. L. F E IN E N D E G E N : I think it is worth while to m ention that a good p a rt of the m a te ria l I presented here today was definitely trigg ered by the panel discu ssio n held here at the IA EA in 1967.
IAEA-PL-615/20
PROBLEMS OF ISOLATED SCIENTISTS OR RESEARCH GROUPS F.W. L I M A Instituto de Energía Atómica, Sao Paulo, SP, Brazil
Abstract P R O B L E M S O F I S O L A T E D S C IE N T IS T S O R R E S E A R C H G R O U P S . In e x a m in in g th e ro le o f h o t a to m ch e m is try in th e A g e n c y , s p e c ific q u estion s dea lin g w ith th e p ro b le m s o f is o la te d scientists o r research grou ps, th e v a rio u s m echanism s f o r tech n ica l assistance t o d e v e lo p in g cou n tries w ere raised. T h is w o rk in g p a p er id e n tifie s so m e o f th e k e y issues and serves as a basis f o r discussion and re c o m m e n d a tio n s fo r th e A g e n c y ’ s program m es.
The problem s of isolated scientists or re se a rc h groups in developing countries are not ne c e ssarily the sam e for a ll developing countries. In a country such as m ine, with an area corresponding to 8 000 000 k m 2, and with a p o litic a l organization that c om p rises 23 states and 4 te rrito r ie s , some of which are la rg e r than m any E uropean cou ntries, the p rob le m s of scientific groups are ra the r different in a northern state, or in the centresouth, o r in the south of the country. H owever, c e rta in kinds of problem s are com m on for p ra c tic a lly a ll isolated re se a rc h groups. These are problem s connected with appropriate u tiliz a tio n of experts supported by in ternatio nal organizations such as the In tern ation al A tom ic Energy Agency. Let us consider some aspects of the role of the experts. F requently, they are either at the start or near the end of a p rofessional c aree r — either they have just received the ir Ph. D. diplom as and are at the beginning of th e ir scientific liv e s, or they are , som etim es, re tire d scientists who are s till active and involved in technological or scientific areas. Both situations may b rin g good re sults to the country to which they are supposed to give expert help. The young Ph. D. w ill b rin g new blood and fresh ideas to the place, while the older expert w ill b rin g a substantial am ount of experience. In both cases, however, som e shortcom ings m ay re su lt if the expert and the in ternatio nal o rganization that supports h im are not aware of the special a n c illa ry conditions of the work in a country quite different fro m the expert's u sual place of work. Let me diverge a m om ent at this point. My com m ents are not meant as deleterious c r itic is m to any in te rn atio n al o rganization that has p ro g ra m m e s in the developing countries. I have been asked to consider the "p ro b le m s " of isolated groups, and not to conceal those prob le m s. If som e c r itic is m may be found in my w ords, it is my intention that they should be understood as being constructive. F ir s t of a ll, let us consider the recently graduated Ph. D s. Frequently they are bright young fellow s, who want, some of them at le a st, to re fo rm everything which they find in the new country and the new place of work — forgetting that they are only going to stay in that country and institutio n for one year o r so. V ery often what they want to change is in the ir specific field of work, which is quite understandable. Not infrequently, however,
317
318
LIMA
some of them want to change m any things outside the ir speciality. F or exam ple, they m ay wish to a lte r the b u reau cratic ru le s, the schedule for m aintenance of equipm ent and its m anner of m aintenance etc., and they are very im patient with delays in requests to buy pieces of equipment or drugs re quired for a specific experim ent. This attitude is quite understandable. A fter a ll, the expert goes to a place fo r a lim ite d tim e , u sually one year, and he keeps an eye on the calendar. But, nevertheless he cannot change the pace of things. The wheels of the a d m in istra tiv e m achine are a ll in te r dependent, and it is not possible to make ju s t one of them go faster than a ll the others. The expert should be aware that m any of the fa c ilitie s to which he is accustom ed in his own country or place of work are sim p ly not available in the new place where he is to live and work for the next year. He should be patient and try to adapt his own method and w orking pace to that of his new surroundings. O therw ise, only half — or even less — of his potential capability w ill be put into actual use. An expert who goes to a developing country m ust be conscious that he is not going to be able to continue his re g ula r w ork with the sam e fa c ilitie s that he has always had before. He is going there to start a new lab orato ry, frequently from the very beginning. O bviously, there is a substantial difference between going to another re search centre to continue one's own sp ecialized field of re search — in which case the expert should have gone to a developed lab o ra to ry in a developed country — and going to a centre in which he is supposed to in itiate activ ities in that new field. The latte r case, in a way, is a situation which involves a c ertain am ount of sa c rific e . The expert w ill probably not have the chance to publish two or three new papers during the year he is going to stay abroad. But, at the end of his stay, if he is successful, he can re tu rn to his own country with a feeling of achievem ent by leaving a new group capable of w orking on a new line of re se a rc h. And that was the sole a im of his expert work. R unning the r is k of becom ing repetitious, I think I m ust in sist again that the idea is not to continue the expert's own work in another environm ent, but to in itiate people on a new lin e of re search, to help setting up new la b o ra to rie s , and to orient them in the jud iciou s choice of equipm ent for the ir lab orato ry. Let us now look to the other extrem e in the age of experts, the case of the older expert. U sually he is a scien tist or engineer with a great am ount of experience and knowledge in his own field. He is very useful in the technological type of work in which m any big problem s are solved by knowledge of details concerning a piece of m ach inery or a step in a c o m p li cated process. This expert has a respected nam e and quite a load of ach ieve m ents; consequently, he re qu ires com patible treatm ent. Not uncom m only, how ever, his is a case in which he does not lik e to abide by the rules of the house. The general regulations of the in stitu tio n m ay re q u ire , for instance, fo r safety reasons, that everybody punches a card in the timeclock at the entrance of the institutio n at the beginning and at the end of every w orking day — and even the d ire c to r of m y institute does that. N ev er theless, some experts w ill do it very re lu ctantly , even should he decide to do it at a ll. If we put ourselves in his shoes, we can see why he m ay be reluctant to punch a card. He has never done that before in h is own place of work. But on the other hand, what is the trouble in punching a c ard? Since he is being paid by the in te rn atio n al supporting institu tio n, nobody is going to make a deduction fro m his sa lary for the day he did not go to
IAEA-PL-615/20
319
work. Let us be good sportsm en and, when in R om e, let us act as the R om ans do. The other people who are punching the clocks every day are the people who are going to be h is colleagues fo r one year or m ore. Another kind of p rob le m , a m ore serious one, that frequently occurs with the very experienced expert is that he is an in ternatio nal c itiz e n with a United Nations passport. But m ost of his technological or scientific experience was acquired in h is own country and quite often by w orking in fields that m ight be considered c la ssifie d . This is often true in atom ic energy and in technological work, when the knowledge of the expert is to be put to use in in d u s tria l or pilot plants of the country to which he has been assigned. D uring his work situations w ill frequently occur in which the expert is asked to solve problem s whose easiest solution is the one that he already knows. But to apply that solution is to disclose s till c la s s i fied w ork in h is own country. Then he faces a very serious d ilem m a : he cannot be u n fa ir to the host country, and refuse to disclose his knowledge to the host country, since knowledge is the m erchandise he is being paid to sell. On the other hand, he cannot disclose that sam e knowledge which is, s till, an in d u s tria l secret in his own country. R e a lly , I do not know how to solve such an im passe . But organizations such as the In tern ation al A to m ic E nergy Agency should give some thought to situations of this kind. They are m ore com m on than one m ight think, and I am aware of situations s im ila r to the ones I have m entioned, which have occurred m ore than once in B r a z il. Next we come to the problem s a ris in g fro m the eventual needs of the expert concerning equipm ent to be im ported. If at a ll possible the expert should try to do h is w ork with the equipm ent the in stitu tio n already has. U su ally the budget fo r im p o rtatio n of m a te ria l m ust be approved durin g the fis c a l year p r io r to the year when the expert is supposed to start his work. The paper work re qu ired fo r im p o rtin g equipm ent is ra the r involved in m ost cou ntries, and an im p o rt p e rm it is ra re ly granted before three o r four months have passed. Shipm ent of m a te ria l and equipm ent cannot always be m ade by a ir , m eaning that about three m onths m o re would be required for the expert to get the equipm ent. A ll these p re lim in a rie s take a total of seven to eight m onths, which is an appreciable frac tio n of a one-year assignm ent. To try to c ircu m v ent a ll of these shortcom ings, the involved in stitu tio ns — both supporting and receivin g institutio ns — should approve the nam e of the expert with at least a six months lead tim e p rio r to the start of his a ctiv ities in the country to which he is being assigned. It is highly advisable that he exchange letters with the people with whom he is going to w ork before m oving to the new country. Then he would be able to plan his a ctiv ities as a function of the available equipm ent and conditions of work. The in stitu tio n supporting and sending the expert should strongly em phasize this point fo r prospective experts. On another line of thought, let us consider one of the v arious beneficial actions that institu tio ns such as the Inte rn atio n al A to m ic Energy Agency have had in developing countries. I re fe r sp e c ific ally to the good re sults that have been obtained through the v arious study group m eetings. O f special use were the study group m eetings on the u tiliz a tio n of re search re acto rs held in variou s countries in South A m e ric a . In Sâo P au lo we have had two of these m eetings — one on general applications of re se a rc h re acto rs in 1963, and another one in 1969 on radioisotope production u sing re search re ac to rs. These m eetings are not expensive and the num ber of young
320
LIMA
people, s ta rtin g th e ir scien tific activ itie s, that may attend the m eetings is high, since tra v e l expenses are low and som etim es non-existent. The benefits that resulted fro m those m eetings were excellent and many good ideas and good suggestions concerning the best use of re search re actors were born in those study group m eetings. The Agency should not stop its support of this type of activity that has helped m any new re se a rc h groups to solve various kinds of problem s that otherw ise would take m onths before a good solution could be found. These m eetings have c ertainly reduced the num ber of problem s of isolated scientists or re se a rc h groups, and have also reduced the am ount of tim e re quired to solve the problem s which do occur. Many of the problem s which a new scientific centre has to face have already subsided in Sâo P aulo, but this is not the case for re search reactor centres at the very beginning of th e ir activ itie s. When we started the construction of the re se a rc h re ac to r at the Instituto de E nerg ia A tĂłm ica 15 years ago, the scientific staff of the institutio n was composed of only 20 people, m ost of them recently graduated with only a bach elo r's degree, and with no previous experience in engineering that m ight help in the c onstruction and assem bly of the re ac to r. Many of us had to do ordinary lab ou r, helping in setting up the re ac to r control table, solde ring , aligning the pool tile s , in s ta llin g pipes for the water treatm ent system , and so on. A fter the assem bly and engineering work for the re se a rc h re actor is finished the tim e com es fo r the fir s t tests of the m achine â&#x20AC;&#x201D; a tim e when experienced people are re a lly re quired. This is a situation in which m any prob le m s, often unpredictable, a rise and when the experience of able experts is of in calcu lab le value. Good orientation at the tim e of the fir s t tests of a re se a rc h re ac to r can avoid quite a large num ber of future headaches, avoiding m istakes with a consequent substantial saving of money, and substantially expediting the tim e when the m achine w ill be re ally put into I actual work. The fir s t m onths, if not the firs t year, of a new re search reactor centre are fu ll of prob le m s fo r isolated re se a rc h groups, and this is a tim e that institutio ns like the Intern ation al A tom ic E nergy Agency, and the experts provided by the Agency, can re a lly help along pragm atic lines. F in a lly , I think that we owe a word of p raise and recognition to the in te rn atio n al institutio ns which have done a wonderful job in m ost of the L a tin A m e ric a n countries, and especially in B r a z il. If today the im portance of re se a rc h is re a lly understood and w ell supported by the governm ent, m uch of this we owe to the pioneer work of in stitu tio ns such as the R ockefelle r Foundation, am ong other private e n terp rises. Twenty years ago the only way we had to send people abroad for tra in in g was, p ra c tic a lly , through p rivate in ternatio nal institutio ns. O nly in very special cases was a governm ent-supported scien tist sent and m aintained abroad for tra inin g . The influence of an in stitu tio n such as the R ockefeller Foundation involved not only su b sid izin g fellow ships in foreign u n iv e rsitie s and places of tra in in g but, very im p o rtan tly , the im planting and in cu lcating of the germ of a good example for governm ental in stitutions them selves. I w ish to re-em phasize that m y re m a rk s should not be understood as c r itic is m to institutio ns which have re a lly done an excellent job in helping developing countries obtain needed expertise. These institutions have helped also, and this is the m ost im po rtan t of a ll, in giving examples and pre p a rin g the ground fo r ra d ic a l tra n sfo rm a tio n s in the philosophy of
IAEA-PL-615/20
321
supporting re search by the governm ent. This has been esp ecially true in B r a z il after 1964, and has brought very b e n eficial consequences and a definite com prehension of the im portance of re search to the p rogress and development of the country.
DISCUSSION D .M . RICH M AN : In your presentation you have em phasized the value of study groups. H owever, the Agency has tended to em phasize the assistance p ro g ram m e — tra in in g p ro g ra m m e s, equipm ent, etc. — and not so m uch the m eetings. Y our discussion of the value of study groups to the isolated scien tist is ra the r new to us. Would you care to elaborate on this in te rm s of fields of basic re se a rc h ? The study groups held by the Agency have u su a lly been in the field of re ac to r u tiliz a tio n . The re la tio n sh ip to the specific field of hot atom c h em istry is ra the r different fro m that of re actor u tiliz a tio n . Do you think that regional study groups can be useful in hot atom or ra d ia tio n c h e m istry ? F .W . LIM A : I have been im pressed by the good results of study group m eetings held in the past in B r a z il, C olo m bia and C hile. In fact, the personnel fro m m y institute have probably got m ore of value out of study group m eetings such as those held in South A m e ric a in the re actor u tiliz a tio n field than we have fro m some of the other p ro g ra m m e s. We can often get m uch good advice and ideas fro m foreign scientists at study groups during the one work week of the study group — in com p arison, fo r instance, with some of the indiv iduals who have gone for longer periods of tim e but take a long tim e to get adapted. G. H A R B O T T L E : W hat are the re la tiv e costs to the agency of experts in the field fo r extended periods versus a study group? D .M . R ICH M AN : I believe that an expert costs fro m $30000 to $50 000 per year in total expense. The cost of a study group to the Agency is now about $10 000 to $12 000 fo r one week. H owever, there is also a financial c o m m itm e n t fro m the host governm ent which is involved in a study group m eeting. F .W . LIM A : U sually in Sáo Paulo the costs of the m eeting fa c ilitie s and in te rn a l transp ortatio n are met by the host governm ent, but the Agency has paid for the external transp ortatio n of those attending the study group. D .M . R ICH M AN : Perhaps a cost to the Agency of $20 000 for six months of technical assistance versus $10 000 - $12 000 fo r a study group m eeting. G. H A R B O T T LE : The c om p arison is n 't ju st financial. It's im portant to point out the large difference in function between the study group and the technical expert. The expert is there as a catalyst to im p a rt knowledge at the beginning o r some c r itic a l point of an operation. The study group is try ing to encourage in teraction am ong scientists of neighbouring or related developing countries in a p a rtic u la r field of study. They m eet each other under ideal conditions for exchanging in fo rm a tio n and discussing problem s. W ith a study group you are buying a lot of in te rn a l expertise and in teraction in which the re gional scientists are helping each other with problem s of which they have m utual understanding, but which are often very difficu lt for an outsider to understand.
322
LIMA
D .M . R ICH M AN : I would like to re tu rn to the specific p roblem of hot atom c h e m istry with re gard to isolated groups. B r a z il, p a rtia lly as the re su lt of Agency assistance, has developed a healthy attitude towards re se a rc h. Does this mean that B r a z ilia n scientists are able to pursue basic re search — to c a rry out the kind of work, such as hot atom ch e m istry , w hich tends to be avoided in some countries if it is not applied; that cannot find support? F .W . LIM A : The enthusiasm of the governm ent, of course, is for the applied re search. Hot atom c h e m istry has not had m uch development in South A m e ric an countries. In B r a z il we have had the benefit fro m the p ioneering work on the p art of Danon and V argas, but the field of develop m ent is not p a rtic u la r ly large. We are a rra n g in g to have in teraction with the Japanese governm ent in the fields of hot atom c h em istry and also r a d ia tion ch e m istry , and expect to receive two or three v is itin g scientists. They w ill be expected to help start lab o ra to rie s along the lines of both applied and pure ra d ia tio n chem istry. A ll w ill be paid by the B r a z ilia n government. K la ra B E R E I: I have the im p re ssio n that alm ost a ll groups engaged in hot atom c h e m istry are isolated to some extent. S m a ll groups in s m a ll countries may feel m ore isolated than those in a big country, but the groups a ll over survive m a in ly at nucle ar centres. The reason for this lie s in the ra the r contradicto ry nature of hot atom ch e m istry which is bounded by many fie ld s, and is sp ecialized and p e rip h e ra l to m any lines of re search. Hot atom c h e m istry appears to be very b a sic , fundam ental science, but at the sam e tim e acts as a tra in in g ground fo r scientists in developing countries. The m ain significance often is that it develops in the scientists a high level of the o re tic a l and ex perim ental research a b ility . The investigations in hot atom c h e m istry are c a rrie d out in s m a ll isolated groups, but these have strong and spontaneous contact with each other, as we have seen in the m eetings arranged by the hot atom chem ists them selves. O ur group in H ungary is s m a ll — three re search workers plus some technical assistants — and s till we survive because we are needed by groups dealing with reactor physics o r biology who come to us for assistance in connection either with nuclear c h e m istry o r with tra c e r techniques. U sually we can help. We try to lessen our iso lation by continuous contact with other hot atom groups fr o m abroad: by v is itin g and w orking there, as we have in Jü lic h , and by in viting foreign scien tists to Hungary to give lectu res. We would n a tu ra lly be very happy if the Agency would contribute to m ore organized and more re g u la r contact am ong the isolated groups of hot atom chem ists. J .P . A D L O F F : I have visited the B r a z ilia n lab o ra to rie s , and they are w e ll equipped. F .W . LIM A : In B r a z il we have good equipment; our difficulty is people, good scien tists. V argas has several people fro m G renoble, and we do have som e v is ito rs in B r a z il. J . DANON: People who are geographically isolated are not necessarily isolated sc ie n tific ally . J .P . A D L O F F : We have two re se a rc h groups in this field in F rance, one in Strasbourg and the other in P a r is . In some ways, they have better in terac tio n with groups in other countries than with each other. D .M . R ICH M AN : W hat do we m ean by iso lated? I think we mean isolated in in te lle c tu al contact. I have the im p re ssio n that the field of hot atom c h e m istry represents one of the better situations in te rm s of in te lle c tu al contacts am ong w orkers in the field in a ll countries because the
IAEA-PL-615/20
323
hot atom chem ists them selves have taken care of organizin g m eetings, and the com m un ication seems to be rather good. The question of iso lation may be m ore im p o rtan t in other fields than it is in hot atom c h em istry . S. A M IE L : Iso lation is a state of m ind only. F .W . LIM A : I had a ra the r d ifficu lt tim e w ritin g this paper, because personally I don't feel isolated. Therefore, I concentrated m ore on the p ro b le m of using Agency experts. G. STOCKLIN: The p roblem of isolated scien tists is not unique for developing countries. It can happen in any country, and develops m ostly in the sc ien tist h im s e lf. Some hot atom chem ists in developed countries are also isolated, and it is often the ir own fault. Some have made the m istake of re m a in in g by them selves as hot atom chem ists at the ir own m eetings. This is re a lly an in te rd is c ip lin a ry field, and it is only recently that hot atom chem ists are beginning to re a liz e that they should extend them selves to in teract with other fields — organic ch e m istry , radiation c h e m istry , life sc ie n tists, solid state p h y sicists, m e dic al people. O urs is not a unique field, but deserves close co-operation with other fields. F or a sc ien tist it is always his own fault if he is isolated. S. A M IE L : Iso lation is synonymous with recognition. R ecognition is based on excellence and this is a closed cycle in developed society. The p rob le m in a developing country is the fa ilu re for excellence in science to be given a high p rio rity . The question being discussed by D r. L im a is re a lly how to tra n sfe r in fo rm a tio n to groups which exist or are trying to develop in an indifferent or even hostile environm ent. When you need help, you need it twice. If the society recognizes that a ce rtain field needs assistance, then they appeal to the Agency or other organizations for support. But usually this has to coincide with appeals w ithin the country to governm ent policy — the governm ent has to want to prom ote the fie ld , too. The other situation is that there is a lo c al group not in accordance with the general policy of its governm ent — with the im m ediate p rob le m of try ing to survive when the governm ent is try ing not to put any of its own money into this development. This is the big p rob le m , and is the m ore general case in my experience with developing centres over the years. I don't think that the Agency has ever c a rrie d out a m ark et study of the best way to tra n sfe r expertise or in form atio n to developing countries — whether to do it through equipm ent plus an expert, usually a rriv in g a year apart. Som e tim es a country asks for equipm ent and also an expert, even though he may not be needed. They are only re ally try in g to find some other source of equipm ent which they c an 't get lo c a lly — only the equipm ent is needed, but it is a package deal and the expert com es along and thinks he is needed, but he is only of secondary im po rtance. This c e rtain ly bring s fru s tra tio n to the expert. I think it would be a good idea for the Agency to ask one of these p rofessional com panies who are studying the tra nsfe r of in form atio n what is the best way to help developing countries. F .S. ROW LA N D : The hot atom chem ists have survived so far in H ungary. However, in countries such as Poland and C zechoslovakia, they have gone off into other fields because they have not been able to survive as hot atom ch em ists. Is that a good thing? If it is not de s ira b le , is there anything that we as a group, or the Agency as an in stitu tio n, have failed to do that has led to th is ? O r is this inevitable — ju st p art of the general trend towards m o re applied w ork? Some people ju s t seem to have been able to r e s is t the trend m ore successfully than others.
324
LIMA
A .G . MADDOCK: I have the experience of going to countries ranging fr o m fa ir ly p rim itiv e to ra the r advanced. I have made it a practice never to give advice that I do not think is in the interest of the needs of the country. Tim es change. The field of hot atom ch e m istry was im po rtan t at one tim e to o ffic ia l nu cle ar energy institutes in these countries fo r several reasons. F ir s t is its inherent re lationsh ip to atom ic energy. But even m ore so because it has provided an excellent method of tra in in g because of the varied ch arac te r of the re search work involved. A t present, although o ffic ia l in terest s till continues, the support is now m a in ly directed towards applied ends. Some fundam ental re search is p erm itted, but the m ain interest in the a re a of hot atom c h e m istry should by now have moved into the u n iv ersitie s. There it is s till quite feasible fo r such work to continue because it s till provides excellent tra in in g fo r a variety of re search careers. And there it works w e ll. I was in Mexico re cently, and I know P ro fe sso r A dloff has been there, and would agree with me that the developments there are exceedingly favourable. The governm ental nuclear re search institute has not lost in terest in the area, but there has been a much m ore rapid expansion in the u n iv e rsity and I think that is the place where the greatest re a l future lie s. Hot atom c h e m istry is now m oving out in se v eral d irec tio ns, and this p a rtia l frag m entatio n has become psy chologically difficult to accom m odate. A .P . W O LF: It seem s to me that we are getting away fro m the re al purpose of this panel to some extent. We are getting into the nebulous area of the re la tive functions of applied and basic re search, and of who foots the b ill. I don't agree with what Maddock is saying. I don't think that hot atom ch e m istry is ne c e ssarily a good field of tra in in g fo r a young person who is going to become a re search sp ec ia list in some field. He needs to understand the basic fundam entals of ch e m istry . But, in the case of the USA, even in the field of nuclear c h e m istry which is m uch m ore w idespread than hot atom c h e m istry , young people who study nucle ar c h e m istry find it hard to get academ ic positions. I think that hot atom c h e m istry is an exciting field of re search — otherw ise I w ouldn't be in it. It's a good disciplin e to study, and I agree with the com m ents made e a rlie r about the in te rd isc ip lin a ry nature of this fie ld . It's up to us to do the interaction. I disagree with the concept of giving applied re se a rc h workers an opportunity to do basic research. I think this is the wrong way to look at it. The proper way to look at it is this: if you have fir s t class scientists in a fie ld , the question is sim ply whether they are doing good work and m aking som e contribution — not whether that contribution is called basic or applied. I think that the field of hot atom c h e m istry is very m uch aliv e , and for the firs t tim e in the past four or five y e a rs, we are beginning to think that hot atom c h em istry is m aking im po rtan t contributions to other fields. This is probably most notable in the area of theory — in the oretical ch e m istry . This is one of the few fields where you see m any papers in the lite ra tu re by people who are not hot atom chem ists — or do not c a ll them selves hot atom chem ists — but who are concerning them selves with problem s of the reactions of atom s in non-B oltzm ann d istrib u tio n s. The sam e is not true of the reaction k inetic ists — there you see very little developm ent fro m people who are reaction kineticists and who are beginning to look into hot atom ch e m istry as a mode for expanding th e ir study of the field. But hot atom c h em istry is beginning to spread. I think this p rob le m of "underdeveloped" o r "developed", or what have you, w ith respect to countries is not so m uch a question of the stage of
IAEA-PL-615/20
325
development but ra the r of the quality of the sc ie n tist, and of the interest of the indiv iduals them selves in creating som ething in science. I don't think that a ll the agencies in the world m eddling in the scientific a ffairs of the other countries w ill be the u ltim a te thing which w ill decide whether or not science in that country w ill survive, and do som ething w orthw hile not only for the scien tist, but for the society in that country. A .G . M ADDOCK: I h ea rtily endorse D r. W o lf's conclusion. J .P . A D L O F F : I have com piled an annual bibliography in hot atom c h e m istry fo r the past ten y e a rs, and there has been a sharp decrease in the num ber of papers published world-wide related to hot atom ch e m istry in the past several years. G. STOCKLIN: This is partly because some of the hot atom chem ists have gone into other fie ld s, and are s till w orking, but are now doing a sort of applied hot atom c h e m istry — in nuclear m edicine, for example. I don't think you can separate fundam ental and applied in the field of hot atom c h e m istry . Of course, you have to know the basics. And as fa r as jobs are concerned, the people in m y laboratory in inorganic c h em istry are having a hard tim e finding jobs, while those in hot atom ch e m istry are not having a hard tim e at a ll — finding jobs in n u cle ar m edicine, for example. I think that there is a need to do at least a lim ite d am ount of hot atom ch e m istry at the nuclear re search centres, because there the opportunity exists very strongly for doing in te rd is c ip lin a ry work. I agree that hot atom c h e m istry is good tra in in g at a u niv e rsity , and I believe that it w ill be possible to do work in nuclear m edicine at some u n iv e rsitie s. S. A M IE L : I wonder if we can a ll agree on a definition of hot atom c h e m istry ? I ra is e this because of the statem ent made by A dloff regarding the decline in the num ber of publications in the field of hot atom ch em istry. A .P . W O LF: I don't think that there has been a decline in the activity of people who at one tim e called them selves hot atom ch e m ists, and that is an im p o rtan t distinction. I think Stocklin put his finger on the key point. A t our own lab o ra to ry m ore and m ore u n iv e rsity people are involving th e m selves in our p rog ram m e s related to nuclear m e dicine, and in p a rtic u la r in our com bination of hot atom work with nuclear m edicine. They do this fo r p re c ise ly the reason given e a rlie r — in th e ir own in stitutions there are b a r r ie r s which prevent facile in te rd is c ip lin a ry interaction. The b io lo g ic a l sc ie n tists, m e dical sc ie n tists, p h y sicists, etc., a ll in teract very sm oothly in a la rg e nu cle ar in stitu te, whereas in the u n iv e rsitie s the tra d itio n is s till for the professo r to be a separate em pire unto h im s e lf — continuing with his own line of re se a rc h, with of course some changes, but ra re ly allow ing his students to in teract strongly with other dis cip lin e s. It is extrem ely ra re to see a P h.D . thesis that genuinely com es fro m an in teraction between several departm ents or fa c u ltie s. I think that in this respect re search in hot atom c h e m istry is ide ally suited to a re search institute. O f course, that doesn't mean that it c an 't also be studied in a u n iv ersity , but the methods of operation are different. N. SAITO: M ost of the hot atom studies so fa r have been concerned with atom s form ed in nucle ar reactions — with the c h e m ic a l effects of n u c le ar tra n sfo rm a tio n s. Hot atom s, how ever, are produced in m any no n nu c le ar events, and I think that we should continue to take a broad approach to the general concept of hot atom ch e m istry . F.S. ROW LA N D: I w ish to make some re m a rk s about the possible roles of the Agency in connection with the field of hot atom c h e m istry . I think as
326
LIMA
an observer here for the past few m onths that it is quite cle ar that the dom inant subjects of Agency concern are n u cle ar power, safeguards in connection with nu cle ar weapons, and studies of the environm ent with special regard to ra d io lo g ic a l effects. These topics are em phasized repeatedly whenever you examine the Agency's p ro g ra m m e . There is not very much c o m m itm e nt on the p a rt of the Agency to basic re se a rc h, with the one conspicuous exception of the 6% of the budget which goes to support theo re tic a l physics in the T rieste laboratory. F or those of us who are interested p r im a r ily in fundam ental science, we have the dual task of persuading the Agency that it is righ t in supporting research in theoretical physics, but that it ought to extend this support to c h em istry and to some other areas which m ay be of m uch m ore p ra c tic a l concern to the Agency in the long run than the one a re a in basic re search which it now supports. In the m e antim e, we can also consider those parts of what we do that m ight have som e application to these areas of m a jo r interest of the Agency — does hot atom c h e m istry in teract in any direct way with nuclear power, with safeguards, or with ra d io lo g ic a l effects in the environm ent? As a p ra c tic a l m a tte r, however, persuading the Agency that it should give support to sym posia that are of a basic nature w ill be difficu lt over the next several years unless they are connected in som e way with these areas of high p rio rity . D .M . R ICH M AN : I want to ra is e a question about hot atom c h em istry and te ch n ic al assistance for the M em ber States of the Agency. If a M em ber State asks for an expert for one year to set up a pro g ra m m e at a univ ersity in hot atom c h e m istry , is this the kind of request to which the Agency can make a m eaningful re ply ? One can draw the p a ra lle l — if it is a p a ra lle l — to a request to the Agency for an expert to come to set up a rad io ch e m istry lab orato ry. I think that there is probably a great difference, and I am interested in your com m ents and responses to such questions. A .G . M ADDOCK: D oesn't the answer to such questions depend on the state of developm ent of the country concerned? A num ber of countries by now should have sufficiently well- established nu cle ar re search centres to cause such a suggestion to be viewed with considerable disfavour. On the other hand, there m ight be some countries which are at a stage of development of their atom ic energy p ro g ra m m e such that it m ight s till be ju stifie d . There is no sim ple answer. G. H A R B O T T LE : I agree. I think D r. L im a has made some valuable points concerning the choice of an expert to go to a developing country. The kind of psychological background fo r selection of an expert is, in my opinion, an a re a in which the agency can use som e help. S. A M IE L : I am re tu rn in g to the question of setting up a fa c ility versus setting up a p ro g ra m m e . There a re n 't any questions about setting up a fa c ility . F or a p ro g ra m m e , if it is an applied pro g ra m m e in which the need is w ell defined, then the decision is also m uch easier. When the need is nebulous, then I wonder if any assistance can be of any re a l use. L . LIN D N E R : In our discu ssio n, we have only considered one side of the solution — sending an expert to a country or sending a group of experts to a m eeting over there. I heard figures ranging fro m $10 000 to $50 000 per year as the cost. We need also to consider what can be gained by having an increased flow of people the other way. F ellow ships are given at about $6000 per year. P erhaps a balance m ight be sought in which m ore people are sent on fellow ships. In our own lab orato ry we have quite often had Agency fellow s. Fre q uen tly this has worked out very w ell, but I m ust add that quite a num ber of these people have had m any prob le m s in adjusting.
C ON C LU SIO N S A N D R E C O M M E N D A T IO N S
T his panel, perhaps m ore so, than any other m eeting that the Agency has held in the past, has made it very clear that hot atom c h em istry is in fact not an iso la te d disciplin e but one w hich im pinges on m any other areas. A m a jo r strength of the Agency is the world-wide o p p ortu nityfo r c o m m u n i cation. There are a num ber of areas in which the Agency could gather and dissem inate in fo rm a tio n , b rin g people together, and provide the im petus fo r people to consider the use of hot atom ch e m istry in solving not only p ra c tic a l p ro b le m s, but also in pursuing new directions of basic research. Thus, an im p o rtan t conclusion is that the Agency should play a decisive role in helping prom ote awareness in other d is ciplin es of the way in which hot atom c h e m istry m ay im pinge on them . The general question of tra n s fe r of expertise is im p o rtan t in this re gard, and m ay m e rit study by the Agency. Six specific areas were suggested in which m eetings m ight be held to exam ine applied activ itie s where hot atom c h em istry m ight be useful. E m p h asis should be on the in teraction of scien tists involved in hot atom ch e m istry with scien tists in the fie ld of activ ity under consideration, but who are not m aking use of hot atom techniques. Study group m eetings m ay be p a rtic u la r ly u sefu l to provide such interaction. The six areas ide ntifie d are: (1) R adiop h arm acy — Role of hot atom c h e m istry in production of lab elled compounds. (2) Ion im p la n tatio n — The hot atom c h em ists' contribution to ion im p la n ta tion has not yet been fu lly appreciated by those people who use ion im plantation ; "c ro s s - fe rtiliz a tio n " among these scientists should be encouraged. (3) F u sion - related prob le m s — In sev eral countries efforts are being made to b rin g the hot atom chem ist into the fusion- related p rob le m area. An id e ntifiable contribution can be made to m a te ria ls damage studies. (4) C h e m ic al synthesis — Hot atom c h e m istry offers p o s s ib ilitie s fo r the p re p a ra tio n of new and unusual c h em ical compounds so fa r not available. The p o s s ib ilitie s offered are yet to be explored and could lead to whole new areas of chem istry. (5) B io lo g ic a l studies — C la rific a tio n of hot atom and ra d ia tio n c o n tri butions to b io lo g ic a l effects. (6) M olecu lar analysis. The question of co-ordinated re se a rc h p ro g ra m m e s was discussed. Such p ro g ra m m e s that the Agency m ight support are generally re stric te d in intent and concerned p r im a r ily w ith the in te rac tio n between developed and developing countries. Such p ro g ra m m e s are good m eans of supporting scien tists in developing countries, and valuable fo r m a in ta in in g com m unication among scientists involved in hot atom chem istry. T echnical assistance was also discussed. It was suggested that the em phasis of the TA p ro g ram m e should be on the p rov ision of experts fo r in tro ducin g hot atom c h e m istry as a fie ld of activ ity to in itia te people on new lines of research.
327
328
C O N C L U S IO N S A N D R E C O M M E N D A T IO N S
The question of nu cle ar data needed by hot atom chem ists was discussed by the panel. It was observed that photonuclear cross-sections, w hich are im p o rtan t, are not as ea sily obtained fro m the lite ra tu re as would be d e s ir able. A lso the search of nucle ar data com pilations is too arduous. The follow ing specific suggestions were offered: (1) The p rin c ip a l m eans of isotope production should continue to be included in the "T ab le of Isotopes", by C.M . L e d erer et al.; (2) There is a need for nucle ar reaction cross-section data over an extensive energy range, e.g. 2-80 MeV, esp ecially on m u ltip le production of secondary particles; (3) C harged-particle nuclear reaction cross-section com pilations are.not fu lly adequate; (4) There is a need for accurate ly determ ined decay schemes, e.g. the 123I decay scheme fo r the calculation of ra d ia tio n dose; (5) There is a need fo r m ore complete tabulations of in te rn a l conversion (IC) coefficients; (6) Data are often not available in evaluated fo rm . It is cle ar that hot atom ch e m istry is im pingin g onm any new areas, and the panel strongly recom m ended that a sym posium on hot atom chem istry be held, suggesting 197 6 or 1977 as appropriate and tim ely. It was fu rth er suggested that the sym posium should involve hot atom c h em istry and related fields ra th e r than focus on the purely the o re tic a l and basic aspects of hot atom chem istry. Such a sym posium would f i ll an im po rtan t gap in in te r national m eetings that are held and deal with hot atom chem istry. O ther m eetings n o rm a lly involve only hot atom chem ists and do not encourage p a rticip ants fro m other fields. The panel, in observing the strong im pingem ent of hot atom c h em istry on m any other scien tific fie ld s, encouraged the Agency to take up a position in support of fundam ental re search in hot atom c h em istry and to support those a ctiv itie s that are appropriate fo r the Agency to encourage in te r  d is c ip lin a ry activity and the continued support of hot atom c h em istry re search as a necessary and appropriate part of atom ic energy prog ram m e s.
LIST OF P A R T IC IP A N T S
A D L O F F , J . P.
L a b o rato ire de C him ie N u clé a ire, (Centre de Recherches N u cléaires), S trasbourg, F rance
A M IE L , S.
Soreq N uclear R esearch Center, Yavne, Is r a e l
B E R E I, K la ra
C en tral R esearch Institute for P hysics, H ungarian A cadem y of Sciences, Budapest 1525, Hungary
DANON, J . (O bserver)
Centro B r a s ile iro de P esquisas F is ic a s , Av. Venceslao B raz 71, R io de Ja n e iro , B r a z il
. D IM O T AK IS, P . N.
U niversity of P a tra s , P a tra s , Greece
F E IN E N D E G E N , L.
Institut fur M edizin, K ernforschungsanlage Jü lic h Gm bH, Postfach 365, 517 Jü lic h 1, F e d e ra l R epublic of G erm any
G E T O F F , N.
In s titut für Theoret. Chem ie und S trahlenchem ie, W â h rin g e rstra s se 38, A - 1090 V ienna, A u s tria
H A R B O T T LE , G.
C h e m istry D epartm ent, Brookhaven N ational L aboratory, Upton, NY 11973, United States of A m e ric a
K A N D E L, R . J .
M olecu lar and Geo-Sciences B ranch, D iv isio n of P h y sic a l R esearch, US A to m ic E nergy C o m m ission, W ashington, DC 20545, United States of A m e ric a
329
330
L IS T OF P A R T IC IP A N T S
L E E , Y.
Ja m e s F ra n c k Institute and D epartm ent of C h em istry , U niversity of Chicago, 5640 E llis Avenue, Chicago, IL 60637, United States of A m e ric a
L IM A , F . W .
Instituto de E n e rg ia A tó m ica , С. P . 11049 P in h e iro s, 01000 Sâo Paolo S. P . , B r a z il
L IN D N E R , L.
Institute for N uclear P hysics R esearch (IKO), O o s te rrin g d ijk 18, A m ste rd a m , The N etherlands
M ADDOCK,. A . G.
The C h em ical L a b o rato rie s, L e n sfie ld Road, C am bridge, England
M A L C O L M E - L A W E S , D .J .
U niversity of Kent at C anterbury, C anterbury, Kent Ct2 7NH, E ngland, United Kingdom
M IG G E , H.
B ereich K ernchem ie und R eaktor, Hahn-M eitner Institut, B e rlin 3 9, G lie nicke r Str. 100, F e d e ra l R epublic of G erm any
NEW TON, M.
C h e m istry D epartm ent, Brookhaven N ational L aboratory, Upton, NY 11973, United States of A m e ric a
PA U LU S, J . M .
L a b o rato ire de C him ie N u clé a ire , В. P. 20, 67037 Strasbourg-Cedex, France
R O S S L E R , K.
Institut für N uklearchem ie, K ernforschungsanlage Jü lic h Gm bH, P ostfach 365, 517 Jü lic h 1, F e d e ra l R epublic of G erm any
SAITO, N.
D epartm ent of C hem istry , U niversity of Tokyo, Bunkyo-Ku, Tokyo, Japan
SH IOKAW A, T.
D epartm ent of C hem istry , Tohoku U niversity, Sendai, Jap an 980
L IS T O F P A R T IC IP A N T S
ST Ü CK LIN , g .
Institut fiir N uklearchem ie, K ernforschungsanlage Jü lic h Gm bH, P ostfach 365, 517 Jü lic h 1, F e d e ra l R epublic of G erm any
W O L F , A . P.
C h em istry D epartm ent, Brookhaven N ational L aboratory , Upton, N Y 11973, United States of A m e ric a
331
IN T E R N A T IO N A L A T O M IC E N E R G Y A G E N C Y C A LA M A N D , A.
N uclear D ata Section, D iv isio n of R esearch and L a b o rato rie s
FU RU T A , T.
In d u s tria l A p p lications and C h em istry Section, D iv isio n of R esearch and L a bo rato ries
LEM LEY, J.
N uclear D ata Section, D iv isio n of R esearch and L a bo rato ries
L O R E N Z , A.
N u clear D ata Section, D iv isio n of R esearch and L a bo rato ries
NISHIW AKI, Y.
D iv isio n of N uclear Safety and E n v iro n m e n tal P rotection
O E T T IN G , F . L .
In d u s tria l A pplications and C h em istry Section, D iv isio n of R esearch and L abo rato ries
SE RV IA N , J . L.
In d u s tria l A pp lications and C h em istry Section, D iv isio n of R esearch and L abo rato ries
Scientific Secretaries R IC H M A N , D .M .
In d u s tria l A pp lications and C h em istry Section, D iv isio n of R esearch and L aboratories, IA E A , Vienna
R O W LA N D , F. S .
D epartm ent of C h em istry, U niversity of C a lifo rn ia, Irv in e , CA 92664, United States of A m e ric a
332
L IS T O F P A R T IC IP A N T S
C ontributors A C H E , H.
D epartm ent of C h e m istry, V ir g in ia Polytechnic Institute and State U niversity, B lacksburg , VA, United States of A m e ric a
CA C A C E , F .
U niversity of R om e, 00100 R om e, Italy
Thefollowingconversion tableisprovidedfor theconvenienceof readersandtoencourage theuseof SI units. FA C TO R S FO R C O N V E R T IN G U N IT S TO SI S Y S T E M E Q U IV A L E N T S * SI base units are the metre (m), kilogram (kg), second (s), ampere (A), kelvin (К), candela (cd) and mole (mol). [For further information, see International Standards ISO 1000 (1973), and ISO 31/0 (1974) and its several parts]
Multiply
by
to obtain
Mass pound mass (avoirdupois) ounce mass (avoirdupois) ton (long) {= 2240 Ibm) ton (short) (= 2000 Ibm) tonne {= metric ton)
= 1 Ibm = 1 ozm = 1 ton 1 short ton = 1t =
4.536 2.835 1.016 9.072 1.00
X X X X X
10_1 101 103 102 103
kg g kg kg kg
km m m m mm
Length statute mile yard foot inch mil (= 10'3 in)
1 mile 1 yd 1 ft 1 in 1 mil
= = = =
1.609 9.144 3.048 2.54 = 2.54
X X X X X
10° 10_1 10“l 10~2 10~2
1 ha 1 mile2 1 acre 1 yd2 1 ft2 1 in2
= = = = = =
1.00 2.590 4.047 8.361 9.290 6.452
X X X X X X
104 10° 103 10_1 10“2 102
1 yd3 1 ft3 1 in3 1 gal (Brit) 1 gal (US) 1I
= = = = = =
7.646 2.832 1.639 4.546 3.785 1.00
X X X X X X
1 0 '1 10-2 104 10“3 10-3 10'3
m3 m3 mm3 m3 m3 m3
1 dyn 1 kgf 1 pdl 1 Ibf 1 ozf
= = = = =
1.00 9.807 1.383 4.448 2.780
X X X X X
10-5 10° 10"1 10° 10'1
N N N N N
= = = = = =
1.054 4.184 1.356 7.46 7.355 7.457
X X X X X X
103 10° 10° 102 102 102
W W W W W W
Area hectare (statute mile)2 acre yard2 foot2 inch2
„2 m km2 m2 m2 m2 mm2
Volume yard3 foot3 inch3 gallon (Brit, or Imp.) gallon (US liquid) litre
Force dyne kilogram force poundal pound force (avoirdupois) ounce force (avoirdupois)
.
Power British thermal unit/second calorie/second foot-pound force/second horsepower (electric) horsepower (metric) (- ps) horsepower (550 ft-lbf/s)
1 Btu/s 1 cal/s 1 ft-lbf/s 1 hp 1 ps 1 hp
* Factors are given exactly or to a maximum of 4 significant figures
Multiply
by
to obtain
Density pound mass/inch3 pound mass/foot3
1 lbm/in3 1 lbm/ft3
= 2.768 X 104 = 1.602 X 10‘
1 Btu 1 cal 1 eV 1 erg 1 ft-lb f 1 kW -h
= = =“ = = =
Energy British thermal unit calorie electron-volt erg foot-pound force kilowatt-hour
1.054 4.184 1.602 1.00 1.356 3.60
X X X X X X
103 10° 10 ' 19 10 ' 7 10° 106
1.00 1.013 1.00 1.333 1.00 2.989 3.386 2.491 9.807 4.788 6.895 1.333
X X X X X X X X X X X
10s 10s 103 10_1 103 103 102 104 10 1 103 102
Pressure newtons/metre2 atmosphere0 bar centimetres of mercury (0°C) dyne/centimetre2 feet of water (4°C) inches of mercury (0°C) inches of water (4°C) kilogram force/centimetre2 pound force/foot2 pound force/inch2 {= psi)* torr {0°C) {= mmHg)
1 N/m2 1 atm 1 bar 1 cm Hg 1 dyn/cm2 1 ftH 2 0 1 inHg 1 Ín H 20 1 kgf/cm2 1 lbf/ft2 1 lbf/in2 1 torr
=
= = = = = = = =
Pa Pa Pa Pa Pa Pa Pa Pa Pa Pa Pa Pa
Velocity, acceleration inch/second foot/second (= fps) foot/minute
1 in/s 1 ft/s 1 ft/min
= 2.54 X 10* = 3.048 X 1СГ1 = 5.08 X 10~3
mile/hour (- mph)
1 mile/h= { ^ °
knot free fall, standard (= g) foot/second2
1 knot 1 ft/s2
*
mm/s m/s m/s ¡ ^ S/h
= 1.852 X 10° = 9.807 X 10° = 3.048 X 10 _1
km/h m/s2 m/s2
Temperature, thermal conductivity, energy/area- time Fahrenheit, degrees— 32 Rankine 1 Btu-in/ft2 -s -° F 1 Btu/ft-s- °F 1 cal/cm-s-°C 1 Btu/ft2 -s 1 cal/cm2 -min
° F — 321 R j = = = = =
5 9 5.189 6.226 4.184 1.135 6.973
= = = = =
2.832 4.719 1.00 2.580 3.70
X X X X X
102 101 102 104 102
f° C [ К W /m -K W /m -K W /m -K W/m2 W/m2
Miscellaneous foot3 /second foot3 /minute rad roentgen curie
1 ft3/s 1 ft3 /min rad R Ci
«atm abs: atmospheres absolute; atm (g): atmospheres gauge.
b|bf/in2 (g) lbf/in2 abs
X X X X X
10’ 2 10 "4 10 ' 2 10 -4 10 10
m 3/s m 3 /s J/kg C/kg disintegration/s
(= psig): gauge pressure; {= psia): absolute pressure.
HOW TO ORDER IAEA PUBLICATIONS
H
Exclusive sales agents for IAEA publications, to whom all orders and inquiries should be addressed, have been appointed in the following countries: U N IT E D K IN G D O M
U N IT E D S T A T E S O F A M E R I C A
■
U N IP U B , P.O. B o x 433, M u rra y H ill Station, N e w Y o rk , N .Y . 1 0 0 1 6
In the following countries IAEA publications may be purchased from the sales agents or booksellers listed or through your major local booksellers. Payment can be m ade in local currency or with UNESCO coupons. A R G E N T IN A A U S T R A L IA B E L G IU M CANADA C.S.S.R. FRANCE HUNGARY I N D IA IS R A E L IT A L Y JAPAN N ETHERLANDS P A K IS T A N POLAND R O M A N IA S O U T H A F R IC A S P A IN SW ED EN U.S.S.R. Y U G O S L A V IA
■
H er M aje sty's Statio n e ry Office, P.O. B o x 569, L o n d o n S E 1 9 N H
C om isión N acional de E n ergía A tóm ica, A v e n id a del Libertador 8 2 50, Bu e n o s A ire s H un te r Publications, 5 8 A G ip p s Street, C ollin gw o od , V icto ria 3 0 6 6 Service du C ourrier de ( 'U N E S C O , 112, R u e du Trône, B -1 0 5 0 Brussels In fo rm atio n Canada, 171 Slater Street, Ottawa, O nt. K 1 A O S 9 S.N .T .L., Spálená 51, C S - 1 10 0 0 Prague Alfa, Publishers, H u rb a n o vo nâmestie 6, C S - 8 0 0 0 0 Bratislava O ffice International de D o cu m en tation et Librairie, 48, rue Gay-Lussac, F -7 5 0 0 5 Paris Kultura, H ungarian T rad in g C o m p a n y fo r B o o k s and Newspapers, P.O. B o x 149, H -1011 B udapest 62 O x fo rd B o o k and Statio ne ry С о т р . , 17, Park Street, Calcutta 16 Heiliger and Co., 3, N athan S trau ss Str., Jerusalem Librería Scientifica, D ott. de B iasio L u c io "a e io u ", V ia Meravigli 16, 1-20123 M ilan M aruzen C om p an y, Ltd., P .O .B ox 5 0 50, 100-31 T o k y o International M a rin u s N ijh o ff N .V., Lange V o o r h o u t 9-11, P.O. B o x 269, T h e Hague M irza B o o k A gency, 65, T h e Mall, P .O .B o x 729, Lahore-3 A r s Polona, Céntrala H andlu Zagranicznego, K ra ko w skie Przedm iescie 7, P L -0 0 -0 6 8 W arsaw Cartim ex, 3-5 13 D ece m brie Stree t, P .O .B o x 134-135, Bucarest V a n S c h a ik 's Bookstore, P .O .B ox 724, Pretoria U niversitas B o o k s (Pty) Ltd., P .O .B o x 1557, Pretoria Nautrónica, S.A ., Pérez A y u s o 16, M a d rid -2 C.E. Fritzes Kungl. H ovb okh an de l, Fredsgatan 2, S - 1 0 3 0 7 S to c k h o lm M e zhd u narod n aya Kniga, Sm o le n ska ya -Se n n a ya 32-34, M o sc o w G -2 0 0 Jugoslovenska Knjiga, Terazije 27, Y U - 1 1 0 0 0 Belgrade
Orders from countries w here sales agents have not yet been appointed and requests for information should be addressed directly to:
^ ¿y ^ ê '^ i='
Publishing Section, International Atom ic Energy Agency, Kàrntner Ring 11, P.O.Box 5 9 0 , A-1011 Vienna, Austria
ub
J
IN T E R N A T IO N A L A T O M IC E N E R G Y A G E N C Y V IE N N A , 19 7 5 P R IC E : U S $ 1 9 .0 0 Austrian Schillings 3 5 0 ,â&#x20AC;&#x201D; (ÂŁ 9 .2 0 ; F .F r. 84 ; D M 4 9 , - )
S U B JE C T G R O U P : IV C hem istry, G eology and Raw M aterials/ R adiation C hem istry