NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 4 n.2, 1999

NOTIZIARIO Neutroni e Luce di Sincrotrone Rivista del Consiglio Nazionale delle Ricerche

SOMMARIO

Cover photo: Representation of a Cu+–(CO)2 adduct in the MFI channel.

EDITORIALE

2

.................................................................................

F.P. Ricci

RASSEGNA SCIENTIFICA The use of Synchrotron Radiation in the Characterization of Zeolite and Zeotype Materials

...

3

C. Lamberti et al.

The Application of Neutron Scattering to the Action of a Pore Forming Toxin . . . . . . . . . . . . . . . . . . . . . . . 14 Il

NOTIZIARIO Neutroni e Luce di Sincrotrone

è pubblicato a

R. J.C. Gilbert and O. Byron

cura del C.N.R. in collaborazione con il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

X-Ray Natural Circular Dichroism

Vol. 4 n. 2 Dicembre 1999 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96

Momentum Distribution Spectroscopy

by Neutron Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

............................

22

L. Alagna et al.

G. Reiter et al.

DIRETTORE RESPONSABILE:

F.P. Ricci COMITATO DI DIREZIONE:

VARIE

.......................................................................................

30

M. Apice, P. Bosi COMITATO DI REDAZIONE:

C. Andreani, L. Avaldi, F. Boscherini, U. Wanderlingh

SCUOLE E CONVEGNI CALENDARIO

.................................................................

36

..............................................................................

42

SCADENZE

..................................................................................

44

FACILITIES

.................................................................................

45

SEGRETERIA DI REDAZIONE:

D. Catena HANNO COLLABORATO A QUESTO NUMERO:

F. Barocchi, P. Bosi, F.Carsughi, R.Giordano, S.Mobilio, G.Vlaic GRAFICA E STAMPA:

om grafica via Fabrizio Luscino 73 00174 Roma Finito di stampare nel mese di Dicembre 1999 PER NUMERI ARRETRATI:

Paola Bosi, Tel: +39 6 49932468 Fax: +39 6 49932456 E-mail: p.bosi@dcas.cnr.it.

PER INFORMAZIONI EDITORIALI:

Desy Catena, Università degli Studi di Roma “Tor Vergata”, Dip. di Fisica via della Ricerca Scientifica, 1 00133 Roma Tel: +39 6 72594364 Fax: +39 6 2023507 E-mail: catenadesy@roma2.infn.it

Vol. 4 n. 2 Dicembre 1999

EDITORIALE

e start this editorial noting the important news that the US Government has recently approved funding for the new spallation neutron source (the SNS) which is based on a 1 MW proton accellerator. At the same time feasibility studies on new spallation sources are in progress in Europe: 1) the European Spallation Source (ESS) project involves the construction of a 5 MW source, with an estimated cost between 1000 and 1500 MECU; the feasibility study of this source has been performed by a consortium of European institutions, the ESS Council.

Recently, the Council named a Project Team whose objective is to perform the design of the source by 2003, in close collaboration with the laboratories associated with the Council, in order to submit it to European Union governments for approval. 2) The Austron project involves the construction of a 0.5 MW proton source; the foreseen investment is 337 MECU over the 7 years necessary for construction and the operating cost will be 36.6 MECU per year after the start of operations in 2007. 3) The ISIS - II project involves the construction of a second target at the Rutherford Appleton Laboratory (UK) where the most intense spallation neutron source, ISIS, is already operational; part of the proton beam from ISIS, with a power of 60 KW, will be directed towards a new target which will be optimized for experiments which require neutrons with a wavelength longer than that at present available at ISIS. The predicted cost is about 150 MECU. The CLRC will fund the new target and related buildings while it will propose to the European partners of ISIS to build and run the new beamlines.

It must be stressed that CNR, which is at the moment member of the ESS R&D Council, has qualified groups of expert researchers who could be involved in the various phases of the ESS project in the next three years; the same groups have worked in the past years to train young scientists in the field of instrumentation at ISIS. For further information the article by F. Barocchi can be consulted.

We recall here that the "Operating Group Grenoble OGG" of Istituto Nazionale per la Fisica della Materia has been recently inaugurated in Grenoble. The main objective of this group is to support the use of the neutron and synchrotron sources present in Grenoble (ILL and ESRF) by Italian research groups from universities, research institutions and industries.

In this period the Users' Meetings of the two third generation synchrotron radiation sources to which Italy provides financial support take place. At the end of November the ELETTRA Users' Meeting took place and was coupled to a workshop on chemical reactions at solid surfaces. In the main session scientific reports were given

W

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

2

•

on subjects ranging from surface physics, to microscopy of polymers, to bio-crystallography, to the development of new radiological techniques; the wide spectrum of topics clearly illustrates the application of synchrotron radiation to numerous scientific fields. About 175 researchers took part in the Meeting, and approximately 70 in the workshop. The plenary session was also the occasion to be informed on the faacility itself. At the moment nine beamlines are operational, one is in advanced commissioning stage and other nine are under construction. The ESRF Users' Meeting takes place in February. As usual, a plenary session is accompanied by three workshops, which this year will be devoted to the time-resolved study of structural transformations, to biocrystallography and to self-organization at interfaces and thin films. The Grenoble Users' Meeting takes place in mid-February so that users can discuss the last details of the proposals for the end of the month deadline with ESRF staff. In the next issue of this journal we will publish extensive reports on both Users' Meetings.

To conclude organizational matters we note that CNR has named a new Coordination Committee for Synchrotron Radiation; for more details the contribution

by P. Bosi con be consulted.

We publish four scientific papers in this issue. The paper

by R.J.Gilbert and O.Bryon presents a fair and well addressed application of Small Angle Neutron Scattering to the study of the action of a pore forming toxin protein on the liposomes membranes. Exploiting contrast variation and the inhibition/activation of the toxin they provide a clear and detailed picture of the arrangements the binding of the toxin protein on the membrane surface and they speculate about the subsequent mechanism of membrane piercing. The paper also contains an overview of recent methods for treating SANS data in complex biological systems. The second paper on neutron spectroscopy illustrates an interesting technique to reconstruct the momentum distribution n(p) from epithermal neutron data. The paper by Alagna et. al. describes the original observation of "natural" dichroism in the X-ray range; this result, an extension to a new energetic domain of the well-known effect in the visible region, was made possible by the advanced characteristics of third generations synchrotron radiation sources (in particular ESRF). Finally, we publish a review on the applications of synchrotron radiation spectroscopy and diffraction to the study of zeolites. This paper demonstrates how synchrotron radiation can provide original and precious infomation on the properties of materials of crucial importance in catalysis.

Vol. 4 n. 1 Giugno 1999

F.P. Ricci

RASSEGNA SCIENTIFICA

Articolo ricevuto in redazione nel mese di Maggio 1999

THE USE OF SYNCHROTRON RADIATION IN THE CHARACTERIZATION OF ZEOLITES AND ZEOTYPE MATERIALS C. Lamberti, S. Bordiga, A. Zecchina Dipartimento di Chimica IFM, Università di Torino, Via P. Giuria 7, 10125 Torino, Italy; and INFM Sezione di Torino

G. Vlaic Dip. di Scienze Chimiche, Via Valerio 28, Trieste, Italy and Sincrotrone Trieste SCpA, SS 14, km 163.5, 34012 Basovizza (TS) Italy

Abstract In the last three decades remarkable progress has been obtained in the synthesis of new zeolite and, more generally, zeotype materials representing a large family of microporous solids characterized by sets of three- bior mono-dimensional (in some cases interconnected) channels. Such materials have gained an increasingly important role in the field of heterogeneous catalysis, where the advantage of having a solid catalyst characterized by three-dimensional surfaces of enormous magnitude (up to 103 m2 g-1) is of great importance. In this field, the most relevant applications are in petrolchemistry, pollution control and in the synthesis of fine chemicals. The important role played by SR facilities

The introduction of a trivalent Al(III) atom in a [TO4] unit (substituting the tetravalent Si(IV) atom), induces a net negative charge to zeolitic framework (x-) which must be compensated by the presence of charge balancing extraframework cations (Xn+x/n). Such cations acts as Lewis acid centers, being electron acceptors, but when Xn+ are protons (H+), the zeolite becomes a Br¯nsted solid acid (i.e. a proton donor).

Starting from the basic [TO4] constituents the framework of any zeolite will be realized by progressively connecting two adjacent [TO4] units by sharing an oxygen atom, which becomes so “bridged” between two T atoms (T-O-T), as intuitively depicted in the scheme: The remarkably great flexibility of the T-O-T angle (from

in both diffraction and absorption experiments in the characterization of zeolite and zeotype materials will be emphasized by selecting some relevant examples.

≈ 100o up to 180o) allows to realize, using the [TO4] unit as the sole building block, an impressively large number of different zeolites [1,2,3], among which we shall recall: faujasite (also indicated as X or Y zeolites depending on the Si/Al ratio), zeolite A, ZSM-5, mordenite, β, L, ferrierite and Ω (the synthetic counterparts of the natural mazzite). Among the above-recalled zeolites a few examples have been reported in Fig. 1, where it is evident that the framework of all zeolites hosts a regular systems of intercrystalline voids and channels of well defined size (usually in the nanometer range, 4-13 Å) accessible through apertures of well defined molecular dimensions.

Beside the remarkable degree of freedom represented by

1. Introduction 1.1 Zeolite molecular sieves Zeolites [1,2,3] are nanoporous crystalline alumosilicates constituted by corner-sharing [TO4] tetrahedra, where T represents a silicon or an aluminum atom, the chemical composition of which can be described by the general formula: Xn+x/n [(Al O2)x(SiO2)y]x- + adsorbed molecules

(1)

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

3

RASSEGNA SCIENTIFICA

supposed to be responsible for the activation and reactivity of adsorbed guests [9,10]. Moreover, the cation and the adjacent negatively charged oxygen atoms provide dual acid-base sites (of the Lewis type) which can also play an important role [11] in many catalytic processes mediated by zeolites.

Fig. 1. Zeolite frameworks represented by sticks connecting T sites: LTA (L); FAU (X or Y); MFI (ZSM-5); MOR (Mordenite); SBEA (β), CHA (Chabazite); MAZ (Mazzite); RHO (ρ).

the great number of zeolitic frameworks, in synthetic zeolites the silicon-to-aluminum ratio (y/x), and thus the cation (or proton) density, can be ad hoc chosen in a considerably large range that, for some structures can vary from one to nearly infinity, thus providing a remarkable mean to modulate the ionicity of the material which increases with decreasing y/x. It is worth recalling that for synthetic zeolites the most common synthesis cations are Na+, K+ or NH4+; however, since zeolites are easily prepared in different cationic forms, cation exchange provides a means to select the nature and to tune acidic strength of the Lewis intrazeolitic centers. An extremely large family of extraframework charge balancing cations has been introduced in zeolites [4]: among which we shall recall: alkali metal cations (Li+, Na+, K+ Rb+, Cs+), alkali-earth cations (Mg2+, Ca2+, Ba2+, Sr2+), transition metal cations (Cu+, Cu2+, Zn2+, Ag+, Au+, Pd2+, Pt2+, Fe2+, Fe3+, Co2+, Co3+, Cr3+ etc... ), lantanide cations, etc. The tunability is even more emphasized when a zeolite is prepared in a bi-cationic form An+-Bm+-Z: in fact in this case, the occupancy of each site is shared by the two cations, and this adds a further degree of freedom to the map of intrazeolite electric fields [5]. This ability represents an important fact, since the role of extra-framework cations as catalytically active sites was already postulated and discussed in the very first papers concerning the catalytic applications of zeolites (see e.g. references [6]).

According to these ideas, it has been shown that such ions possess a remarkable polarizing power: in fact, depending on the zeolite structure, the cation charge, radius, and location, extra-framework cations expose adsorbed guest molecules to local electric fields of the order of 109-1010 Vm-1 [7,8,9] These intense fields are

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

4

•

1.2 Zeotypes molecular sieves Zeotypes are nano- or meso-porous materials structurally related to zeolites, in which silicon or aluminum atoms (or both) have been replaced by other atoms [2,3,12,13].

As far as silicalite (the Al-free zeolite of ZSM-5, or MFI, structure) [14] is concerned, B(III) [13,15,16], Ti(IV) [13,16,17,18,19,20,21], V(IV-V) [13], Cr(III-V) [13], Fe(III) [2,13,16,23], Co(II) [13], Ga(III) [13,24], Ge(IV) [13], As(III) [13], Zr(IV) [13], Sn(IV)[13] Sb(III) [13], can be isomorphically substituted in a very small percentage (13 wt. %) inside the framework yielding new zeotype materials which are counterparts of the ZSM-5 zeolite.

These new materials show specific catalytic properties in oxidation reactions related to the coordination state of the heteroatom. Moreover, as far as trivalent metal are concerned, the zeolite framework has a net negative charge which can be balanced by a number of bridged [Si(OH+)M(III)]- (M = B, Al, Cr, Fe, Ga, As, Sb) protons or extraframework charge-balancing cations [Si(OXn+)M(III)]-, thus giving rise to microporous solids with Brønsted or Lewis acidity respectively. As far as protonic metallosilicates are concerned, the following Brønsted acidic strength scale has been established Si(OH)B < Si(OH)Fe < Si(OH)Ga < Si(OH)Al [25].

The family of zeotypes materials is far from being only restricted to MFI structure and is, since some years impressively growing in number, atomic composition, pore size and solid acidity/basicity strength. Without any presumption of exhaustivity we shall mention [13]: the Ti-ZSM-11, Ti-ZSM-12, Ti-ZSM-48 and Ti-β titanosilicates; the Fe-faujasite, Fe-β ferrosilicates; and the Ga-ZSM-11, Ga-faujasite, Ga-mordenite, Gaferrierite, and Ga-β gallosilicates. In this latter list only zeotypes where the tetrahedral TO4 site is shared between silicon and one single heteroatom (Ti, Fe or Ga) are mentioned: it is thus evident that by removing this constraint the number of possible zeotypes materials will be rapidly divergent. We can imagine boronaluminumsilicates (MCM-22), boron-titanosilcates, alumino-gallosilicates, etc.

On a parallel line, as silicalite was the analogous microporous material of quartz, having the same SiO2 chemical formula, also aluminum-orthophosphate AlPO4 has several crystalline microporous counterparts forming a new generation of aluminoposphates indicated as AlPO-n (where n is a conventional integer number indicating the crystal structure) [26,27]. For sake of

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

brevity ALPO’s will not be discussed in this contribution, nor will silicon doped ALPO’s (where Si(IV) substitutes a P(V) atom) giving so rise to SAPO-n materials [26,28,29] or divalent metal ion doped ALPO’s (where Me(II) (Me = Mg, Zn, Co, Ni, Mn, etc) substitutes an Al(III) atom so engendering Me-APO-n zeotypes). Both SAPO-n and Me-APO-n materials need the co-presence of couter ions to assure the electrostatic neutrality of the negatively charged framework Finally, a new generation of amorphous silicates exhibiting regular one dimensional channels of mesoscopic size (20-150 Å) was born with the discovery of MCM-41 [30]. It is so evident that the growth of number of available mesoporous systems allows to open a new way to prepare specifically designed catalysts, similar to what was done in the past for high surface area oxide (MgO, Al2O3, SiO2, TiO2 etc...) supported catalysts, with the further advantages coming from the regularity of the pore system. In fact, it has been demonstrated that accessible single active catalytic sites can be grafted on or transplanted into mesoporous hosting systems. As an example B, Al, Ti, V, Cr, Mn, Fe etc ... can be grafted on MCM-41 [31].

earth science also; however, a discussion of natural zeolites is beyond both the purposes of the present review and the scientific expertise of the authors (the use of SR and neutron sources in the characterization of minerals has been recently reviewed by Artioli [34] and Pavese [35], respectively).

1.3 Applications of zeolites and zeotypes molecular sieves The absolute regularity in channel dimensions and accessibility makes zeolites and zeotypes materials much more molecularly selective in the adsorption of well desired molecules if compared to amorphous carbon or silica gel, which have irregular pore systems. This is the reason for their wide use as molecular sieves. The same characteristics explain the continuously increased role that zeolites and zeotypes have in heterogeneous catalysis (e.g. petrochemical industry, pollution control and fine chemistry).

Moreover, their ability to encapsulate organized molecules, crystalline nano-phases and supramolecular entities inside their channels and pores makes zeolites promising materials in the field of low-dimensional physics, where the quantum effects due to the spatial confinement become observable. Semiconductor Quantum wires and quantum dots can so potentially be obtained by hosting semiconductor crystalline nanophases inside zeolite channels or cages, so obtaining interesting applications in the fields of optoelectronic, non linear optics, photochemistry, and chemical sensors [32]. The same driving idea applies for metal and bimetallic dots: the incorporation of such nano-particles inside the pores/channels of zeotype materials opens a new frontier in the chemistry of metal supported catalysts [33].

Finally it is worth recalling that some tens of different zeolite structures have been found in natural deposits.

This makes zeolites important materials in the field of

2. The unique role played by SR techniques in the characterization of Zeolite and zeotype materials.

Due to the great complexity of zeolite structures (most of them being characterized by low symmetry unit cells containing up to some hundred atoms) and due to the fact that often the material characteristics are strongly related to the introduction of "dopant" atoms, either in the framework (isomorphically substituting silicon), or as extraframework charge-balancing cations it is evident that an accurate characterization of such materials is a difficult task. For this reason there is an increasing need to support the information obtained from conventional laboratory techniques (IR, Raman, UV-Vis and luminescence spectroscopies, resonant solid state NMR and EPR spectroscopies, conventional XPS and XRPD, microcalorimetry, etc.) with data obtained using nonconventional sources like neutrons or synchrotron radiation. Important structural information can be obtained from powder diffraction (PD) of both neutrons and X-rays. The Rietveld refinement of high quality PD data, collected in situ, can give the location of extraframework atoms, occupancies and their evolution upon interaction with adsorbates as well as the location of the adsorbed molecule, i.e. key information in understanding the catalytic activity of the material. As far as SR is concerned, its atomic selectivity can be used to obtain information on a single atomic species thus providing a fundamental tool in the characterization of framework and extraframework "dopant" species. This is the field of anomalous XRPD and, on the local scale, of

1.4 In situ experiments: an ineluctable need During a SR or neutron experiment zeolites should be measured after a thermal treatment able to remove all the pre-adsorbed molecules coming from the ambient atmosphere (activation process). Once this step has been achieved, measurements can be performed, in situ, either on the as activated sample (i.e. zeolite under vacuum conditions) or after having dosed a well defined amount of high purity gas on the sample [36]. The comparison between the data collected before and after the adsorption of a given molecule will allow to extract important information on the interaction process. The typical molecules employed in these studies are either very simple molecules [37] such as CO, NO, N2, H2O, NH3, ... or those involved in the chemical reactions catalyzed by the zeolite.

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

5

RASSEGNA SCIENTIFICA

EXAFS and XANES spectroscopies. In some cases, the high collimation of the beam and the focusing capability of some ad hoc conceived beamlines allow to perform single crystal XRD acquisition and structure refinement even on crystals having the dimensions of few tenths of a nm along the three directions [38].

As far as energetic aspects are concerned, in some cases, UPS and XPS measurements can take benefit of the much higher energy resolution and flux intensity available with SR; however, care must be taken to correctly compensate charging effects due to the policrystalline and insulating nature of zeotype materials. The spectroscopy of vibrational modes and their perturbation by interaction with adsorbates can be monitored using the Far-IR radiation of the low-energy tail of the spectrum emitted by wiggler in a low energy storage ring, or by performing INS measurements.

3. Characterization of metal centers isomorphically substituted into [TO4] centers Among the remarkable variety of heteroatoms that have been successfully isomorphically substituted into zeolites (see introduction), in the two following subsections we shall focus our attention on titanium (3.1), and on iron and gallium (3.2) in the MFI framework. Subsection (3.3) is devoted to pure silicalite, in order to single out its defective nature. This choice is due to both the relevant catalytic interest of the materials and to the experience of the authors.

3.1 Ti-silicalite (TS-1) The unique efficiency yield and molecular selectivity of Ti-silicalite-1 (TS-1, discovered at the beginning of the eighties [17]) in catalytic oxidation reactions involving hydrogen peroxide as oxidant and in ammoximations of ketons and aldeydes to oximes (see e.g. ref. [39]), makes TS-1 one of the most important industrial catalysts discovered in the last twenty years. Since the parent Tifree silicalite in not an active catalyst in such reactions, the hypothesis that Ti species must be present in the catalytic center of TS-1 was inferred in a straightforward manner. Therefore it is immediately evident that the determination of the nature of the incorporated Ti species is of paramount importance in order to understand the catalytic properties of this important material. This is the reason for the lively debate observed in the literature about the structural nature of the Ti centers in TS-1 [16, 18, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52]: titanyl groups, extraframework defect sites, monomeric and dimeric Ti species, Ti species incorporated in edge sharing type structures forming bridges across the zeolite channels and isomorphic substitution, have been inferred by different authors; the same holds for the local geometry, where Ti species

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

6

•

having local coordinations like tetrahedral, square pyramidal, and octahedral have been hypothesized. At present there is a general consensus that the Ti(IV) atoms are incorporated as isolated centers in the framework and substitute Si atoms in the tetrahedral positions. The first evidences indicating that Ti(IV) are framework species come from XRPD and IR-Raman spectroscopies. The X-Ray powder diffraction (XRPD) measurements of Millini et al. [18] have evidenced that the unit cell volume increases linearly with the Ti loading of the sample. A band at 960 cm-1 appears in both IR and Raman spectra [16,18,52,53] of TS-1. It has been asigned to the ν(Si-O) mode perturbed by the presence of an adjacent framework Ti species. Some information on the local geometry around Ti(IV) comes from UV-Vis spectroscopy of TS-1 in vacuo since the band at about 49.000 cm-1 can explained in terms of the O → Ti ligand to metal charge transfer (LMCT) transition in isolated 4-fold coordinated species [TiO4] [53,54] (the LMCT of octhaedral coordinated Ti species,

Fig. 2. XANES spectra of TS-1 in vacuo and in presence of adsorbed NH3.

The arrows point out the evolution of the XANES features upon increasing NH3 pressure (up to ≈ 60 Torr). The inset shows the magnification of the pre-edge peak reported in the figure (solid line) as well as the XANES spectrum obtained after outgassing at room temperature the adsorbed NH3 (dotted line). Spectra collected at LURE DCI (EXAFS3 beam line).

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

like anatase or rutile, occurs in the 30.000-33.000 cm-1 interval).

From this brief picture, it is evident that x-ray absorption spectroscopy is the technique of choice to give the final answer to the nature of Ti centers in TS-1, because its atomic selectivity is able to overcome the difficulty related to the very low content of Ti (less than 3 TiO2 wt.%).

XANES spectroscopy is a self-explaining technique to distinguish between tetrahedral and octhaedral coordinated Ti species. In fact, since unperturbed Ti(IV) is a 3p6 3d0 ion, no dipole allowed pre-edge transitions from 1s into 3p orbitals is observable, while the dipole fobidden 1s2 3d0 → 1s1 3d1 transition is expected to be very low. By introducing Ti(IV) ion inside a crystalline matrix, electron levels are perturbed by ligand field effects and a 3p/3d mixing occurs, the magnitude of which depends upon the symmetry around Ti(IV). In other words, the presence of an inversion center in the octahedral geometry makes the A1g → T2g and A1g → Eg transitions Laporte forbidden. On the contrary, in the tetrahedral geometry, where no inversion point is present, the A1 →T2 transition becomes Laporte allowed and its intensity is very high, overshadowing the weaker A1 → E transition. XANES experiments, performed at the PULS beamline, [44,45] show in TS-1 measured in vacuo conditions a strong pre-edge peak at 5967 eV similar to that of BaTiO4 model compound, thus unambiguously indicating that Ti(IV) is in a tetrahedral geometry. The strongly reduced pre-edge peak, observed in TS-1 upon contact with air means that Ti(IV) is able to expand its coordination sphere reaching a six-fold coordination similar to that of Ti in anatase.

EXAFS measurements indicates Ti-O distances in the range 1.79-1.81 Å and a first shell coordination number ranging between 4.1 and 4.5 (with error bars in the 0.250.6 interval) [44,45,46,47,48,49]. Also EXAFS results support the hypothesis of isomorphous substitution and are able to explain, at a local level, the unit cell increase measured by XRD: in fact the Ti-O distance is much greater than the corresponding Si-O (typically in the order of 1.60 Å). As far as the second shell is concerned, an accurate EXAFS analysis can be done only in the framework of the multiple scattering approach because in some cases the T-O-T angle is near 180o thus enhancing the focusing effect of the central atom. The data analysis on the second shell environment is in progress [55].

In order to investigate the interaction of Ti(IV) with ammonia, a molecule present in the reaction ambient of TS-1, dosages of increasing amounts of NH3 have been in situ followed by both XANES and EXAFS spectroscopies.

Fig. 2 shows the progressive reduction of the 4967 preedge peak and the parallel white line increases upon

Fig. 3. EXAFS data collected at LURE DCI (EXAFS3 beam line) on TS-1 in vacuo (solid line) and in presence of ≈ 60 Torr of NH3 (dotted line). Part a): averaged k x χ(k). Part b): k3-weighted FT of the EXAFS data reported in section a). Part c): first shell filtered EXAFS signals (solid lines) and corresponding fits (dotted lines).

increasing the NH3 equilibrium pressure. This is the clear manifestation of the stepwise TiO4 → (TiO4) NH3 → (TiO4)(NH3)2 process. This phenomenon has been qualitatively observed also by IR-Raman and UV-Vis spectroscopies: the 960 cm-1 band progressively shifts to higher frequency as a consequence of the reduced perturbation that Ti has on the Si-O stretching mode [16,18,52,53], while the O→Ti LMCT band progressively shift to lower frequencies, where the LMCT of six-fold coordinated Ti are expected [53,54]. From a quantitative point of view, EXAFS data analysis of the TS-1 in presence of 50 Torr of NH3 has evidenced that, within experimental errors, two ammonia molecules are coordinated to Ti(IV), locating the N atoms at 1.95 Å. The ammonia adsorption has the parallel effect of stretching the Ti-O bond length by 0.05

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

7

RASSEGNA SCIENTIFICA

Fig. 4. For XRPD TS-1 activated under dynamic vacuum at 400 K was transferred (in vacuo) into a borosilicate glass capillary sealed and mounted on the sample spinner on the ω axis of the diffractometer. Data have been collected, for a little over 6 hours, at the ESRF (BM16) with λ = 0.85018(1) Å in a continuous scanning mode. Parts a), b) and c) reports the observed, calculated and difference profiles and the reflection positions. Rietveld refinement was performed (by G. L. Marra and G. Artioli) in space group Pnma using the program GSAS over the 2θ angular range of 6-60o (0.85 < d < 8.12 Å). The inset reports the Refined cell volume V vs. Ti content x (x = [Ti]/([Ti]+[Si]) ).

Å in order to allow the insertion of two NH3 molecules inside the coordination sphere of Ti(IV) [56]. Comparison between raw EXAFS spectra, corresponding FT and first shell filtered data of TS-1 before and after interaction with NH3 is reported in Fig. 3. The number NH3 molecules adsorbed per Ti site has been confirmed by independent microcalorimentric measurements, where the energetic effects of the phenomenon have also been investigated [56].

As far as subsequent desorption experiments are concerned, IR-Raman, UV-Vis, microcalorimetric, EXAFS and XANES experiments indicates that the initial conditions are nearly totally restored upon a prolonged outgassing at room temperature [16,18,44,45,46,48, 51,53,54], see inset of Fig. 2. The ability of Ti(IV) to modify in a reversible (or nearly reversible) way its local environment upon interaction with adsorbates is probably the key property explaining the remarkable catalytic ability of this important material.

Synchrotron radiation has recently been employed to perform high quality XRPD measurements on carefully dehydrated TS-1 samples with increasing Ti-loading (0-

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

8

•

2.2 wt% TiO2). The aim of the experiment was twofold: (i) improve the important work of Millini et al. [19] performed with a conventional x-ray source on hydrated samples; (ii) verify the existence of some preferential substituting sites among the 12 T centers of the orthorhombic MFI cell, as recently claimed by some authors on the basis of molecular dynamics simulations [57]. Typical data are reported in Fig. 4, together with corresponding theoretical pattern and residual (in the inset the linearity between Ti content and unit cell volume is shown). Rietveld refinement on very high quality powder diffraction data leads us to conclude that the presence of preferential substitution tetrahedral sites for Ti, is very unlikely. Our experimental results [58] agree with the outcome of the quantum mechanical calculations of Jentys and Catlow [59] and of Millini et al.

[60], namely that the Ti is homogeneously distributed on the MFI framework, or it may be slightly partitioned on different sites in different samples. It is worth underlining that this conclusion is supported by recent micro-calorimetric data of NH3 absorption on TS-1 [56], where the evolution of the heat of adsorption with

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

Fig. 5. k3-weighted, phase uncorrected, FT of EXAFS data collected, in transmission mode ,at LURE DCI, on EXAFS3 beamline for Fe-S and EXAFS13 beamline for Ge-S: parts (a) and (b) respectively. Full lines with template, dashed and dotted lines after template burning at lower and higher temperatures.

acidic centers) has been proved to be interesting from a catalytic point of view. Among all we shall recall the conversion of light alkanes, methanol conversion and hydrocarbon cracking. It is thus evident that the role of post- synthesis treatments on the catalytic properties of Ga-S and Fe-S is of fundamental importance in the determination of the acidic, catalytic and shape-selective properties of the material, since it determines the ratio between framework and extra-framework metal species.

With this aim we have investigated by means of EXAFS spectroscopy, the evolution of the local structure around the metal heteroatom M in M-silicalites (M = Fe and Ga) starting from a virgin sample still containing the template up to a final samples calcined at increasing temperatures, see Fig. 5. A consistent reduction of the magnitude of the first shell peak upon burning the template at increasing temperatures is evident for both Ga- and Fe-silicalites, parts (a) and (b) respectively.

The interpretation of such experimental data is as follows: when M heteroatoms occupy tetrahedral framework positions they have a well defined and ordered first shell environment, characterized by 4 oxygen ligands at a well established M-O distance. This ordered situation gives rise to a constructive interaction of the EXAFS signal coming from the different absorbing M sites, yielding (within experimental errors) to a M-O

coverage was found to be typical of heterogeneous surfaces. Neutron powder diffraction data on TS-1 are planned in order to further improve the structural knowledge of this material [61].

3.2 Fe-and Ga-silicalite As already mentioned in section 1.2, the insertion of a trivalent metal atom (as Al, Ga or Fe) in the SiO2 framework implies the appearance of charge balancing cations. When protons are used to compensate the negative charge of the framework, bridged [Si(OH+)M(III)]- (M = Al, Fe, Ga) Brønsted groups appear.

While the stability of both aluminum and titanium as a heteroatom in the MFI framework is very high (so allowing the ZSM-5 zeolite to work at high temperature and TS-1 in H2O2 aqueous solution) both Ga(III) and Fe(III) show, upon increasing the template burning temperature, an evident tendency to migrate from framework into extraframework positions in forms of small oxidic nano aggregates trapped inside the zeolitic channels. This progressive migration implies a reduction of the number of Brønsted acidic [Si(OH+)M(III)]- sites with a parallel increase of new acidic MiOj (M = Fe, Ga) centers of Lewis nature.

As already exhaustively discussed in the literature (see refs. [13,23,62,63] for Fe- and refs. [13,24,63,64] for Gasilicalite) the co-existence of different Ga species (thus giving rise to the co-presence of Brønsted and Lewis

Fig. 6. Periodic reconstruction of defective silicalite obtained after elimination of one six-membered Si(7)-Si(7)-Si(11)-Si(10)-Si(10)-Si(11) ring in a cell: [101] view. The dangling bonds on the oxygens have been saturated by protons. The Connolly surface (defined by dots), obtained using a probe molecule of 2.8 Å in diameter (M. L. Connolly, Science, 221 (1983) 707), clearly show the second set of sinusoidal channels. In correspondence of the defects the available volume is increased.

Results obtained at the HRPD beamline of the ISIS facility, in collaboration with G. Artioli and G.M. Marra.

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

9

RASSEGNA SCIENTIFICA

coordination number of 4. The situation is completely different when a fraction of framework atoms M migrates into extraframework positions forming small oxidic nano aggregates (MiOj) of different size, geometry and anchoring sites to the zeolitic channels. Such heterogeneity implies that the local environment of extraframework M atoms has a consistent spread in both M-O bond distances and coordination numbers (please note that with O we mean oxygen atoms of both oxidic nanocluster and zeolitic framework at the anchoring site). As a consequence, the EXAFS signal coming from extraframework oxidic MiOj nanoparticles is affected by such a large Debye-Waller factor (of static origin) that it becomes practically irrelevant and the observed EXAFS oscillation are due only the complementary fraction of M atoms still occupying framework positions. The EXAFS results reported in Fig. 5 represent the direct proof of the migration of Ga heteroatoms into extraframework positions thus confirming what previously inferred from indirect IR evidences [64]. This evolution has also been followed by XANES spectroscopy where both Fe [62] and Ga [62,65] near edge features are strongly modified by an evolution from tetrahedral to distorted octahedral symmetry.

3.3 Characterization of defects in silicalite As a final remark it is worth noticing that MFI zeotypes are rather defective materials, exhibiting the presence of internal defects: the vacancy of one or more adjacent silicon atoms will give rise to the presence of oxidrylated nano-cavities in the framework, also indicated as hydroxyl nests [66]. The crystal structural and chemical role of internal OH groups in the MFI lattice is still under debate, although it is known that their presence dramatically improves the framework interaction with guest molecules and increases the absorption capacity.

Models for the location and clustering of the hydroxyl groups in silicalite have been proposed on the basis of spectroscopic and volumetric observations aided by parallel molecular dynamic simulations [66].

Surprisingly, recent neutron powder diffraction data have evidenced that preferential location of Si atoms vacancies was found (within 3 esd) on four out of twelve independent T-sites in the orthorhombic silicalite (Si(6), Si(7), Si(10), and Si(11)) [67]. The fact that three out of the four observed defective T sites are adjacent to each other implies that in principle vacancy clusters up to 6 Si defects are possible (i.e. Si(7)-Si(7)-Si(11)-Si(10)-Si(10)Si(11) loop). This is in agreement with the model of hydroxyl nests in silicalite put forward on the basis of spectroscopic evidence. In fact, it can explain: i) the increased adsorption properties of defective silicalites [66] (not accounted by isolated T vacancies); ii) the presence of an IR absorption band in the skeleton

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

10

•

stretching mode region at about 900 cm-1, due to a double oxygen bridge between two adjacent Si atoms located in proximity of an hydroxyl nest (also supported by an ab initio study [68]). Fig. 6 represents the model of defective silicalite emerging from our study.

4. Characterization of extra-framework cations 4.1 Copper exchanged zeolites Cu+-ZSM-5 has attracted much interest for the direct conversion of NO into N2 and O2 [69]. Cu+ ions hosted in ZSM-5 are known to have a high reactivity, as demonstrated by: (i) the formation of end-on Cu+(N2) dinitrogen stable complexes at RT [70,71]; (ii) the formation, upon interaction with NO at 77 K, of Cu+(NO) and Cu+(NO)2 stable complexes, which evolve spontaneously with formation of nitrous oxides and oxidized copper species when temperature is increased [70,72]; (iii) the formation, upon contact with CO at 77 K, of well defined and stable Cu+(CO), Cu+(CO)2 and Cu+(CO)3 complexes [70,73]. The high activity of Cu+ZSM-5, as compared to other supported copper catalysts, is probably related to the high coordinative unsaturation of extra-framework Cu+ ions hosted in the MFI structuretype framework. The formation of a di-carbonilic complex is represented in Fig. 7.

Fig. 7 Representation of a Cu+-(CO)2 adduct in the MFI channel.

The direct proof of this coordinative unsaturation, has been given by in situ EXAFS experiments [70] where it has been shown that Cu+ cations are coordinated to 2.5 ± 0.3 framework oxygen atoms at 2.00 ± 0.02 Å. Interaction with 40 Torr of CO (1 Torr ≈ 133.3 Pa) at RT strongly modifies the first local environment of CO, as

Vol. 4 n. 1 Giugno 1999

RASSEGNA SCIENTIFICA

recognized that a single well defined peak at 8983-8984 eV is the fingerprint of copper species in the oxidation state one. This peak is due to the dipole-allowed 1s → 4p electronic transition of Cu+ [80]. On the contrary, Cu2+ species exhibit: i) a weak absorption at about 8976-8979 eV, attributed to the dipole-forbidden 1s → 3d electronic transition ii) a shoulder at about 8985-8988 eV and an intense peak at about 8995-8998 eV, both due to the 1s→4p transition [80]. Fig. 9a reports the XANES spectra of a Cu2+-ZSM-5 zeolite activated in situ at increasing temperatures (from RT to 400 oC): from this experiment the progressive Cu2+ → Cu+ reduction is evident. Fig. 9b shows the effect of in situ dosage of pure O2 and subsequent dosage of H2O: it thus emerges that only the cooperative action of both O2 and H2O is active in the Cu+→Cu2+ re-oxidation process. These XANES data have been strongly supported by parallel EXAFS, UV-Vis, EPR and IR studies [81].

Fig. 8. Part (a): k3-weighted, phase uncorrected, FT of EXAFS data collected at RT, in transmission mode ,at LURE DCI, on EXAFS1 beamline for Cu+-ZSM-5 before and after interaction with CO dotted and full curves respectively. Part (b) k3-weighted, phase uncorrected, FT of EXAFS data collected at 20 K, in transmission mode, at the ESRF on BM29 beamline for Ag+-ZSM-5 before and after interaction with CO dotted and full curves respectively.

documented in Fig. 8a: under these thermodynamic conditions IR spectroscopy has evidenced the formation of Cu+(CO)2 complexes [70,73]. EXAFS data analysis results in the coordination of N = 2 CO molecules with a Cu-C distance of 1.95 ± 0.05 Å, while the distance between Cu+ cations and framework oxygen atoms increases by 0.16 ± 0.04 Å [74]: this is the first direct evidence of the solvatation effect that CO molecules have on cuprous cations in zeolites and support IR evidences based on both C-O [74] and [74,75] Si-O framework stretching regions. In such conditions an OC-Cu+-CO angle of 130o has been inferred.

In a recent experiment performed at the GILDA beamline we were able to dose CO on Cu+-ZSM-5 at liquid nitrogen temperature: in such thermodynamic conditions we have observed the formation of Cu+(CO)3 complexes with CO linearly bonded to Cu+ [76]. The EXAFS data analysis (performed in collaboration with F. D’Acapito using GNXAS software [77]) strongly supports previous indirect IR evidences [70,73]. In the frame of the same collaboration, the simulation of the near edge region (using the CONTINUUM package developed at the INFN National Laboratories in Frascati (I) [78]) indicates that the Cu+(CO)3 complex is in a nearly C3V symmetry [76]. The study of the RED/OX chemistry (Cu+ ⇔ Cu2+) occurring in copper exchanged zeolites is of paramount importance and XANES spectroscopy is one of the most powerful techniques [70,79]. In fact, it is widely

4.2 Silver exchanged zeolites As far as Ag+-exchanged zeolites are concerned [82], several catalytic and photocatalytic processes have been performed by exploiting the presence of both isolated Ag+ ions and aggregated Agn clusters. Among them we can mention the photochemical dissociation of H2O into H2 and O2 [83a,b], the disproportionation of ethylbenzene [83c], the oxidation of ethanol to acetaldehyde [84], the aromatization of alkanes, alkenes and methanol [85], the selective reduction of NO by ethylene [86] and the photocatalytic decomposition of NO [87].

Fig. 9. XANES spectra (collected in transmission mode at the ESRF on GILDA BM8 beamline) of Cu2+-ZSM-5 thermally treated at increasing temperatures (part a) and after interaction with adsorbates (part b). In both parts, the inset in the upper left corner reports the magnification of the 1s → 3d electronic transition while the inset in the lower right corner of part b) reports superimposed the seven calibration XANES spectra of Cu metal collected by measuring the photon flux (Φ2) after the sample.

Part a) shows the Cu2+→ Cu+ oxidation process, while the Cu+ Æ Cu2+ reoxidation by interaction with an atmosphere of O2 and H2O is reported in part b).

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

11

RASSEGNA SCIENTIFICA

EXAFS studies of Ag+-exchanged zeolites (see Fig. 8b) show that Ag+ cations are coordinated to 2.5 ± 0.5 framework oxygen atoms at 2.30 0.03 Å. Within experimental errors, Ag+ cations have the same environment as Cu+, being the framework oxygen located at an increased distance quantitatively compatible with the increased cationic radius. Interaction with CO at low temperature (see Fig. 8b) yields to the formation of Ag+(CO)2 with a OC-Ag+-CO angle very close to 180o: in such a geometry the MS contributions are maximized. This fact is also supported by parallel IR evidence [88].

Acknowledgments An exhaustive list of all colleagues and friends having contributed to the here reviewed, or just quoted, results is very long and cannot be extensively given. We are particularly indebted to all beamline scientists and technicians who have always allowed us to operate under optimal conditions, in particular: the PULS group (ADONE); the EXAFS1, EXAFS3 and EXAFS13 groups (LURE DCI); the SIRLOIN staff (LURE SUPERACO); the GILDA BM8, BM16 and BM29 groups (ESRF); the SUPERESCA staff (ELETTRA). Investigated materials have been synthesized in the laboratories of EniChem in Novara (Istituto G. Donegani) or of EniTecnologie in S. Donato (Mi). Finally, S. B. and C. L. want to thank the organizers and the professors of the “Scuola Nazionale di Luce di Sincrotrone”, periodically held in S. Margherita di Pula (Ca), since their participation (as Ph.D. students) to the first edition has engendered their progressive entry in the SR community. The use of SR and neutron sources is a key point of the project coordinated by A. Zecchina and co-financed by the Italian MURST (Cofin 98, Area 03).

References 1. W. M. Meier, D. H. Olson and Ch. Baerlocher, Atlas of Zeolite Structure Types, (Elsevier, London), 1996.

2. R.M.Szostak,Molecular Sieves,(Van Nostrand Reinhold,N.Y.), (1989).

3. J. M. Thomas, R. G. Bell and C. R. A. Catlow, in Handbook of Heterogeneous Catalysis, (G. Ertl, H. Knˆzinger and J. Weitkamp Eds.), VCH, Weinheim (D), (1997), p. 286-310.

4. W. J. Mortier, Compilation of Extra Framework Sites in Zeolites; Butterworth: London, (1981); and references therein.

5. G. L. Marra, A. N. Fitch, A. Zecchina, G. Ricchiardi, M. Salvalaggio, S. Bordiga and C. Lamberti, J. Phys. Chem. B, 101 (1997) 10653.

6. J. A. Rabo, C. L. Angell, P. H. Kasai and V. Shoemaker, Discuss. Farady Soc., 41 (1966) 328; R. M. Barrer and R. M. Gibbons, Trans. Faraday Soc., 61 (1965) 948; H. W. Haynes, Catal. Rev. Sci. Eng., 17 (1978) 273; J. A. Rabo, Catal. Rev. Sci. Eng., 23 (1981) 293.

7. See e.g.: C. Lamberti, S. Bordiga, F. Geobaldo, A. Zecchina and C. Otero Arean, J. Chem. Phys., 103 (1995) 3158; A. Zecchina, S. Bordiga, C. Lamberti, G. Spoto, L. Carnelli, L. and C. Otero Arean, J. Phys.

Chem., 98 (1994) 9577; S. Bordiga, G. Lamberti, F. Geobaldo, A. Zecchina, G. Turnes Palomino, G. and C. Otero Arean, Langmuir, 11 (1995) 527.

8. Bàse and H. Fàrster, J. Mol. Struct., 218 (1990) 393; T. Yamazaki, I. Watanuki, S. Ozawa and Y. Ogino, Y., Langmuir, 4 (1988) 433.

9. J. B. Uytterhoeven, D. Dompas and W. J. Mortier, W. J., J. Chem. Soc. Faraday Trans., 88 (1992) 2753.

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

12

•

10. C. Mirodatos, A. Biloul and D. Barthomeuf, J. Chem. Soc. Chem.

Commun., (1987) 149.

11. W. W. Kaeding and S. A. Butter, J. Catal., 61 (1980) 155; T. Mole, J. Catal., 94 (1983) 423.

12. P. Ratnasamy, and K. Kumar, Catal. Today, 9 (1991) 329 and refs. therein.

13. B.Sulikowski, HeterogeneousChem.Rev.,3 (1996) 203 and refs. therein.

14. E. M. Flaningen, J.M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, R. M.Kirchner and J. V. Smith, Nature 271 (1978) 512.

15. M. Taramasso, G. Perego, B. Notari, in Proc. 5th Int. Conf. Zeolites, (L. V. C. Reeds Ed.) Heyden, London, 1980, p. 40.

16. D. Scarano, A. Zecchina, S. Bordiga, F. Geobaldo, G. Spoto, G. Petrini, G. Leofanti, M. Padovan and G. Tozzola, J. Chem. Soc., Faraday Trans.

89 (1993) 4123.

17. M. Taramasso, G. Perego and B. Notari, U.S. Patent 1983, 4, 410, 501.

18. M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud.

Surf. Sci. Catal. 48 (1989) 133.

19. R. Millini, E. Previde Massara, G. Perego and G. Bellussi, J. Catal. 137 (1992) 497.

20. R. Millini and G. Perego, Gazzetta Chimica Ital. 126 (1996) 133.

21. G. N. Vayssilov, Catal. Rev. Sci. Engl. 39 (1997) 209.

22. B Notari, Adv. Catal, 42 (1996) 253.

23. R. Szostak and T.L. Thomas, J. Catal., 100 (1986) 555; R. Szostak, V. Nair and T.L. Thomas, J. Chem. Soc., Faraday Trans 1, 83 (1987) 487; G. Calis, P. Frenken, E. de Boer, A. Swolfs and M. A. Hefni, Zeolites, 7 (1987) 319; R. B. Borade, Zeolites, 7 (1987), 399.

24. L. Xin-Sheng and J. M. Thomas, J. Chem. Soc. Chem. Commun, (1985) 1544; Hayashi, S.; Suzuki, K.; Shiu, S.; Hayamizu, K.; Yamamoto, O.; Bull. Chem. Soc. Jpn. 58 (1985) 52; J. Weitcamp, H. K. Beyer, G. Borbèly, V. Cortès and S. Ernst, Chem. -Ing. -Tech., 58 (1986) 969; R. Szostak and T.L. Thomas, Inorg. Chem., 25 (1986) 4311; G. P. Handreek and T. D. Smith, J. Chem. Soc., Faraday Trans 1, 85 (1989) 3215; Liu, X.; Klinowski, J. J. Phys. Chem., 96 (1992) 4303; Chu, C. T. W.; Chang, C. D. J. Phys. Chem., 89 (1985) 1569; Lalik, E.; Liu, X.; Klinowski, J. J. Phys. Chem., 96 (1992) 805; Kosslick,H.;Tuan,V.A.;Parlitz, B. Fricke, R.; Peuuuuker, C.; Storker, W. J.Chem. Soc. Faraday Trans., 89 (1993) 1131.

25. W. Langenaeker, N. Coussement, F. De Proft ane P. Geerlings, J. Phys.

Chem., 98 (1994) 3010; A. Corma, F. Llopis, P. Viruela, C. ZicovichWilson, J. Am. Chem. Soc., 116 (1994) 134; P. Strodel, K. M. Neyman, H. Knˆtzinger, N. Rˆsch, Chem. Phys. Lett., 240 (1995) 547; A. Chatterjee, T. Iwasaki, T. Ebina and A. Miyamoto, Micrporous Mesoporous Mater. 21 (1998) 421.

26. M. Hartmann and L. Kevan, Chem. Rev., 99 (1999) 635, and ref. threin.

27. S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flaningen, J. Am. Chem. Soc., 104 (1982) 1146.

28. L. Smith, A. K. Cheetham, R. E. Morris, L. Marchese, J. M. Thomas, P. A. Wright and J. Chen, Nature, 271 (1997) 799.

29. B.M. Lok, , C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flaningen, J. Am. Chem. Soc., 106 (1984) 6092.

30. T. C. Kregse, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Back, Nature, 359 (1992) 710; J. S. Back, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, T. C. Kregse, D. K. Schmidt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. Mccullen, J. B. Higgings and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834.

31. G. Sankar, F. Rey, T. Maschmeyer, J. M. Thomas, G. N. Greaves, A. Corma, B. R. Dobson and A. J. Dent, J. Chem. Soc. Chem. Commun., (1994) 2279; T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 378 (1995) 159; F. Rey, G. Sankar, T. Maschmeyer, J. M. Thomas, R. G. Bell and G. N. Greaves, Top. Catal., 3 (1996) 121; R. D. Oldroyd, G. Sankar, J. M. Thomas and D. ÷zkaya, J. Phys. Chem. B, 102 (1998) 1849.

32. G. A. Ozin, Adv. Mater., 4 (1992) 612; G.A. Ozin, A. Kuperman and A. Stein, Angew. Chem Int. Ed. Engl., 28 (1989) 359; A. Trave, F. Buda and A. Fasolino, Phys. Rev. Lett., 77 (1996) 5405; P. Armand, M. Saboungi, D.L. Price, L. Iton, C. Cramer and M. Grimsditch, Phys. Rev. Lett., 79 (1997) 2061; E. Borello, C. Lamberti, S. Bordiga, A. Zecchina and C. Otero Arean, Appl. Phys. Lett., 71 (1997) 2319; C. Lamberti, Microporous Mesoporous Mater., 30 (1999) 155; S. Bordiga, G. Turnes Palomino, A. Zecchina, G. Ranghino, E. Giamello and C. Lamberti, J. Chem. Phys., 112 (2000), in press.

33. L. Sordelli, G. Martra, R. Psaro, C. Dossi and S. Coluccia, J. Chem. Soc. Dalton Trans., (1996) 765; C. Dossi, R. Psaro, A. Bartsch, A. Fusi, L. Sordelli, R. Ugo, M. Bellatreccia, R. Zanoni and G. Vlaic, J. Catal., 145 (1994) 377; M. Bellatreccia, R. Zanoni, C. Dossi, R. Psaro, S. Recchia and G. Vlaic, J. Chem. Soc. Faraday Trans., 91 (1995) 2045; F. Cimini and R. Prins, J. Phys. Chem. B 101 (1997) 5277; G. Schultz –Ekloff and S. Ernst, in Handbook of Heterogeneous Catalysis, (G. Ertl, H. Knˆtzinger and J. Weitkamp Editors) VCH, Weinheim (D), 1997, pp.374-387.

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

63.

G. Artioli, Notiziario Neutroni e Luce di Sincrotrone, 2 (1997) 4.

A. Pavese, Notiziario Neutroni e Luce di Sincrotrone, 2 (1997) 20.

J. M. Thomas, Chem. Eur. J. 3 (1997) 1557.

A. Zecchina and C. Otero Arean, Chem. Soc. Rev., 25 (1996) 187 G. Vezzalini, S. Quartieri, E. Galli, A. Alberti, G. Cruciani and O. Kvick, Zeolites 19, (1997) 323.

W. Holderich, M. Messe and F. Na?mann, Angew. Chem. Int. Ed. Engl., 27 (1988) 26; P. Roffia, G. Leofanti, A. Cesana, M. A. Mantegazza, M. Padovan, G. Petrini, S. Tonti and P. Gervasutti, Stud.

Surf. Sci. Catal., 55 (1990) 543; G. M. Clerici, G. Bellussi, and U. Romano, J. Catal. 129 (1991) 159; G. M. Clerici, Appl. Catal. 68 (1991) 249; G. Bellussi, A. Carati, G. M. Clerici, G. Maddinelli and R. Millini, J. Catal. 133 (1992) 220; A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrini, G. Leofanti, M. Padovan, M. Mantegazza and P. Roffia, P. in: New Frontiers in Catalysis, L. Guczi, F. Solymosi, P. Tètènyi, Eds., Elsevier, Amsterdam (1993), p. 719.

D.R.C.Huybrechts,L.De Bruyker and P.A.Jacobs,Nature,345(1990) 240.

E. Scultz, C. Ferrini and R. Prins, Catal. Lett., 14 (1992) 221.

D. Trong On, A. Bittar, A. Sayari, S. Kaliaguine, and L. Bonneviot, L. Catal. Lett., 16 (1992) 85; D. Trong On, A. Bittar, A. Sayari, S. Kaliaguine, and L. Bonneviot, J. Mol. Catal., 74 (1992) 233.

S. Pei, G. W. Zajac, J. A. Kaduk, J. Faber, B. I. Boyanov, D. Duck, D. Fazzini, T. I. Morrison and D. S. Yang, Catal. Lett., 21 (1993) 333.

S. Bordiga, F. Boscherini, S. Coluccia, F. Genoni, C. Lamberti, G. Leofanti, L. Marchese, G. Petrini, G. Vlaic and A. Zecchina, Catal.

Lett., 26 (1994) 195.

S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, and G. Vlaic, J. Phys. Chem., 98 (1994) 4125.

S. Bordiga, F. Geobaldo, C. Lamberti, A. Zecchina, F. Boscherini, F. Genoni, G. Leofanti, G. Petrini, M. Padovan, S. Geremia and G. Vlaic, Nucl. Instr. Meth. B, 97 (1995) 23.

L. Le Noc, C. Cartier dit Moulin, S. Solomykina, D. Trong On, C. Lortie, S. Lessard and L. Bonneviot, Stud. Surf. Sci. Catal., 97 (1995) 19; L. Le Noc, D. Trong On, S. Solomykina, B. Echchahed, F. Bèland, C. Cartier dit Moulin and L.Bonneviot,Stud.Surf.Sci.Catal.,101(1996) 611.

C. Lamberti, S. Bordiga, A. Zecchina, G. Vlaic, G. Tozzola, G. Petrini and A. Carati, J. Phys. IV France, 7 (1997) C2-851.

C. Lamberti, S. Bordiga, D. Arduino, A. Zecchina, F. Geobaldo, G .Spanù, F. Genoni, G. Petrini, A. Carati, F. Villain and G. Vlaic J. Phys.

Chem. B, 102 (1998)6382.

U. Romano, A. Esposito, F. Maspero, C. Neri and M. Clerici, Stud.

Surf. Sci. Catal. 55 (1990) 33.

F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leofanti and G. Petrini, Catal. Lett. 16 (1992) 109.

A. Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti and G. Petrini, G., Stud. Surf. Sci. Catal. 65 (1991) 65 671; Camblor, M. A. Corma, A. and PÈrez-Pariente, J. Chem. Soc. Chem. Commun, (1993) 557; G. Tozzola, M. A. Mantegazza, G. Ranghino, G. Petrini, S. Bordiga, G. Ricchiardi, C. Lamberti, R. Zulian and A. Zecchina J. Catal., 179 (1998) 64.

A. Zecchina, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, M. Padovan and G. Leofanti, Stud. Surf. Sci. Catal. 69 (1991) 251.

Zecchina, S. Bordiga C. Lamberti, G. Ricchiardi, D. Scarano, G. Petrini, G. Leofanti and M. Mantegazza, Catal. Today 32 (1996) 97.

Collaboration among University of Turin (I), EniChem company (I) and Royal Institution (UK), J. Phys. Chem. B, manuscript in preparation.

V. Bolis, S. Bordiga, C Lamberti, A. Zecchina, A. Carati, G. Petrini, F. Rivetti, and G. Spanù, Langmuir, 15 (1999) 5753; Microporous Mesoporous Mater. 30, (1999) 67.

Y. Oumi, K.Matsuba, M Kubo, T. Inui, and A. Miyamoto, Microporous Materials, 4, (1995) 53; S. L. Njo, H. van Koningsveld and B. van de Graaf, J. Phys. Chem. B, 101, (1997) 10065.

C. Lamberti, S. Bordiga, A. Zecchina, A. Carati, A. N. Fitch, G. Artioli, G. Petrini, M. Salvalaggio and G. L. Marra, J. Catal., 183 (1999) 222.

A. Jentys, and C. R. A. Catlow, Catal. Lett., 22 (1993) 251.

R. Millini, G. Perego and K. Seiti, Stud. Surf. Sci. Catal., 84 (1994) 2123.

G. Artioli, G. L. Marra and C.Lamberti, ISIS, HRPD beamline, Jun.4-6, 1999.

S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola and G. Vlaic, J. Catal., 158 (1996) 486, and refs. therein; F. Geobaldo, C. Lamberti, S. Bordiga, A. Zecchina, G. Turnes Palomino and C. Otero Are·n, Catal. Lett., 42 (1996) 25, and refs. therein.

C. Lamberti, G. Turnes Palomino, S. Bordiga, D. Arduino, A. Zecchina, G. Spanù and G. Vlaic, presented at Second International Conference on Synchrotron Radiation in Materials Science, Kobe (Japan), October

64.

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

31-November 3, 1998; Jpn. J. Appl. Phys. 38-1 (1999) 55; C. Lamberti, G. Turnes Palomino, S. Bordiga, A. Zecchina, G. Spanù and C. Otero Arean, Catal. Lett., in press.

Otero Arean, G. Turnes Palomino, F. Geobaldo A. Zecchina, J. Phys.

Chem, 100, (1996) 6678.

K. Nishi, K. Shimizu, M. Takamatsu, H. Yoshida, A. Satsuma, T. Tanaka, S. Yoshida and T. Hattori, J. Phys. Chem. B, 102, (1998) 10190.

A. Zecchina, S. Bordiga, G. Spoto, L. Marchese, G. Petrini, G. Leofanti and M. Padovan, J. Phys. Chem., 96, (1992) 4985; J. Phys. Chem., 96, (1992) 4991; A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G., Leofanti, M. Padovan, and C. Otero Arean, J. Chem. Soc. Faraday Trans., 88, (1992) 2959; G. L. Marra, G. Tozzola, G. Leofanti, M. Padovan, G. Petrini, F. Genoni, B Venturelli, A. Zecchina, S. Bordiga and G. Ricchiardi, Stud. Surf. Sci. Catal., 84, (1994) 559; S. Bordiga, G. Ricchiardi, C. Lamberti, D. Scarano, G. Spoto and A. Zecchina, Materials Engineering, 5 (1994) 197.

G. Artioli, C. Lamberti and G. L. Marra, Acta Cryst. B, (1999) in press.

A. G. Pelmenschikov, G. Morosi, A. Gamba, A. Zecchina, S. Bordiga, and E. A. Paukshtis, J. Phys. Chem. 97, (1993) 11979.

M. Iwamoto and H. Hamada., Catal. Today, 10 (1991) 57; Y. Li and W. K. Hall, J. Phys. Chem., 94 (1990) 6145; M. Shelef, Chem. Rev., 95 (1995) 209 and refs. therein.

C. Lamberti, S. Bordiga, F. Geobaldo, M. Salvalaggio, G. Spoto, A. Zecchina, G. Vlaic and M. Bellatreccia, J. Phys. Chem. B, 101 (1997) 344 and refs. therein.

G. Spoto, S. Bordiga, G. Ricchiardi, D. Scarano, A. Zecchina and F. Geobaldo, J. Chem. Soc. Faraday Trans., 91 (1995) 3285.

G. Spoto, S. Bordiga, D. Scarano and A. Zecchina, Catal. Lett., 13 (1992) 39; G. Spoto, A. Zecchina, S. Bordiga, G. Ricchiardi, G. Martra, G. Leofanti and G. Petrini, Appl. Catal. B, 3 (1994) 151.

A. Zecchina, S. Bordiga, M. Salvalaggio, G. Spoto, D. Scarano and C. Lamberti, J. Catal., 173 (1998) 500.

A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, C. Lamberti and M. Salvalaggio, J. Phys. Chem. B, 103 (1999) 3833.

J. Sarkany and W. M. H. Sachtler, Zeolites, 14 (1994) 7; J. Sàrkàny, J. Mol. Struct., 410 (1997) 95.

C. Lamberti, S. Bordiga, G. Turnes Palomino, A. Zecchina and F. D’Acapito, manuscipt in preparation.

A. Filipponi, A. Di Cicco and C. R Natoli,. Phys. Rev. B, 52, (1995) 15122; A. Filipponi and A. Di Cicco, Phys. Rev. B, 52, (1995) 15135.

T. A. Tyson, K. O. Hodgson, C. R. Natoli and M. Benfatto, Phys. Rev. B 46 (1992) 5997.

C.Lamberti, G.Spoto, D.Scarano, C.Pazè, M.Salvalaggio, S. Bordiga, A. Zecchina, G.Turnes Palomino and F.D'Acapito,Chem.Phys.Lett., 269 (1997) 500; C. Lamberti, S. Bordiga, A. Zecchina, M. Salvalaggio, F. Geobaldo and C.Otero Arean, J.Ch.Soc.Faraday Trans.,94(1998) 1519.

D. LuBien, M. E. Winkler, T. J.Thamann, R. A. Scott, M. S. Co, K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 103 (1981) 7014; L. S. Kau, D. J. Spira-Solomon, J. E. Penner-Hahn, K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 109 (1987) 6433; L. S. Kau, K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 111 (1989) 7103; N. J. Blackburn, R. W. Strange, J.Reedijk, A. Volbeda, A. Farooq, A. Karlin and J. Zubieta, Inorg. Chem., 28 (1989) 1349; B. J. Reedy and N. J. Blackburn, J. Am. Chem. Soc., 116 (1994) 1924.

G. Turnes Palomino, P. Fisicaro, E. Giamello, S. Bordiga, C. Lamberti and A. Zecchina, J. Phys. Chem. B, submitted.

T. Sun and K. Seff, Chem. Rev., 94 (1994) 857. (a) P.A. Jacobs, J.B. Uytterhoeven and H.K. Beyer, J. Chem. Soc. Chem.

Commun., (1977) 128; (b) G. Calzaferri, S. Hug, T. Hugentobler and B. Sulzberger, J. Photochem. 26 (1984) 109; (c) T. Baba and Y. Ono, Zeolites, 7 (1997) 292.

D. Lai, J. Li, P. Huang, and D. Wang, J. Mater. Gas Chem., 3 (1994) 211.

Y. Ono, K. Osako, G.-J. Kim and Y. Inoue, Stud. Surf. Sci. Catal., 84 (1994) 1773 Y. Inoue, K. Nakashiro and Y. Ono, Microporous Mater, 4 (1995) 379.

S. Sato, Y. Yu-u, H. Yahiro, N. Mizuno and M. Iwamoto, Appl. Catal., 70 (1991) L1.

M. Matsuoka, E. Matsuda, K. Tsuji, H. Yamashita and M. Anpo, Chem.

Lett.,(1995)375 M.Matsuoka, E.Matsuda, K.Tsuji, H. Yamashita and M. Anpo, J.Mol.Catal.A, 107(1996)399 K.Masuda, K.Tsujimura, K. Shinoda and T. Kato, Appl. Cat.B, 8 (1996) 33 M. Anpo, M. Matsuoka and H. Yamashita, Catal.Today, 35(1997) 177 M. Anpo, G. Z. Shu, H.Mishima, M.Matsuoka and H.Yamashita, Catal.Today, 39 (1997)159 M.Flytzani-Stephanopoulos and Z.Li,Appl.Catal.A,165 (1997) 15.

S. Bordiga, C. Lamberti, G. Turnes Palomino, F. Geobaldo, D. Arduino and A. Zecchina Microporous Mesoporous Mater., 30 (1999) 129; S. Bordiga, G. Turnes Palomino, D. Arduino, C. Lamberti, A. Zecchina and C. Otero Arean, J. Mol. Catal. A, 146 (1999) 97.

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

13

RASSEGNA SCIENTIFICA

Articolo ricevuto in redazione nel mese di Luglio 1999

THE APPLICATION OF NEUTRON SCATTERING TO THE ACTION OF A PORE FORMING TOXIN Robert J. C. Gilbert Division of Structural Biology, University of Oxford, Wellcome Trust Centre for Human Genetics, Roosevelt Drive, Headington, Oxford, OX3 7BN.

Olwyn Byron Division of Infection and Immunity, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, G12 8QQ.

Introduction In this article we describe investigations by us concerning the structure and mechanism of the pore-forming bacterial protein toxin pneumolysin. In doing this we have made use of scattering data obtained from soluble toxin and from liposomes with pneumolysin added, in combination with contrast variation. We describe the nature of our system, the experimental approaches we have taken to address it, and the analytical strategies we have employed.

Finally we summarize our findings and speculate on their significance for understanding the mechanism by which pneumolysin forms pores and the nature of protein/membrane interactions in general.

A high resolution structure for pneumolysin has so far proved elusive because the protein does not readily crystallise. However, on the basis of its high sequence homology with the toxin perfringolysin 0 (from Clostridium perfringens) for which an X-ray diffraction structure has recently been obtained [6], an atomic model for pneumolysin has been proposed [7]. This structure suggests a possible dynamic conformation for the protein. In the third domain appears to be poorly packed against the second and so it has been suggested that the third domain might adopt a dynamic conformation or a structure different in solution from that suggested by homology modelling to high resolution. We sought to address this using small-angle neutron scattering (SANS) of soluble toxin.

The other problem thrown up by the high resolution model of the toxin was the unknown method by which it forms pores and what the effect of the action of

Pneumolysin is a bacterial virulence factor Pneumolysin is an important virulence determinant of the human bacterial pathogen Streptococcus pneumoniae.

Pneumolysin is a 53 kDa protein produced within the pneumococcus and released on bacterial cell lysis.

Pneumolysin activates the complement system through non-immunospecific association with immunoglobulin proteins and damages membranes as a result of pore formation [1,2]. These two processes together underpin the role of pneumolysin in pneumococcal disease [3].

Pore formation involves attacking the membrane by binding to cholesterol in its surface, self-association coupled to membrane insertion of the toxin, and ultimately oligomerization of pneumolysin into large, ring-shaped complexes consisting of 30-50 monomers [4].

The pore-forming oligomers appear to sit perched above the membrane surface with portions of two of their domains inserted into the lipid bilayer, as determined by fluorescence spectroscopy and cryo-electron microscopy of liposomes bearing toxin oligomers [4,5]. It is at present uncertain what the location of the actual pore is relative to the pneumolysin oligomer since the centre of the toxin ring is apparently filled with lipid, while the density minimum in the bilayer occurs just outside the oligomer [4]. The structures of pneumolysin as a monomeric, soluble protein and when inserted into the lipid bilayer are shown in figure 1.

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

14

â&#x20AC;˘

Fig. 1. (a): Structure of pneumolysin as determined by high resolution homology modelling [7]. The amino- (N) and carboxy- (C) termini of the molecule and its four domains (1 to 4) are labelled. (b): Model of the pneumolysin oligomer represnted by a curved hexamer of subunits sitting on a phospholipid bilayer, seen from the side. (c): The same model as in part (b) seen from above

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

their instability or their size. An atomic model for the ribosome is in sight [12] but in the past SANS has played an important part in structural work on this large and complex machine responsible for the production of protein in cells [13-15]. The flexible glycosylated regions of proteins and the fluid lipid bilayers that form boundaries of cells are other structures for which SANS may present an informative approach.

Fig. 2. (a): Scattering curves at two concentrations of Ply-TNB. (b): Distance distribution function (——, Rg = 34.3 ± 0.4 Å) for Ply-TNB at 3.72 mg ml-1. (c): p(r) of 3.72 mg ml-1 Ply-TNB (❍)compared to calculated p(r) functions for the unaltered pneumolysin structure (——) and the structure with domain 3 rotated up by 15o (——). (d): Bead models of pneumolysin unaltered (left) and with domain 3 rotated up by 15˚ and 19˚.

pneumolysin on the structure of the membrane is. This we investigated by comparing the structural characteristics of liposomes prior to and following the addition of pneumolysin.

Small-angle neutron scattering and biology When SANS is applied to biological systems, a range of questions may be in the researcher's mind. Information concerning the structure and chemical composition of macromolecular complexes (protein, nucleic acid, lipid, carbohydrate etc.) is derived from the calculation of the radius of gyration of the sample, or more informatively the real-space vector population between its scattering nuclei (the distance distribution function, see below).

Data may be compared with simulated scatter from models of the supposed conformation of the macromolecule formed of beads and iterative improvement in the congruence between real and simulated data may lead to an understanding of the structure of the complex and the disposition of species of varying scattering length density within it [8-10].

Software developed recently for use with small-angle Xray and neutron scattering data presents the possibility of obtaining an inverse-Debye solution and hence an objective description of the structure of a molecule in solution at ~10 Å resolution [11,11b]. This approach relies on a homogeneous sample and may be especially useful for molecules which have so far eluded structure determination by X-ray crystallography or nuclear magnetic resonance due to their structural flexibility,

Investigating the conformation of soluble monomeric pneumolysin Our first experiment to investigate the structure of pneumolysin in solution was dogged by the previously unknown ability of pneumolysin to undergo the transition from soluble monomer to oligomeric aggregate in the absence of membranes or cholesterol [16]. As a result of this we developed a method for preventing the self-association of toxin by derivatizing its single cysteine amino acid with a benzyl ring. We accomplished this using Ellman's reaction: reacting dithio(bis)nitrobenzoate (DTNB) with the free thiol group presented by the pneumolysin cysteine to produce pneumolysinthionitronbenzoate (Ply-TNB) [16]. We demonstrated that Ply-TNB was monodisperse by analytical ultracentrifugation and proceeded to scatter neutrons from it in 100% D2O buffer at station LOQ of the Rutherford Appleton Laboratory, Chilton, UK. The scattering curves obtained at 2 concentrations are shown in figure 2(a). We subjected these curves to Guinier analysis [17] but prefered to analyze the data over the full Q range using the distance distribution (p(r)) function [18]. This is the real-space Fourier transform of the scattering curve and is analogous to the Patterson function of crystallography. It therefore describes the population of real-space vector lengths between points i and j in a scattering species, thus: p(r) =

1 2π 2

∞

∫ I(Q) 0

•

Qr • sin Qr • dQ

(1)

where Q is the reciprocal space scattering vector in Å-1, I(Q) is the intensity of scatter of vector Q in cm-1 and r is the real-space vector length in Å. The p(r) function was calculated using the program GNOM [19] in which the upper limit of the integration in equation (1) is the length of the scattering species (Dmax) and is fixed for each run of the program. We therefore carried out the Fourier transformation between limits of 0 and a range of upper values and plotted the resulting p(r) functions to obtain a solution with zero or negligible amplitude beyond the Dmax of the scattering species. The p(r) function shown in figure 2(b) describes the shape of the Ply-TNB (the population of real-space vectors ij within the volume of Ply-TNB) determined for the higher

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

15

RASSEGNA SCIENTIFICA

concentration scattering curve shown in 2(a).

In order to set the p(r) and Rg we calculated for Ply-TNB in context we resorted to bead modelling of pneumolysin. The basis of bead modelling is the representation of the molecule by an assembly of spheres each of which contains a proportion of the nuclei from which neutrons are scattered. This we accomplished using the program AtoB [20] which converts the atomic coordinates of a protein into bead coordinates in terms of location and size of bead. The scattering curves were then calculated using the program SCT [21,22] which uses the equation of Crichton et al. [23] in calculating the scattering curve ofmbeads [23],   sin Qr I(Q) of 2an assembly j = φ (QR) n −1 + 2n −2 ∑ Aj  

I(0)

j =1

QRj 

(2)

where φ2(QR) is the squared form factor for a sphere of radius R and φ 2 (QR) = 3

(sin QR − QRcosQR)2 = I(Q) (QR)3

(3)

while n is the number of spheres filling the body, j signifies a defined sphere within the scattering body, Aj is the number of distances rj for that value of j, rj is the distance between the spheres, and m is the number of different distances rj. Finally we transformed the scattering curves into p(r) functions using GNOM for direct comparison with the experimental p(r) functions.

The comparison between the experimental and theoretical scattering curve is made in figure 2(c). We did not explicity allow for a hydration layer around the surface of the pneumolysin molecule. The hydration layer of a macromolecule is denser than the bulk water and thus has an apparent shrinking effect on a protein molecule when observed as a function of its negative contrast with 100% D2O [24]. We noted the apparent discrepancy between the p(r) function calculated for the dry bead model of pneumolysin and that determined experimentally. Since domain 3 was predicted to be flexible and/or to adopt a different conformation in solution from that predicted by high-resolution modelling we rotated this domain and discovered that the agreement between model and experimental curves was significantly improved if the third domain was reoriented 15˚-19˚ upwards. With rotations of 5˚ to 15˚ the agreement improved on each further rotation; with rotations >19˚ the agreement between experimental and modelled curves deteriorated. Rotation of other potentially flexible regions of the molecule such as domain 4 did not improve the fit and therefore we conclude that domain 3 may well adopt a position

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

16

different in solution from that suggested by high resolution homology modelling. Furthermore, since rotations of between 15˚ and 19˚ agreed equally well we conclude that the position of domain 3 may in fact be dynamic. This might explain the still imperfect agreement between modelled and experimental curves despite our manipulation of the bead model in terms of rigid body rotations of domains.

This analysis represents a fairly crude method for extracting structural information from SANS curves. A more exciting and potentially revolutionary method has been described by Chacón et al., who demonstrate that the non-existence of an inverse-Debye solution can be circumvented objectively if one applies a genetic algorithm to the search for the conformation of a scattering species described in terms of a body of beads, as described above [11]. Their elegant and perlucid paper represents a major advance, augmented by the recent description by Svergun of a similar approach driven by simulated annealing rather than a selective algorithm [11b].

•

Fig. 3. (a): Guinier plots for scattering curves from liposomes using the sheet Guinier approximation. ❍ at 100% v/v D2O/H2O: RTH = 11.16 ± 1.6 Å, T (thickness of the sheet i.e. the liposome bilayer) = 38.67 ± 5.5 Å, QmaxRTH = 0.79; ● at 75% D2O: RTH = 11.57 ± 1.4 Å, T = 40.08 ± 4.8 Å, QmaxRTH = 0.82; ✧ at 50% D2O: RTH = 10.15 ± 4.2 Å, T = 35.16 ± 14.5 Å, QmaxRTH = 0.69; ▲ at 0% D2O with positive slope, which if fitted with a straight line would yield a negative value for RTH. The arrows mark the limits of the fits. The positive slope at 0% D2O is due to the proximity of this contrast to the contrast match point of the liposome sample (12.4 % D2O). (b): Nested solutions to the thickness distribution function for the 100% and 75% D2O scattering curve plotted in part (a). —— at 100% D2O: T = 33 Å, RTH = 10.45 ± 0.02 Å. - - at 75% D2O: T = 35 Å, RTH = 11.26 ± 0.03 Å. (c): Model for a liposome constructed from beads viewed in 3 planes. (d): Scattering curve calculated from the liposome model shown in part (c). Compare to the curves shown in Figure 4.

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

Investigating the interaction of pneumolysin with membranes The interaction of pneumolysin with membranes involves insertion into and disruption of the bilayer. This leads to pore formation, the exact structural nature of which is still a mystery. We sought to understand something about the interaction between bilayer and toxin from SANS curves from liposomes with and without toxin, which we obtained at station LOQ as above and at station D11 of the Institut Laue-Langevin in Grenoble, France. We explored various possible methods of analyzing the data including the calculation of thickness radii of gyration (RTH) from sheet-averaged Guinier plots (where the area factor of the Guinier equation, Q2 is multiplied out of the scattering curve) and performing the same trick with p(r) functions within GNOM to obtain thickness distribution functions pTH(r) [19,25]. We were uncertain how appropriate it was to model scattering occurring from the liposome as occurring from a sheet since the liposome surface would possess curvature. In some cases it appeared appropriate and yielded a value for the thickness of the liposome surface in the region one would expect (~40 Å); in others the anomalous nature of the fits obtained indicated that these were not suitable analytical tools. With the pTH(r) functions we frequently obtained unique solutions in terms of a thickness suggesting that describing the liposome surface as a sheet was appropriate, yet the values obtained for the sheet thickness from pTH(r) analysis were consistently ~8 Å lower than those obtained by Guinier analysis. We attempted to resolve the matter by constructing liposome models from beads using a specially developed version of the bead manipulation program AtoB (see above) and backcalculating the scattering curves to obtain Guinier plots and pTH(r) functions for these hollow spherical liposome models. This exercise succeeded only in undermining our confidence in the sheet-approximation applied to the liposome scattering curves still further and so we abandoned these two analytical approaches. Figure 3 shows examples of SANS curves for liposomes analyzed using the sheet-average Guinier and pTH(r) approaches; the imperfect liposome model we constructed from beads; and a scattering curve calculated for it using SCT as above.

a solid ring were the models used to represent liposomes and toxin oligomers respectively.

Since SANS reveals only a rotationally averaged, low resolution structure a pairwise sum over all possible atomic coordinates may be reduced to a sum over scattering volumes containing averaged scattering length densities. For a dilute system of N particles, the SANS is due entirely to interference terms within the volume V of sin( ri − r j Q) one I(Q) =particle NV 2 (∆ρ )2 F 2 (Q) = N ∫ ∫ (ρ (ri ) − ρs )(ρ (r j ) − ρs ) dVi dV j V V

ri − r j Q

(4)

where F(Q) is the form factor of the particle, ρ(rp) is the neutron scattering length density within the particle and ρs the scattering length density of the solvent.

For a sphere of radius r, F(Q) is analytic: F(Q,r) =

3(sin(Qr) − Qr cos(Qr)) (Qr)3

(5)

For a hollow shell of uniform scattering length density ρp of inner radius R and shell thickness ∆.

I(Q) = N(ρ p − ρ s)2 {V1 F(Q, R1 ) − V2 F(Q, R1 + ∆)}

2

(6)

In the model used here the inner radius R1 was summed over a Schultz distribution. The oscillation seen at small Q in figures 3 to 5 is due to interference between the terms in equation (4), effectively between opposite bilayers across the diameter of the liposome. This shows that the liposomes are reasonably rigid, and by least squares fitting makes both R1 and ∆ well determined.

Given the scattering length densities and calibration of I(Q) against standard scatterers, the absolute value of I(Q) provides a check on the amount of sample in the beam.

The SANS for dilute, randomly oriented rods or discs of length L and radius R requires numerical integration over angle γ between Q and the rod axis, I(Q) = N(∆ρ )2 V 2 ∫

π /2

0

F 2 (Q)sin( γ )dγ

(7)

where A novel method of analysis for a biological system We found in the end that the best way to analyze our data made use of the direct fitting of scattering equations to the SANS curves obtained from liposomes before and after toxin addition. This we accomplished using the versatile and powerful program FISH [26]. FISH fits data with the scattering equations of geometric models for simple structures using a standard iterative least-squares method. Here, a hollow sphere with a scattering shell and

F(Q) =

sin( 12 QL cos γ ) 2J1 (QRsin γ ) 1 QRsin γ 2 QL cos γ

(8)

and J1(x) is a first order Bessel function of the first kind.

In a way analogous to equation (6) the scattering for a hollow cylinder or ring may be computed inside the integral of equation (7).

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

17

RASSEGNA SCIENTIFICA

model indicate an internal radius of curvature of ~266 Å for a spherical shell of ~40 Å thickness. The parameters for this and successive fits are listed in Table I. The agreement between the volume of the spherical shell and the calculated molecular volume of scattering molecules in the sample indicates that the shell fit accounts for essentially all of the sample. The trend of I(0) with contrast indicated that the D2O compositions were as listed (not shown).

Figure 4(b) shows scattering from the same samples with pneumolysin added at a molar ratio to cholesterol of 1:200. The fits are again good, and it is clear from the parameters listed for this fit in Table I that the spherically averaged surface of the liposome has reduced in thickness to ~36 Å. There has been a concomitant rise in the radius of curvature of the inner surface of the

Fig. 4. (a): Scattering curves obtained at RAL for liposomes alone with contrast variation fitted with the scattering equation for a two-shell hollow sphere using FISH (Heenan, 1989). ❍ 100% D2O, ● 75% D2O, ❒ 62.5% D2O, r 50% D2O and ◆ 40% D2O. The scattering lenght density distribution and parameters for these fits are listed in Table I. (b): Scattering curves for liposomes with pneumolysin:cholesterol 1:200 obtained at RAL fitted with the scattering equation for a two-shell hollow sphere using FISH (Heenan, 1989). ❍ 100% D2O,+ 87.5% D2O, ● 75% D2O, ❒ 62.5% D2O and r 50% D2O. The scattering length density distribution and parameters for these fits are listed in Table I. (c): Scattering curves for liposome in the presence of Ply-TNB obtained at RAL at 75% (●) and 50% (▲) D2O fitted with the scattering equation for a two-shell hollow sphere using FISH (Heenan, 1989). The pneumolysin was subsequently activated in situ by the addition of DTT. The scattering curves for the activated system are also shown at 75% (❍ ) and 50% (r) D2O, again fitted using FISH (Table I). (d): Scattering curves for liposome alone (❍), and with pneumolysin:cholesterol 1:200 (●), 1:100 (r), 1:80 (+), and 1:67 (❒) fitted with the hollow spherical shell model using FISH. The parameters of the fits (----) are listed in Table I.

We carried out the following experiments: scattering from liposomes at a variety of contrasts, from liposomes and toxin at the contrast match point of the liposomes, from liposomes and toxin at a series of high contrasts, from liposomes and Ply-TNB (inactive toxin that cannot form pores due to its inability to self-associate or insert into the bilayer) which we later activated in situ by reductive breakage of the disulphide bond joining pneumolysin to TNB, and from liposomes at a variety of pneumolysin concentrations.

Viewing the structures of liposomes before and after their interaction with pneumolysin Figure 4(a) shows scattering curves obtained from liposomes alone at a series of high contrasts fitted with the hollow spherical model using FISH. There is an excellent agreement between the fitted curve and the experimental data, and the fitted dimensions of the

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

18

•

Table I: Parameters from the fitting of scattering equations to curves obtained at RAL. ρ is scattering length density, while ∆ρ is the difference between ρsample and ρbuffer. The scattering length densities were calculated from the chemical compositions of the samples using published values. R is the radius of curvature of the liposome and σ/R is the polydispersity in R . Vshell is the volume fraction of the sample occupied by the shell of the hollow sphere with which the data were fitted. The calculated volume fraction of the sample occupied by the lipid was 0.263 % for the liposome sample, was 0.216 % for the 1:200 Ply:cholesterol sample of liposomes with toxin, for 1:100 was 0.170 %, for 1:80 was 0.145 % and for 1:67 was 0.121 %.

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

spherical shell to ~272 Å, while there is again a good agreement between the volume of the spherical shell and the calculated molecular volume of scattering molecules in the sample. This indicates that pneumolysin bound to the liposome surface could be accommodated within the shell model and that there was no significant sample (protein or lipid) not accounted for by the spherical shell.

Thus attack by pneumolysin does not destroy the liposomal structures but alters the nature of their bilayer surfaces. The trend of I(0) with contrast was linear, indicating again that the D2O compositions were as listed (not shown).

We next investigated whether the thinning of the liposome bilayer was the result of the action of pneumolysin or due to some systematic error in the sample not accounted for by our analysis. We mixed PlyTNB which cannot form pores or self-associate with liposomes and obtained scattering data at 75% D2O, where both lipid and protein would be well contrasted against the background buffer, and at 50% D2O where the scatter from protein would be mostly matched out (its contrast against the buffer background would be low).

These fits are shown in figure 4(c). As indicated in Table I at 75% D2O the surface of the bilayer was thicker than when measured alone in 4(a) above at ~46 Å, while at 50% D2O is was 40 Å as before. This suggests that some Ply-TNB has bound to the membrane but has not undergone insertion or caused the membrane to change in structure due to pore formation, making the liposome surface appear thicker. At 50% D2O this thickening was

Fig. 5. Small-angle neutron scattering curve of liposomes with pneumolysin:cholesterol 1:100 at 12.4% D2O obtained at ILL. The scattering curve has been fitted with the scattering equation for a ringshaped structure. The fitted dimensions of the ring were a radius of 248.0 ± 6.5 Å, a height of 85.4 ± 11.0 Å, and a radial thickness of 69.0 ± 5.2 Å.

not apparent, presumably due to the low contrast of the protein against the buffer background.

When we added dithiothreitol in situ to reduce the disulphide bond linking the inactivating benzyl group to the base of domain 4 of pneumolysin the scattering data indicated a change in liposome structure comparable to that observed when active toxin was added directly to the liposomes (figure 4(b) above)). Thus at both 75% and 50% D2O the surface of the liposome became thinner while the internal radius of curvature of the shell increased. This proves that the surface thinning and the rise in liposomal dimensions are due to the action of pneumolysin.

We also looked to see what the effect of higher toxin concentrations was on the liposome surface. This experiment used a different liposome preparation to the one described in figures 4(a) to (c): the resultant liposomes had the same chemical composition but happened to be smaller. This provided further evidence that we were observing pneumolysin sitting on the bilayer since as well as the surface thinning at a molar ratio pneumolysin:cholesterol of 1:200 we observed a converse thickening of the membrane at molar ratios from 1:100 up to 1:67 (figure 4(d) and Table I). This represents the effects on the average liposome bilayer thickness of an oligomer 105 Å tall mostly perched above the lipid bilayer. The interior radius of curvature was in addition at all toxin concentrations higher than for liposomes alone, as before.

The final experiment we performed was to scatter neutrons from the liposomes with toxin at the contrast match point of the liposomes, which we had previously determined to be 12.4% D2O. We fitted this scattering curve with a scattering equation describing a ring (in fact a short hollow cylinder). The fit for this is shown in figure 5, and indicates a good agreement between the shape of the model and the scattering species observable at the contrast match point of the lipid. The dimensions of the ring fit were an internal radius of 248.0 Å, a height of 85.4 Å and a radial thickness for the ring itself of 69.0 Å.

Pneumolysin oligomers have radii of 175-250 Å, a radial thickness of ~65 Å, and are 105 Å tall. This suggests we have successfully observed the pneumolysin oligomer in isolation when the scatter form the liposome surface in which it resides is matched out.

What does the effect of pneumolysin on liposome membranes mean?

The effects we have observed when pneumolysin binds to and attacks the bilayer surfaces of liposomes can be summarized thus: toxin binding and partial insertion brings about a thinning of the bilayer, which is masked at higher toxin concentrations by the oligomers associated with pore formation which have a height substantially

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

19

RASSEGNA SCIENTIFICA

greater than the thickness of the membrane. In addition the radius of curvature of the inner surface of the spherical shell increases on toxin attack, indicating a rise in the surface area of the liposome.

Membrane attack by pneumolysin involves initial binding of the membrane by a site at the base of domain 4 close to a region containing 3 tryptophan residues. We believe that binding to cholesterol may elicit the ejection of this "Trp-rich loop" to form a hydrophobic dagger [7].

Insertion of the dagger would be followed by oligomerization of the protein which we have shown to result in refolding of domain 3 [4] which would then enter the membrane causing pore formation [5]. The only cause we can think of for the reduction in bilayer thickness we observe is the compression and/or intercalation of the acyl chains within the hydrophobic core of the membrane. There are two mechanisms by which this could occur, which would both rely on a hydrophobic mismatch existing between the membraneinserted portion of pneumolysin and the hydrophobic portion of the bilayer [27]. Both possible mechanisms would also harness the preference demonstrated by tryptophan residues for location at the interface between hydrophobic and hydrophilic environments [27]. If the Trp-rich loop does extend a dagger-like conformation into the hydrophobic core of the bilayer then it would be expected to have a functional length of ~15 Å whereas the depth of the hydrophobic chains of a bilayer is ~30 Å [28].

One tryptophan would lie at each polar/apolar interface, while the third might interact with cholesterol in the membrane in a ring-stacking interaction. This form of compensation for the existence of mismatch can lead to aggregation of the mismatched protein [29], which would tie in with the fact that aggregation of pneumolysin on the membrane is much accelerated compared to that which occurs in solution [4]. The increase in the internal radius of curvature of the liposomes following attack by pneumolysin suggests the partial insertion of toxin into the lipid bilayer, increasing its surface area and thus its diameter. This would occur whether pneumolysin adopted a trans-bilayer orientation as just described or in fact remained at the surface of the membrane. With this second possibility only the Trp-rich loop in its native fold (i.e. not as a dagger) would be inserted into the hydrophobic portion of the bilayer, leaving the base of domain 4 sitting in the polar headgroup region. This position for pneumolysin would maintain the tryptophan residues at an interface and lead to thinning in the bilayer due to the conservation of the molecular volume of the phospholipids following expansion of the surface area of the membrane upon protein insertion [30].

One well-known effect of hydrophobic mismatch is the induction of non-bilayer lipid phases such as inverted hexagonal (HII) phases [27]. This would bring about a

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

20

•

change in the permeability of the lipid bilayer and may lead to the formation of a pore i.e. a localized region of solute leakage within the plane of the membrane. We have evidence, gained from solid-state NMR experiments, that pneumolysin induces HII-like phases when it attacks the liposomes on which we performed SANS in the experiments described in this article (B. Bonev, R.J.C.G., O.B. and A. Watts, unpublished results).

There are furthermore suggestions of a non-lamellar structure in the membranes analyzed by cryo-EM [4], and in figure 6 we show the densities obtained from that work. This presents the working model for the relative dispositions of pneumolysin oligomer and membrane [4] and we hope may provide a useful interpretative framework for the observations we have made from our SANS experiments.

Our study of the interaction of pneumolysin with a model liposomal bilayer membrane represents a novel experimental strategy. It is a development of an approach used by Hunt and co-workers [31] to investigate the behaviour of a monodisperse protein within the surface of a liposome. In the Hunt experiments the aim was to obtain conditions in which the signal from the lipid component of the sample was masked by the buffer in order to observe the protein in question in isolation. To this end they made use of chain-perdeuterated

Fig. 6. (a): View of the 3D reconstruction obtained from electron cryomicroscopy of pneumolysin oligomers viewed tangentially at the lipid bilayer surface. Top surface rendered view of the reconstruction; bottom density variation in the lipid/protein complex revealed as a central slice through the reconstructed density. These images demonstrate the presence of a density minimum just outside the oligomer (indicated by *), of density variations within the bilayer, and the position of the oligomer almost entirely outside the membrane [4]. (b): A closer view of the fitted toxin domains inside the electron density envelope. Domain 4 contacts the bilayer, accompanied we believe by an extended portion of domain 3 which in the crystal structure possesses a helical conformation [4].

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

phospholipids which formed a bilayer with homogeneous scattering length density, in order to eliminate interference effects. Our strategy is qualitatively different in that we are interested simultaneously in the lipid and protein components of the model system and have therefore modelled the entire liposome/protein complex as a spherically-averaged body. The information we have obtained is comparable to that generated using reflectometry of neutrons at a solidliquid or liquid-air interface [32]. However our approach has the advantage that the dynamic random orientations of the liposomes bearing pneumolysin isotropically averages the scatter from each liposome over its whole surface area and thus circumvents the possibly nonrandom location of pneumolysin oligomers on the bilayer. Reflectometry applied to the same system may be less informative since the planar nature of the sample excludes orientational averaging and relies on lateral averaging of signal in the bilayer. Nevertheless, we plan to build on the experiments described in this article using reflectometry and further small-angle scattering experiments with more intense neutron sources and chain-perdeuterated phospholipids. We hope these future experiments will permit the differentiation of phospholipid from cholesterol from protein in the sample and thus aid the interpretation of the data in terms of the effect of membrane attack by the toxin on the distribution of all three components.

Acknowledgements We are grateful to a number of colleagues for discussion concerning our work and assistance with carrying out experiments. In particular we thank Richard Heenan, Peter Timmins, Tony Watts, Boyan Bonev, Helen Saibil, Jamie Rossjohn and Michael Parker. In addition we thank Dmitri Svergun for GNOM and Stephen Perkins for SCT. The molecular models in figures 1 and 6 were drawn using Bobscript [33,34] and rendered in Raster3D [35].

Note Some of these data have been described in a recent article: Gilbert, R.J.C., Heenan, R.K., Timmins, P.A., Gingles, N.A., Mitchell, T.J., Rowe, Studies on the structure and mechanism of a bacterial protein toxin by analytical ultracentrifugation and small-angle neutron scattering. J. Mol. Biol. 293, 1145-1160.

References 1. Morgan, P.J., Hyman, S.C., Byron, O., Andrew, P.W., Mitchell, T.J. and Rowe, A.J. (1994) J. Biol. Chem. 269, 25315-25320.

2. Mitchell, T.J., Andrew, P.W., Saunders, F.K., Smith, A.N. and Boulnois, G.J. (1991) Mol. Microbiol. 5, 1883-1888.

3. Alexander, J.E., Berry, A.M., Paton, J.C., Rubins, J.B., Andrew, P.W. and Mitchell, T.J. (1998) Microb. Pathogen. 24, 167-174.

4. Gilbert, R.J.C., Jimenez, J.L., Chen, S., Tickle, I.J., Rossjohn, J., Parker, M.W., Andrew, P.W. and Saibil, H.R. (1999) Cell 97, 647-655.

5. Shepard, L.A., Heuck, A.P., Hamman, B.D., Rossjohn, J., Parker, M.W., Ryan, K.R., Johnson, A.E. and Tweten, R.K. (1998) Biochemistry 37, 14563-14574.

6. Rossjohn, J., Feil, S.C., McKinstry, W.J., Tweten, R.K. and Parker, M.W. (1997) Cell 89, 685-692.

7. Rossjohn, J., Gilbert, R.J.C., Crane, D., Morgan, P.J., Mitchell, T.J., Rowe, A.J., Andrew, P.W., Paton, J.C., Tweten, R.K. and Parker, M.W. (1998) J. Mol. Biol. 284, 449-461.

8. Trewhella, J. (1997) Curr. Op. Struct. Biol. 7, 702-708.

9. Chamberlain, D., Keeley, A., Aslam, M., Arenas-Licea, J., Brown, T., Tsaneva, I.R. and Perkins, S.J. (1998) J. Mol. Biol. 284, 385-400.

10. Perkins, S.J., Ashton, A.W., Boehm, M.K. and Chamberlain, D. (1998) Int. J. Biol. Macromol. 22, 1-16.

11. Chacรณn, P., Moran, F., Diaz, J.F., Pantos, E. and Andreu, J.M. (1998) Biophys. J. 74, 2760-2775.

11b. Svergun, D.I. (1999) Biophys. J. 76, 2879-2886.

12. Ban, N., Freeborn, B., Nissen, P., Penczek, P., Grassucci, R.A., Sweet, R., Frank, J., Moore, P.B. and Steitz, T.A. (1998) Cell 93, 1105-1115.

13. Svergun, D.I., Koch, M.H.J., Skov Pedersen, J. and Serdyuk, I.N. (1994) J. Mol. Biol. 240, 78-86.

14. Svergun, D.I., Burkhardt, N., Skov Pedersen, J., Koch, M.H.J., Volkov, V.V., Kozin, M.B., Meerwink, W., Stuhrman, H.B., Diedrich, G., Nierhaus, K.H. (1997) J. Mol. Biol. 271, 588-601.

15. Wadzack, J., Burkhardt, N., Junemann, R., Diedrich, G., Nierhaus, K.N., Frank, J., Penczek, P., Meerwinck, W., Schmitt, M., Willumeit, R. and Stuhrmann, H.B. (1997) J. Mol. Biol. 266, 343-356.

16. Gilbert, R.J.C., Rossjohn, J., Parker, M.W., Tweten, R.K., Morgan, P.J., Mitchell, T.J., Errington, N., Rowe, A.J., Andrew, P.W. and Byron, O. (1998) J. Mol. Biol. 284, 1223-1237.

17. Guinier, A. and Fournet, G. (1955) Small-angle scattering of X-rays.

Wiley, New York.

18. Pilz, I., Glatter, O. and Kratky, O. (1979) Methods Enzymol. 61, 148264.

19. Semenyuk, A.V. and Svergun, D.I. (1991) J. Appl. Cryst. 24, 537-540.

20. Byron, O. (1997) Biophys. J. 72, 408-415.

21. Perkins, S.J. and Weiss, H. (1983) J. Mol. Biol. 168, 847-866.

22. Perkins, S.J., Smith, K.F. and Sim, R.B. (1993) Biochem. J. 295, 101-108.

23. Crichton, R.R., Engelman, D.M., Haas, J., Koch, M.H.J., Moore, P.B., Parfait, R. and Stuhrmann, H.B. (1977) Proc. Nat. Acad. Sci. USA 74, 5547-5550.

24. Svergun, D.I., Richard, S., Koch, M.H.J., Sayers, Z., Kuprin, S. and Zaccai, G. (1998) Proc. Nat. Acad. Sci. USA 95, 2267-2272.

25. Perkins, S.J. (1988) in: Modern Physical Methods in Biochemistry, Part B, pp. 143-265 (Neuberger, A. and van Deenen, L.L.M., Eds.) Elsevier Science Publishers B. V., Amsterdam.

26. Heenan, R.K. (1989) RAL Report 89, 129.

27. Killian, J.A., Salemink, I., de Planque, M.R.R., Lindblom, G., Koeppe, R.E.K., II and Greathouse, D.V. (1996) Biochemistry 35, 1037-1045.

28. Lewis, B.A. and Engelman, D.M. (1983) J. Mol. Biol. 166, 211-217.

29. Harroun, T.A., Teller, W.T., Weiss, T.M., Yang, L. and Huang, H.W. (1999) Biophys. J. 76, 937-945.

30. Heller, W.T., He, K., Ludtke, S.J., Harroun, T.A. and Huang, H.W. (1997) Biophys. J. 73, 239-244.

31. Hunt, J.F., McCrea, P.D., Zaccai, G. and Engelman, D.M. (1997) J. Mol. Biol. 273, 1004-1019.

32. Johnson, S.J., Bayerl, T.M., McDermott, D.C., Adam, G.W., Rennie, A.R., Thomas, R.K. and Sackmann, E. (1991) Biophys. J. 59, 289-294.

33. Kraulis, P.J. (1991) J. Appl. Cryst. 24, 946-950.

34. Esnouf, R.M. (1997) J. Mol. Graphics 15, 132-134.

35. Merritt, E.A. and Murphy, M.E.P. (1994) Acta Cryst. D 50, 869-873

Vol. 4 n. 2 Dicembre 1999

โ ข

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

21

RASSEGNA SCIENTIFICA

Articolo ricevuto in redazione nel mese di Luglio 1999

X-RAY NATURAL CIRCULAR DICHROISM L. Alagna, S. Turchini, T. Prosperi ICMAT-CNR AdR di Roma, CP 10, 00016 Monterotondo Stazione, Italy

B. Stewart Department of Chemistry and Chemical Engineering, University of Paisley, Paisley PA1 2BE, UK

R.D. Peacock Dep. of Chemistry, Univ.of Glasgow, Glasgow G12 8QQ, UK

"For Louis Pasteur, the two distinctive properties of dissymmetric systems, optical activity and chiral discrimination, provided prime evidence for a divine origin to the universe. Handedness appeared to be built into the macrocosm of the galaxies, each with a non superposable mirror image by virtue of its rotation, as well as the microcosm of each molecule of most natural products."

S. F. Mason Introduction When a beam of polarized light is rotated upon passage through matter, the substance is said optically active.

Pasteur was the first to associate this phenomenon with structural dissymmetry. In his celebrated work on optical activity he noticed that there were two types of crystals of an optical active substance, which could be separated with the aid of a microscope. A careful inspection of the two crystals found them to be mirror images of one another. A necessary condition for optical activity is that the smallest characteristic unit of a substance is not superimposable on its mirror image. More rigorously the unit cell or the molecule symmetry group contain no improper rotations, i.e. reflection planes, rotation reflection axes, and center of inversion. Physical techniques giving molecular chirality information have been developed using the differential interaction of circularly polarised radiation with chromophores throughout the electromagnetic spectrum. The extremes of this spectrum are less accessible because the circular differential effect decreases in magnitude for very long or very short wavelengths in comparison to the dimensions of the chiral molecular structure.

The X-ray absorption spectroscopy plays a central role in defining short range structural and electronic information. Via the core-level absorption the technique is element sensitive, and via selection rules the final state is in a definite angular momentum channel.

The natural CD in the X-ray region, by examining nearedge absorption features, combines element-specific local chirality information and, more in general, information on the mixing of even-odd components in the excited state wawefunction (and therefore in the ground state) for many materials of fundamental and technological

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

22

•

interest. The leading term in X-ray absorption is the atomic absorption cross-section. The fine structure modulation is related to the local environment of atoms surrounding the photoabsorber. After subtraction of the atomic contribution, the remaining absorption is dominated by single scattering of the photoelectron by neighbouring atoms. The Fourier analysis of this contribution leads to a radial distribution function for backscatterers in the vicinity of the absorber. True threedimensional information is however more difficult to extract and depends on Multiple Scattering (MS) events which require careful analysis. The technique developed here selectively allows direct experimental access to the MS contributions to photoabsorption since the Circular Differential absorption has no single-scattering part. The technique, like all CD effects, is sensitive to absolute chirality around the photoabsorber and represents the opening up of a new area of X-ray optics. Chirality is an important property of natural and synthetic asymmetric catalysts (as in enzymes and zeolites, for example) for the delivery of designed drugs and other metabolites.

Additionally chirality is a significant factor in all oddparity linear and non-linear optic phenomena.

Natural Circular dichroism is the differential absorption between left and right circular polarized light. The effect is due to the interference of odd/parity terms due to different photon-molecule interaction operators. Two are the possible mechanisms: i) electric dipole-magnetic dipole, ii) electric dipole-quadrupole dipole.

Writing the absorption cross section (in atomic units) as σ (ω ) = 4παω f ε ⋅ r +

α α ( k ∧ ε )(l + 2 s) + i ω ( k ⋅ r )(εˆ ⋅ r ) i 2 4

2

(1)

where ω is the incident photon energy, ε the polarization vector, α the fine structure constant, k the photon wave vector and introducing spherical components of the various vectors:

ε ± = m( xˆ ± iyˆ) / 2 ; r ± = m( x ± iy) / 2 ; l ± = m(l x ± il y ) / 2

(2)

the transition operator is, assuming the propagation vector of the light parallel to the z axis, :

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

interaction is more favourable for shallow edges. In case of low perturbation of the core states summing over the allowed transitions gives zero dichroism. The E1-E2 interaction vanishes for non oriented systems and, because in virtually all valence to valence transition the magnetic moment term is dominant, it is difficult to evaluate in optical transition. Dimensionally the ratio between the E1-E2 and the E1-M1 term scales with photon energy, and it could be expected that E1-E2 is dominant in the Natural Circular Dichroism in the X-Ray region.

Fig. 1. Experimental Nd L3 edge absorption and circular dichroic signal (x200) for enantiomorphic crystals

± T± = r ± i

α ± α M + i ωzr ± 2 4

(3)

where the first term is the electric dipole (E1), the second term is the magnetic dipole (M1) and the third the electric quadrupole (E2). Note that T+ corresponds to left(-) photon circular polarisation (and viceversa) and ( M = l + 2s) is the magnetic dipole operator.

The difference between the absorption cross section of different helicity is ∆σ ∝ −ω

nj

Re  n µ j j Q n − n µ j j Q n  + x yz y xz  

Im  n µ j j m n − n µ j j m n  x x y y  

(E1-M1)

(E1-E2)

(4)

Where µ is the magnetic dipole and Q is the eletric quadrupole The E1-M1 term is the leading term in valence to valence transition.

The order of magnitude of the g dissymmetry factor (g=2(I+ - I- )/ (I+ + I- )) are about 10-3-10-2 for optical transitions. In the core valence transition the E1-M1 term is forbidden because of the selection rule on the principal quantum number.

The measures of this effect should be particularly challenging because it is a probe also for core relaxation effect, removing the orthogonality between core and valence states and allowing small magnetic dipole.

Several attempts were done to give an estimation of the electric dipole-magnetic dipole interaction. The g-factor was of the order of 10-3. The existence of this term in non oriented systems is related to the crystal field splitting of the atomic core level. The electric dipole-magnetic dipole

Experimental Dichroism is related to the lack of a mirror image symmetry in the experiment. In Natural Circular Dichroism the source of the dissymmetry is the sample itself.

In a XNCD experiment the two helicity of the photons must be used, while for X-Ray Magnetic Circular Dichroism it is possible to break the symmetry applying a magnetic field with opposite orientations, holding the same polarization. This could add systematic errors in the spectra due to irreproducibility in the monochromator position, and, for a bending magnet, another cause of errors is due to the change of the geometry of the source sampling different portions of the beam above and below the orbit plane. For this reason third generation high brillance sources made these experiments feasible, providing high flux and, with the insertion of specialized helical undulators, the two helicity polarized beam available along the same optical axis.

The XNCD spectra reported in this paper were carried out at the ESRF beamline ID12A which is dedicated to polarisation-dependent XAFS studies [1]. Circularly or elliptically polarised X-ray photons were generated with helical undulator Helios 2 [2] that has the capability to flip the helicity of the emitted photons. The latter option is essential for X-NCD measurements. The spectra were obtained at room temperature for Nd L3 edge and Liquid Nitrogen temperature for Co K-edge by monitoring the fluorescence yield (FY) as a function of energy.

Hexagonal crystals of Na3[Nd(digly)3].2NaBF4.6H2O (digly = the dianion of diglycolic acid) [3] and 2[Co(en)3Cl3].NaCl.6H2O were grown from aqueous solution by slow evaporation. The unique axis of the crystals was identified by optical microscopy and confirmed by measurement of the axial CD in the visible region.

XNCD at Nd L3 edge and Multiple Scattering approach Figure 1 shows the Nd L3 edge absorption spectrum of the complex together with the CD spectra (multiplied by 200) of enantiomorphic single crystals [4]. It clearly

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

23

RASSEGNA SCIENTIFICA

shows that enantiomorphic single crystals have enantiomeric circular dichroism spectra.

A major feature is a sizable dichroism (g = 3x10-3) exactly where the predicted electric quadrupole-allowed 2p -> 4f transitions are expected. The position of the transition, some 9 eV to low energy of the white line, is similar to that of equivalent transitions in other rare earths [5], [6].

The quadrupolar character of this transition was also confirmed by the disappearance of the CD in the powder spectrum of one of the enantiomers. Enantiomeric features with 10-3 are also seen under the white line and in the near edge region of the spectrum with weaker features evident in the extended fine structure region.

The white line absorption is due to the 2p ->ε d and 2p > ε s transitions, with the former being predominant.

The nearest magnetic dipole transition is 2p -> ε p which is located to higher energy of the 2p -> 4f, ε d and ε s transitions. The 2p -> ε p transition is formally magnetic dipole forbidden by the ∆n = 0 selection rule and is allowed only by core-hole relaxation. Therefore the magnetic dipole - electric dipole CD is expected to be rather weak. However, for CD, an advantage is the presence of electric quadrupole-allowed 2p -> 4f transitions, predicted [7] and recently observed in the XMCD spectrum of Gd3+ [5] and in the resonant inelastic X-ray scattering of Yb metal [6] some 5 - 10 eV to low energy of the L3 edge. This transition is quadrupoleallowed and electric dipole-forbidden, thus maximising the dissymmetry factor. The region after the white line also contains final states of ε f character [8] which can form the basis for the observed CD through the quadrupole mechanism.

In order to simulate the observed dichroic signal, the absorption cross section σ(ω) has been calculated in the framework of the one-electron multiple scattering theory (MST) with effective (complex) optical potential of the Hedin-Lundquist (HL) type as described eg in ref.[8]. The cluster taken into account consisted of 60 atoms within a radius of 6.3 Å from the central Nd, slightly over the average mean free path of the final state photoelectron and enough to reach cluster size convergence. In this approximation we have, using atomic units,

incoming wave boundary conditions, calculated at the energy E = ω - Ic of the photoelectron, which in configuration space and in the notation of reference[10] is given by: − r r r 00 r r r G (r , r ' ; E ) = ∑ RL (r )τ LL' RL' (r ) − ∑ RL (r <)SL (r ' >).

LL'

L

(6)

where τ is the usual scattering path operator of MST. Using Equations (5) and (3) and indicating by Md and Mq the radial dipole and quadrupole matrix elements multiplied by the corresponding Gaunt coefficients with transition operators the X-NCD signal is given by: 00 ∆σ (ω ) ≡ σ − (ω ) − σ + (ω ) = 2πα 2ω 2 ∑ ∑ Im[ M Ld ( m)τ LL ' (ω ) jz

) M Lq' ( ± ) − ( + ↔ − )]

LL'

(7)

taking only the quadrupole contribution by way of an example. A similar formula holds for the electric dipolemagnetic dipole interference term. Notice that the singular term in Eq. (6) being diagonal in L, does not contribute in both cases.

Figure 2 shows the theoretical X-NCD from Eq. (7) as a function of photon energy, normalised to the theoretical atomic cross section (0.12 Mb), plotted against the measured experimental signal normalised in the same way. The agreement is excellent as far as the phase of the oscillations is concerned, not so in absolute magnitude, the calculated theoretical signal being a factor 4.5 bigger.

The pre-edge feature is not at the correct energy position but this is understandable due the non self-consistent character of the potential used and to the use of the

σ m (ω ) = 4παω ∑ Φ c T m Im G − (ω − Ic )T ± Φ c jz

(5)

where ω is the incident photon energy, Ic the core ionization potential, α= 1/137 is the fine structure constant, |Φc> is the initial spin-orbit coupled L3 state (|Φc> = R p3/2(r)|Jjz >). The spherical components of the transition operator T under the chosen experimental conditions (incident photon direction parallel to the crystal z axis) are given by equation (3).

Moreover G- is the Green's function of the system with

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

24

•

Fig 2. Comparison between experimental and theoretical XNCD signals, normalized to the respective atomic absorption. The exprimental curve has been magnified by a factor of 4.5. The inset shows the calculated (dashed line) in Mb and measured (full line) Nd L3-edge absorption normalized to the white line peak, taken as the zero of the energy scale.

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

muffin-tin approximation. Of the two allowed quadrupole transitions to f and p states, the latter gives a contribution which is on average 10 times smaller than the transition to f states mainly due to the smaller radial matrix element.

We have also simulated the effect on the spectra coming from the the partial or total absence of crystallization water. We found that its absence mainly affects the phase of the signal in the energy region around and above 50 eV, in a way which is compatible with the slight discrepancy between theory and experiment in fig. 2. The inset compares the measured and calculated dipole allowed absorption. A similar calculation for the magnetic dipole contribution gives a signal which is on average 1000 times smaller. This was to be expected since on one hand the radial matrix elements of the two mechanisms are in the ratio 1/10 in favour of the quadrupole transition, due to the ∆n = 0 selection rule for the magnetic dipole transition which is broken only when account is taken of the final state relaxation around the core-hole, while on the other hand the quadrupolar contribution is enhanced by a factor ω (455 Ryd)compared to the magnetic one. Both the smaller quadrupole and the magnetic dipole contribution do not show the same phase as the main contribution, indicating that the observed CD is mainly due to quadrupole transtions to f-like final states. As apparent from Equations (6) and (7) the X-NCD signal is proportional to the imaginary part of the amplitude probability of creating the final state photoelectron into an angular momentum state selected by the electric dipole operator at the photoabsorbing site, times the full MS amplitude for returning at the same site to be annihilated in an angular momentum state selected by the quadrupole operator (or viceversa). In a real spherical harmonic representation this amplitude is a contracted cartesian odd tensor of rank two, transforming as the products (x,yz-y,zx). Therefore it is zero for any symmetry point group that contains either an inversion centre or a reflection plane or a roto-reflection axis, and so does not allow the mixing of both dipole and quadrupole allowed wavefunction components.

As a consequence, in a MS path analysis, the X-NCD signal bears information only on those paths that are transformed into each other by the operations of a chiral symmetry group (in the case under study the D3 group) and not of another (not chiral) group. In fact, even in a chiral molecule, subsets of MS paths may possess a symmetry greater than that of the full system and in particular may possess an achiral symmetry. They may be either intrinsecally achiral or occur as enantiomorphically-related equivalent sets. In both cases, either individually or as a set, they do not contribute to the CD signal. Stated in more physical terms, one

observes that our absorption experiment conserves parity, therefore it is invariant to a mirror reflection.

The effect of this latter operation is to interchange enantiomers and to interchange the hands of circular polarisation, thus any circular dichroism is unchanged by this operation. On the above basis, a test for the contribution of a MS path may be formulated: Apply a symmetry operation which: a) inverts the helicity of the photon and b) preserves the orientation of the radiation wave vector in the molecular axis system. Since these operations interconvert enantiomers, any X-NCD contribution must change sign. Therefore, a) if the effect of the operation is to transform a MS path into itself, such a path must be intrinsically achiral and contributes zero to the X-NCD, b) if the path is converted into an equivalent path then the set of such paths has a net zero contribution.

For example, all single scattering paths are transformed into themselves by vertical mirror planes. Thus they are intrinsically achiral. Notice that this is true also in general (i.e. even in the case the incident photon wavevector does not coincide with the rotation axis of the molecule) since it is always possible to find a mirror plane containing the photon wavevector and the two atoms.

Thus one finds that the lowest order contributing paths of shortest length are the double scattering paths involving, besides the Nd central atom, an oxygen in first shell and a carbon in second shell in chiral position (R = 7.0 Å) or the two first neighbours oxygen atoms above and below the xy plane (R = 8.2 Å) again in chiral position.

This is born out by a preliminary study of the amplitude function of the sine Fourier Transform. Since the absolute intensity of the dichroic signal is highest in the near edge region of the absorption spectrum, one might wonder whether a careful structural analysis will not be hampered by the similar well known difficulties met in the EXAFS analysis. However the excellent agreement between theory and experiment in Fig. 2 is not fortuitous.

What makes the XANES region barely usable for direct structural analysis is the presence of the background “atomic” absorption containing inelastic intrinsic processes whose energy dependence distorts the pure diffractive signal coming from coherent MS processes, the only ones bearing structural information. However in the dichroic signal this contribution disappears and one is left with the purely diffractive contribution that the present status of theoretical art is able to describe with very high accuracy. This fact is also confirmed by the success of photoelectron diffraction analysis at low (30 eV) electron kinetic energy (ke), where again the photocurrent modulations as a function of the electron escape direction, being measured at fixed ke, are well described by MS processes[11].

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

25

RASSEGNA SCIENTIFICA

Fig 3. The Co K-edge absorption spectrum of an oriented single crystal of 2[Co(en)3Cl3].NaCl.6H2O together with the XNCD spectra (multiplied by 100) of the Λ and ∆ enantiomers.

XNCD at the Co K edge, large XNCD in the pre-edge region The occurrence of a well-resolved pre-edge (1s -> 3d) feature some 18 eV to low energy of the Co K edge made this system a natural choice for our extension of XNCD studies to the transition metals. Transition metal pre-edge features are commonly used as diagnostics of both oxidation state and coordination geometry in interpreting the X-ray spectra of metallobiosites and their model compounds. The possibility of providing additional local (element-specific) chirality information is one of the important potential applications of XNCD. The most noticeable feature of the present work [12] as reported in figure 3 is the spectacular size of the 1s -> 3d CD observed below the Co K edge. The dissymetry factor is 12.5% if the raw CD and absorption are used; if the white line “tail” is subtracted from the absorption spectrum, the dissymmetry factor is nearer to 20%.

The 1s -> 3d transition is electric quadrupole allowed and electric dipole and magnetic dipole forbidden in the octahedral parent symmetry and becomes partially electric dipole allowed in the C3 site symmetry of the complex in the crystal. The transition would remain magnetic dipole forbidden in all symmetries due to the ∆n= 0 selection rule except that this selection rule is not rigorously enforced when there is orbital relaxation in the presence of the core hole. As a comparison, the dissymetry factors for the 3d -> 3d 1A1 -> 1E(T1g) and 1A1 -> 1E(T2g) transitions which are electric dipole/magnetic dipole and electric dipole/electric quadrupole allowed respectively are approximately 22% and 2%. In order to investigate the efficacy of the E1-E2 mechanism for the 1s-3d transition, we have performed both frozen core and relaxed core HF calculations in a Gaussian orbital basis

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

26

•

[14]. Such calculations of the absorption spectra of transition metal complexes by ab initio methods are rare [15] and there is only one example of an ab inito calculation of transition metal NCD [16]. We used as our model complex the D-[Co(en)3]3+ ion in a D3 geometry optimisation, starting from the crystal structure 21 of 2[∆Co(en)3Cl3].NaCl.6H2O. As a check, the NCD of the magnetic dipole allowed 1 A1 -> 1E(T1g) valence transitions were was reproduced satisfactorily, with similar agreement to experiment found in refs [17] and[16]. The present work extends the ab initio approach to the calculation of core-valence CD for the first time. The results confirm the importance of the quadrupole-dipole interference term in the mechanism of the CD in this oriented crystal. The sign of the pre-edge signal is correctly reproduced as is the order of magnitude of the Kuhn dissymmetry factor. The source of electric dipole transition moment for the preedge CD is ca. 97% Co-based, as expected for a transition emanating from the 1s core orbital. In regard to the large magnitude of the CD in the pre-edge excitation, it appears that the E1-E2 mechanism is particularly efficient in this system. The transition has ca. 10% electric dipole activity and essentially all of this borrowed transition moment is effective in the CD. Since current measurements can now detect g factors of order 10-4 this gives scope for the study of more dilute systems of biological interest.

Conclusions Hence the prospects for a successful structural analysis for the dichroic signal look very promising. Moreover bond angles and lengths are not the only physical information present in the dichroic signal. Use of the generalized optical theorem in MST [18] shows that Im τ in Eq. 8 is directly the product of the amplitudes in an angular momentum expansion of the excited state photoelectron wavefunction, therefore leading to a direct measurement of the mixing of even and odd parity components. A sum rule similar to the one used in magnetic CD can then relate this property to that of the ground state [19].

One can therefore map this mixing versus energy gaining insight into the electronic wavefunction in all those cases where its direct experimental assessment was until now impossible (eg transition metals in glasses and oddparity non-linear optical media).

In conclusion we have shown that the mechanism of the phenomenon discussed in this paper gives direct insight into the interference between different moleculeradiation interaction channels and in particular is a unique method for gaining element-specific local chirality structural information in materials of basic and applied interest.

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

References 1. J. Goulon, N.B. Brookes, C. Gauthier, J. Goedkoop, C. Goulon-Ginet, M. Hagelstein, A. Rogalev, Physica B208-209, 199 (1995) 2. P. Elleaume, J. Synchrotron Rad. , 1, 19 (1994) 3. F. R. Fronczek, A. K. Banerjee, S. F. Watkins and R. W. Schwartz, Inorg.

Chem. 20, 2745 (1981) 4. L. Alagna, T. Prosperi, S. Turchini, J. Goulon, A. Rogalev, C. GoulonGinet, C.R.Natoli, R.D.Peacock, B. Stewart, Phys. Rev. Lett. , 80, 4799 (1998) 5. C. Giorgetti, E. Dartyge, C. Brouder, F. Baudelet, C. Meyer, S.Pizzini, A. Fontaine, R. M. Galera, Phys. Rev. Lett., 75, 3186 (1995) 6. M.H. Krisch, C.C. Kao, W.A. Calieke, K. Hamalainen and J.B. Hastings, Phys Rev Lett., 74, 4931 (1995) 7. P. Carra, M. Altarelli, Phys. Rev. Lett., 64, 1286 (1990) 8. G. Materlik, J.E. Muller, J.W. Wilkins, Phys. Rev. Lett., 50(4), 267-70 (1983) 9. Z.Y. Wu, F. Lemoigno, P. Gressier, G. Ouvrard, P. Moreau, J. Rouxel and C. R. Natoli, Phys. Rev. B 34, 11009 (1996-II) and references therein.

10. T. A. Tyson, K. O. Hodgson, C. R. Natoli and M. Benfatto Phys. Rev. B 46, 5997 (1992-II) 11. S. Gota, R. Gunnella, Z.Y. Wu, G. Jezequel, C.R. Natoli, D. Sebilleau, E.L. Bullock, F. Proix, C. Guillot and A. Quemerais,Phys. Rev. Lett. 71, 3387 (1993); E.L. Bullock, R. Gunnella, L. Patthey, T. Abukawa, S. Kono, C.R. Natoli and L.S.O. Johansson, Phys. Rev. Lett. 74, 2756 (1995)

12. B. Stewart , R.D.Peacock, L. Alagna, T. Prosperi, S. Turchini, J. Goulon, A. Rogalev, C. Goulon-Ginet, submitted to Jour. Amer. Chem. Soc. (1999) 13. Scf eigenvectors produced by the GAMESS-UK program suite (Guest, M. F; Sherwood, P. ,EPSRC Daresbury laboratory, 1995) were used to construct the transition density matrices needed to evaluate all oneelectron integrals of the electric and magnetic dipole and electric quadrupole operators as well as the relaxed-frozen overlap integrals required for the relaxed core HF (RCHF) calculation. The computational methodology closely follows previous work on K-shell excitations in Cu(II) complexes [19] and the work of Schirmer et al [20] for the RCHF part. Using an extended basis set, scf convergences were carried out for the ground state and the K-shell ionised cation.

14. L. G. Vanquickenborne, B. Coussens, A. Ceulmans, K. Pierloot, Inorg.

Chem. 1991, 30, 2978-2986.

15. M.C. Ernst, D.J. Royer, Inorg Chem., 1993, 32, 1226-1232.

16. R.S. Evans, A.F. Schriener, P.J. Hauser, Inorg. Chem., 1974, 13, 218517. C.R. Natoli, M. Benfatto and S. Doniach, Phys. Rev. A 34, 4682, (1986) 18. C.R. Natoli, C. Brouder, P. Sainctavit, J. Goulon, C. Goulon-Ginet and A. Rogalev, Eur. Phys. J., B4, (1998), 1-11.

19. T. Yokoyama,.N. Kosugi, H. Kuroda, Chemical Physics 1986, 103, 101109; N. Kosugi, T. Yokoyama, K. Asakura, H. Kuroda, Chemical Physics 1984, 91, 249-256.

20. J. Schirmer, M. Braunstein, V. McKoy, Phys. Rev. A, 1990, 41, 283-300 ; A. Schmitt, J. Schirmer, Chem. Phys. , 1992, 164, 1-9.

Vol. 4 n. 2 Dicembre 1999

â&#x20AC;˘

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

27

RASSEGNA SCIENTIFICA

Articolo ricevuto in redazione nel mese di Luglio 1999

MOMENTUM DISTRIBUTION SPECTROSCOPY

BY NEUTRON COMPTON SCATTERING Didcot Oxfordshire 0X11 0QX UK J. Noreland, R. Delaplane Uppsala University, Sweden

G. Reiter University of Houston, Texas, USA J. Mayers ISIS Facility, Rutherford Appleton Laboratory, Chilton

The measurement of proton momentum distributions by neutron scattering is analogous to the measurement of electron momentum distributions by Compton scattering and is known as Neutron Compton scattering (NCS).

Both techniques rely upon the fact that when the momentum transferred to the target particle is much greater than the initial momentum of the particle, the impulse approximation (IA) can be used to interpret the data. In the IA, the total momentum and kinetic energy of

Fig. 1. Distance along hydrogen bond.

extended to exploit the much more detailed behaviour on proton dynamics, which can be obtained from single crystal data.

Measurements of the proton momentum distribution n(p), can provide very detailed information about the microscopic dynamics. According to elementary quantum mechanics, n(p) is related by Fourier transform to the proton wave function. The link between n(p) and the proton wave function is formally identical to that between a diffraction pattern and scattering density, so that if n(p) can be measured, the proton wave function can be reconstructed by crystallographic techniques. For example figure 1 shows a model of a proton wave function in a hydrogen bond, where the proton wavefunction is distributed unevenly between two sites separated by a distance 2a=1Å. Simulated eVS data for momentum transfer along the bond is shown in figure 2.

The tails on the data in the region 10-20 Å-1 are due to “interference effects” between components of the wavefunction in the two wells. These effects are similar to the well-known interference fringes, which can be observed in a Young’s slit experiment.

In fact NCS provides information only about the component of p along the scattering vector, measuring

the neutron and proton are conserved during the collision process. From a measurement of the change in energy and momentum of the neutron, the initial momentum of the proton along the direction of the momentum transfer can be determined. NCS measurements on protons have only become possible since the construction of intense accelerator sources such as ISIS, which allow inelastic neutron scattering measurements with energy transfers in the electron volt (eV) region. At lower energy transfers, corrections to the IA become large and it is difficult to relate the intensity of inelastically scattered neutrons to the momentum distribution of the protons. The eVS spectrometer at ISIS has been performing measurements of proton momentum distributions in isotropic samples for a number of years and the technique has now been

Fig. 2. Wave vector transfer in Å-1

-0.5

0

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

28

0.5

•

Vol. 4 n. 2 Dicembre 1999

RASSEGNA SCIENTIFICA

Fig. 3

the “Radon transform” of n(p). The problem of inverting the Radon transform and reconstructing n(p) from NCS data is similar to the reconstruction of 3-D images from, for example, NMR scans and was solved mathematically a number of years ago. Programs for analysing eVS data on single crystal samples were

Fig. 4

installed last year and can now be used to reconstruct n(p). The procedure consists of fitting the data to a complete set of Hermite polynomials and spherical harmonics, convoluted with the instrument resolution function. Typically 20 coefficients are sufficient to accurately fit a data set. An advantageous by-product of the procedure is that instrumental effects are removed and a data set of typically 105 numbers can be reduced to 20 coefficients. The momentum distribution can be reconstructed from the coefficients, essentially by replacing the Hermite polynomials in the expansion by Laguerre polynomials. The dotted line in Figure 2 shows an example of the procedure. It was obtained by fitting the simulated eVS data, and using 20 fit coefficients to reconstruct n(p). The differences between the reconstruction and the analytic n(p), shown as the solid line, are very small.

A real eVS data set on a single crystal of Oxalic acid (KHC2O4) is shown in figure 3.

The plot was constructed by superimposing measurements along all directions in a single plane of the crystal. Three similar measurements of perpendicular planes were made and fitted simultaneously. A particular advantage of the data analysis procedure is that if the potential is harmonic, then only the coefficient of lowest order Hermite polynomial will be non-zero. The presence of higher order coefficients indicates that the potential energy well is anharmonic.

Figure 4 shows the anharmonic components of the momentum distribution of KHC2O4 in the same plane of the crystal.

Unique features of the technique are that information on the ground state is obtained directly, whereas conventional spectroscopy measures transitions between the ground state and excited states. A reconstruction of the wave function allows the determination of the spatial distribution of the proton on very short time scales, whereas diffraction techniques determine an average spatial distribution over much longer time scales. In combination with diffraction measurements, the NCS technique can distinguish between static disorder and dynamic disorder due to quantum tunnelling. There are many areas of technological and scientific interest to which this new technique can be applied, such as the study of hydrogen bonds, which are essential for biological processes and the study of metal hydrides, which have great potential for clean energy storage.

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

29

VARIE

Attività della European Neutron Scattering Association nel 1999 L’ENSA (European Neutron Scattering Association) è stata creata alla fine del 1994 come una aggregazione a livello europeo delle varie associazioni di “neutron users” nazionali, questo allo scopo di promuovere la ricerca in neutronica attraverso una stretta connessione e collaborazione tra i grandi laboratori nazionali ed internazionali europei e gli “user” stessi.

Di questa associazione fanno parte attualmente 15 membri effettivi in rappresentanza delle seguenti nazioni: Austria, Repubblica Ceca e Slovacchia, Danimarca, Francia, Germania, Italia, Norvegia , Olanda, Polonia, Regno Unito, Russia, Spagna, Svezia, Svizzera, Ungheria. Alle riunioni dell’ENSA sono inoltre invitati come osservatori i rappresentanti del Belgio e del Portogallo, delle sorgenti europee, ILL, ISIS, LLB, dei progetti Europei, FMRII, AUSTRON, ESS, della Ruond-Table CEE e dell’ ESF. Durante il 1999 ci sono state due riunioni dell’ENSA, una nel Gennaio 1999 a S.Sebastian in Spagna ed una nel Settembre 1999 in concomitanza con il Congresso ECNS 99 di Budapest. A parte la normale informazione sulle novità ed interventi nazionali che in occasione delle riunioni i vari membri si scambiano, gli argomenti rilevanti trattati durante questo anno sono stati: 1. Congresso ECNS-99 di Budapest; 2. Premio "Walter Halg" per la Neutronica; 3. Affiliazione dell’ENSA alla ESF come “Comitato Associato”; 4. Risposta ai quesiti inviati dall’ESF sul progetto ESS; 5. Politica europea per la neutronica; 6. Rinnovo delle cariche di Presidente, Vice-Presidente e Segretario dell’ENSA. Qui di seguito sono riportate in breve alcune informazioni su questi 6 punti.

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

30

1. Congresso ECNS-99 di Budapest La novità rispetto al congresso precedente ha riguardato la decisione di dare spazio all’attività ed alle proposte dei giovani, intendendo come giovani i ricercatori con meno di ~35 anni. Questo è stato fatto non solo rinnovando anche in questa occasione i premi alle migliori relazioni brevi fatte da giovani, ma anche istituendo una commissione di discussione di strumentazione avanzata in cui giovani, scelti nelle varie nazioni ed aree di ricerca, arrivassero a relazionare al congresso con proposte tra loro discusse. Lo scopo di questa iniziativa è stato di coinvolgere il piu’ possibile nelle discussioni scientifiche i giovani che saranno poi i veri realizzatori ed utilizzatori di quei laboratori e quelle macchine che sperabilmente verranno realizzate in un futuro, che non si presenta in questo momento come immediato. La commissione è stata coordinata da Ushi Stegenberger di ISIS ed ha in effetti prodotto delle relazioni di grande successo a Budapest dove un gruppo di oratori ha riportato i risulatati del lavoro svolto. Per il resto l’organizzazione del Congresso di Budapest ha seguito la linea della conferenza precedente, in particolare l’assegnazione delle relazioni è stata fatta tenendo conto delle proposte ma anche della consistenza numerica della neutronica nei vari paesi europei. Si può certamente affermare che la presenza italiana alla conferenza è stata adeguata sia per partecipanti che per relazioni svolte, ottenendo anche riconoscimenti.

2. Premio "Walter Halg" per la Neutronica Durante l’ultima riunione del 1998 dell’ENSA era stata avanzata, da parte di alcuni membri, la proposta di istituire un premio europeo per ricerche nel campo della neutronica,

•

Vol. 4 n. 2 Dicembre 1999

questo perché un tale riconoscimento avrebbe permesso non solo di premiare uno dei tanti ricercatori di altissimo livello presenti nel campo, ma avrebbe anche dato una positiva ulteriore visibilità alla neutronica in campo europeo. L’ENSA aveva fatta propria questa proposta invitando i membri a trovare possibili strade di reperimento di fondi dedicabili a questo scopo.

Nella riunione di S.Sebastian Albert Furrer, Presidente dell’ENSA, comunica che sono stati resi disponibili dei fondi in Svizzera per la istituzione del premio. In particolare il Prof.

Walter Halg, che è stato l’iniziatore dell’attività neutronica in Svizzera, ha deciso di fornire fondi bancari, gli interessi dei quali potranno essere adoperati per l’istituzione di tale premio. I fondi permetteranno all’ENSA di assegnare un premio di 10.000 Franchi Svizzeri ogni due anni, quindi in concomitanza con i congressi di neutronica.

L’ENSA decide quindi di istituire il premio con le seguenti modalità: -il premio sara intitolato “Walter Halg Prize of the European Neutron Scattering Association”; -sarà un premio di 10.000 Franchi Svizzeri da assegnare ogni due anni; -sara’ assegnato ad un ricercatore che abbia effettuato “outstanding coherent work in neutron scattering; with long-term on scientific and/or technical neutron scattering applications”; -ENSA nominerà in ogni occasione un comitato per l’assegnazione del premio ai candidati proposti; - i candidati possono essere proposti da individui, gruppi ed associazioni.

Il primo premio “Walter Halg” è stato assegnato in Budapest durante il congresso ECNS-99 a Ferenc Mezei per il suo trentennale lavoro di altissimo livello in neutronica, con particolare riferimento agli studi sull’uso della polarizzazione dei neutroni, ai dispositivi di “spin flip” ed alla realizzazione degli spettrometri ad eco di spin che hanno permesso lo stu-

VARIE

dio di particolari proprietà di materiali quali i materiali magnetici, i vetri, i polimeri e la materia biologica, oltre alla realizzazione dei “supermirrors” per neutroni.

3. Affiliazione dell’ENSA alla ESF come “Comitato Associato” All’atto della costituzione non fu ritenuto opportuno conferire all’ENSA uno stato giuridico preciso, come appare nello statuto firmato dai primi associati, questo perché non ci sono stati mai fondi da gestire ed inoltre uno stato giuridico richiede la scelta della nazione dove ottenere tale posizione. Negli ultimi tempi, in particolare con la sopravvenuta necessità di gestire i fondi del premio “Walter Halg”, si è discusso nuovamente della possibilità di dare uno stato giuridico all’ENSA. L’idea che è risultata per ora più perseguibile, allo scopo di ottenere il riconoscimento prima menzionato, è quella di dare all’ENSA anche la qualifica di “Comitato Associato” dell’ESF (European Science Foundation), il ché automaticamente darebbe all’ENSA uno stato giuridico essendo ESF un organismo riconosciuto giuridicamente a livello europeo. Questa associazione permetterebbe inoltre di entrare in contato diretto con l’organo europeo preposto a coordinare tutte le agenzie di finanziamento scientifico nazionali.

Allo scopo di ottenere questo riconoscimento da parte dell’ESF, è stata quindi intrapresa una azione con richiesta ufficiale all’ESF stessa che ha iniziato a prendere in considerazione la proposta. Si presume di ottenere una risposta ufficiale entro la prima metà del 2000.

4. Risposta ai quesiti inviati dall’ESF sul progetto ESS Il Consiglio dell’ESS (European Spallation Source Project) alla fine del lavoro triennale 1996-97-98 di realizzazione del primo progetto di massima per la nuova sorgente a spallazione europea ha inviato tale progetto al-

l’ESF per un giudizio scientifico.

Questa è in genere la prassi seguita da tutti i progetti europei prima della stesura definitiva e la richiesta di finanziamento ai governi nazionali interessati. Il progetto è stato giudicato da una apposita commissione dell’ESF che ha posto all’ESS, ed anche all’ENSA in quanto associazione degli “user”, alcune domande su ESS che richiedono precisa risposta.

Le domande sono le seguenti (riportate qui nella formulazionme originale): I. Which are - in general - the most important new facts and arguments for the ESS case arising from the recently elaborated reference basis, including new developments such as the up-coming/projected facilities, FRM2 and AUSTRON ?

II. “To mantain the Europe’s lead” is one strong and general statement used for arguing the ESS case. But: What does mantain Europe’s lead mean concretely ?

III. “Averting a nutron drought in Europe in the next decade” is another strong and general argument for the upgrading of existing neutron facilities, and for the construction of new and advanced neutron sources, culminating in the ESS project. What is the project positioning and role of the ESS as the largest neutron source in the European network? What will be the unique positioning and role of the ESS for European ( and global) science and research as the highest-brillance pulsed source?

IV. "ESS will serve outstanding “small science” R&TD in many fields of physical, life, and technical sciences" is another strong and general argument widely used. But: What is the supporting evidence for that?

V. If the answers to the question before do not project a substantial role of the ESS for life sciences R&TD: Can ESS’s 1 builion Euro-case be credibly argued and defended with the use of ESS solely for physical and technical sciences research?

Vol. 4 n. 2 Dicembre 1999

•

VI. In terms “value for money”: What are the approximate costs of “typical experiments” at advanced neutron sources in Europe and envisaged at the ESS, in comparison to the costs of “complementary experiments” at existing and projected photon sources?

Da queste domande è chiaro che per la commissione ESF il caso scientifico presentao da ESS ha alcuni punti deboli che devono assolutamente essere risolti. Le risposte che il Consiglio dell’ESS darà a queste domande saranno di cruciale importanza per il proseguimento del progetto stesso.

Poiché richiesta, l’ENSA ha risposto a questi quesiti dal punto di vista degli “users” inviando all’ESF una risposta dettagliata e circostanziata di appoggio incondizionato al progetto ESS, che è stata discussa ampiamente nelle ultime due riunioni.

In questa risposta si fa’ inoltre notare l’inutilità dell’accostamento in termini competitivi e non di sostegno complementare delle due tecniche, quella neutronica e quella fotonica (raggi X), per le ricerche di scienze della vita. Ricerche queste che sembrano essere, in modo un po’ forzato, le più importanti attualmente per la commissione istituita da ESF sul caso ESS. 5. Politica europea per la neutronica L’ENSA ha discusso, nella riunione tenutasi in S.Sebastian, la strategia globale europea che le varie nazioni, aderenti all’ENSA, hanno per il futuro, questo per avere un quadro totale europeo ed anche per elaborare eventualmente una politica autonoma. Ogni delegato nazionale ha dato notizia della strategia nazionale per quanto a sua conoscenza, gli interventi sono stati tutti schematici, ma nondimeno interessanti, e sono riassunti molto brevemente qui di seguito nei punti fondamentali: Austria Non esiste attualmente una vera e propria sorgente nazionale, anche se

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

31

VARIE

è in funzione il piccolo reattore di Vienna. L’Austria è membro dell’ILL in consorzio con la Repubblica Ceca e tale associazione continuerà certamente fino al 2003. Inoltre l’Austria sta cercando di realizzare un accordo tra: Austria, Repubblica Ceca, Italia, Polonia, Slovenia, Svizzera e Ungheria, per la realizzazione del progetto AUSTRON. Belgio Non c’è per ora nessuna strategia della comunità neutronica belga.

Repubblica Ceca e Slovacchia Il reattore di Rez ha la licenza fino al 2006 e potrà andare avanti fino al 2010, se ristrutturato. L’associazione con l’ILL è iniziata nel 1999 in consorzio con l’Austria.

La comunità Ceca è favorevole ad un accordo per AUSTRON. Danimarca Il reattore DR3 a Riso andrà avanti il più possibile, in accordo con le scelte scientifiche tecniche ed economiche.

La prossima revisione avverrà nel 2000 ( ogni 4 anni). Se lo stato tecnico è approvato la licenza verrà rinnovata per dieci anni. Non si vedono ragioni perché questo non accada.

Inoltre la maggioranza degli esperti di neutroni danesi lavora attivamente per il progetto ESS. Francia Le prospettive sono di mantenere in funzione il più possibile ILL ed Orphee e di partecipare attivamente a ESS. Si spera che il contratto tra CEA ed CNRS per Orphee verrà rinnovato quest’anno per i prossimi tre anni. Per quanto riguarda la partrecipazione ad ESS questa andrà valutata in relazione con le difficoltà politiche attuali di partecipare a grandi iniziative.

Germania L’ operatività delle attuali sorgenti nazionali è la seguente: BER2 in Berlino opererà fino al 2010-15. FRJ2 a

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

32

Julich opererà fino al 2005-06. FRG1 a Geesthacht opererà forse fino al 2005. L’inizio dell’operatività del FMR-II di Monaco sarà nel 2001-02.

L’associazione con l’ILL al 30% sarà continuata fino alla eventuale chiusura, che potrebbe avvenire attorno al 2013. Si partecipa attivamente al progetto ESS con l’obiettivo di una operatività per il 2010. ESS è ritenuta essenziale per rimpiazzare ILL nel periodo 2010-15.

Italia L’Italia non ha sorgenti nazionali.

L’Italia ha una associazione sia con ISIS che con ILL che si prevede possa continuare, con il presente impegno, anche oltre il 2003 almeno fino al 2010 se non intervengono fatti nuovi e determinanti. La strategia della comunità neutronica italiana è quella di continuare possibilmente a partecipare a progetti internazionali riguardanti la neutronica. In particolare l’Italia è attualmente associata al progetto ESS, mentre è in discussione una possibile partecipazione ad AUSTRON. Norvegia Si sta aspettando a breve termine la nuova licenza di funzionamento per il reattore da 2 MW di Kjeller che durerà per 10 anni. Si continuerà anche la presente collaborazione con Dubna.

Olanda Il reattore di Delft andrà avante almeno per i prossimi 10 anni, c’è ora un nuovo laboratorio ed una sorgente fredda verrà installata tra breve. Il reattore di Petten è ritornato disponibile per la neutronica ma il futuro non è chiaro. L’Olanda continua la sua associazione con ISIS. La stategia futura è quella di mantenere la partecipazione ad ISIS e contribuire alla realizzazione di ESS. Polonia Il reattore MARIA a Swierk opererà ancora parzialmente per i prossimi 5

•

Vol. 4 n. 2 Dicembre 1999

anni, cioè finche ci sarà carburante, dopo non è chiaro cosa succederà, probabilmente verrà chiuso. Si continua inoltre la collaborazione con Dubna.

Portogallo In linea di principio il reattore da 1 MW potrà operare per i prossimi 10 anni, in particolare è disponibile il carburante per i prossimi 6 anni. Si potrebbe anche maggiorare la potenza fini a 5 MW con spesa non eccessiva.

Regno Unito A breve termine si prevede di continuare con l’associazione ad ILL fino al 2003. Inoltre si progetta la realizzazione di una seconda stazione di misure , chiamata ISIS II, per la realizzazione completa della quale si prevede un termine di 5-10 anni. Tale stazione prevede la maggiorazione della sorgente e la realizzazione di un nuovo parco strumenti dedicati particolarmente ai neutroni freddi. A lungo termine c’è un supporto molto grande per la realizzazione di ESS. Russia Per quanto riguarda il reattore pulsato IBR-2 di Dubna, questo lavorerà con fascio neutronico migliorato fino al 2006. Una ristrutturazione è prevista tra il 2007 ed il 2010, mentre la fine dell’operatività è prevista per il 2030-35. Si spera inoltre che il retatore PIK di Gatchina inizierà l’operatività nel 2005. Il commissioning della sorgente “Moscow Meson Factory Neutron Source” è in stato avanzato ed i primi neutroni sono stati rivelati già nel 1998. La Russia è attualmente associata all’ILL e si prevede possa presto partecipare al progetto ESS. Spagna La Spagna non ha sorgenti nazionali, attualmente è associata all’ILL e tale associazione verrà mantenuta per il futuro.

La comunità neutronica è interessata a partecipare a progetti di stru-

VARIE

mentazione. Inoltre partecipa al progetto ESS. Svezia Il reattore di Stusvik avrà supporto finanziario certo per i prossimi 3 anni. Esso ha la licenza di funzionamento per i prossimi 14 anni, mentre potrà operare senza maggiori ristrutturazioni per i prossimi 20 anni. La Svezia ha una associazione ad ISIS e la comunità spera di poter partecipare allo sviluppo di ESS. Svizzera La sorgente neutronica SINQ, di recente costruzione, potrà funzionare per almeno 30 anni. L’Associazione all’ILL è stata rinnovata fino al 2003.

La Svizzera è stata interpellata per una partecipazione ad AUSTRON, l’eventuale partecipazione a questo progetto prevede però una uscita da ILL. La Svizzera è associata al progetto ESS al quale partecipa attivamente per lo sviluppo del target.

Ungheria Il reattore di Budapest ha licenza di funzionamento e carburante fino al 2014. L’Ungheria sta attualmente considerando la possibilità di associarsi ad una grande sorgente europea, potrà essere ILL o AUSTRON in dipendenza dalle scelte politiche e dell’evoluzione dei progetti europei.

Sulla base delle notizie avute dalle comunità neutroniche, dai laboratori internazionali, dall’evoluzioni dei progetti europei, l’ENSA ha osservato che per quanto riguarda l’impostazione di una politica europea per la neutronica si puossono trarre alcune conclusioni.

Attualmente ci sono circa 4700 ricercatori europei interessati alla neutronica e questo numero cresce tipicamente di 2-4-% all’anno, questo incremento continuerà così se non interverra un decremento effettivo della disponibilità di neutroni in Europa. Si prefigura inoltre una disponibilità di sorgenti del seguenti tipo:

Sorgenti Nazionali: 13 delle 17 comunità nazionali interpellate possiedono sorgenti nazionali( in alcuni paesi ce ne sono più di una, ad esempio Germania, e Russia), cioè attualmente circa il 90% degli utilizzatori di neutroni è servito da sorgenti nazionali. Questo numero calerà al 70% nel 2005 ed al 30% nel 2015 a causa dell’esaurimento di tali sorgenti. Questo è un taglio drastico della fornitura di neutroni da parte di sorgenti nazionali, specialmente se si pensa che circa i due terzi degli esperimenti sono fatti alle sorgenti nazionali.

ILL Grenoble: Attualmente oltre il 90% della comunità di utilizzatori europei ha accesso all’ILL su base contrattuale, mentre circa un quarto di tutti gli esperiemnti sono fatti all’ILL. Quindi una chiusura dell’ILL, che può avvenire nel 2013, avrà un effetto gravemente negativo sulla fornitura di neutroni in Europa se provvedimenti compensativi non verranno presi.

ISIS Didcot Attualmente circa un terzo della comunità europea ha accesso ad ISIS su base contrattuale, mentre circa il 16% degli esperimenti europei e fatto ad ISIS. Poiché ISIS è complementare ad ILL, essa giuoca un ruolo fondamentale non solo per la ricerca in neutronica ma anche per mantenere viva la metodologia di costruzioni di strumenti per sorgenti pulsate in vista dello sviluppo delle sorgenti di terza generazione come ESS. In conclusione, in vista del taglio drastico di disponibilità di neutroni che ci sarà dopo il 2005, per il decremento delle sorgenti nazionali in funzione, e nel 2013, per la possibile chiusura di ILL, mentre la comunità neutronica potrebbe sperimentare nello stesso periodo un incremento fino a circa 6500 addetti, non c’è dubbio che ci sia un estremo bisogno di realizzare nuove sorgenti neutro-

Vol. 4 n. 2 Dicembre 1999

•

niche in Europa e sia quindi giustificata la richiesta di inpegno costruttivo a livello europeo. Il progetto AUSTRON riempirebbe il “gap” che si verificherà intorno al 2005, mentre la sorgente ESS sarà indispensabile se si vuole almeno mantenere la supremazia europea in questo campo.

Senza la realizzazione del progetto ESS l’Europa si troverebbe intorno agli anni 2015 in una grave crisi per quanto riguarda la neutronica con grave danno per tutta la comunità scientifica europea. Questa strategia deve anche essere vista rispetto alle recenti decisioni di USA e Giappone riguardo alla realizzazione delle loro sorgenti pulsate di neutroni.

6. Rinnovo delle cariche di Presidente, Vice-Presidente e Segretario dell’ENSA A norma di statuto dell’ENSA le cariche direttive di Presidente , VicePresidente e Segretario hanno durata di due anni. Inoltre è prassi consolidata che il Vice-Presidente venga eletto Presidente per i due anni successivi. Le cariche nel biennio passato 98-99 sono state ricoperte da : Presidente: Prof. A. Furrer (Svizzera) Vice-Presidente: Prof. R. Cywinski (U.K.) Segretario: Prof. B. Lebech (Danimarca) In Budapest è stata eletta per gli anni 2000-01 la nuova composizione degli organi direttivi come segue: Presidente: Prof. R. Cywinski (U.K.) Vice-Presidente: Prof. F. Barocchi (Italia) Segretario: Prof. L. Borjesson (Svezia)

Prof. Fabrizio Barocchi Presidente della SISN Delegato italiano nell’ENSA

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

33

VARIE

Commissione CNR di Spettroscopia Neutronica Fin dal 1975 il CNR è fortemente impegnato a sostegno di attività di ricerca e di sviluppo di strumentazione nel campo della spettroscopia di neutroni. Queste ricerche, a carattere fortemente interdisciplinare, coinvolgono la chimica, la fisica, le scienze della terra, le scienze della vita e alcuni tipi di applicazioni industriali.

Nella metà degli anni settanta il CNR sottoscrisse una Convenzione con il CNEN (oggi ENEA) per l'utilizzo del Reattore TRIGA presso il centro della Casaccia per attività di ricerca in questo campo, convenzione che si concluse nell'anno 1991. In seguito, nella metà degli anni ottanta, l'Ente stipulò un accordo con ESRC (Engeneering and Science Research Council) inglese per l'utilizzo della sorgente di neutroni a spallazione ISIS che proprio in quegli anni entrava in funzione presso il Rutherford Appleton Laboratory (UK). Tale accordo, ancora in vigore fino all'anno 2002, permette ai ricercatori italiani l'utilizzo di una quota pari al 5% annuo su tutto il parco

Comitato di Coordinamento Luce di Sincrotrone Il CNR, a seguito del decreto di riordino e nelle more di una revisione dei propri programmi e di una predisposdizione di nuovi modelli organizzativi, ha disposto lo scioglimento della Commissione Radiazione di Sincrotrone, costituita nel marzo 1995 e presieduta dal Dr. Natoli.

In attesa delle decisioni del nuovo organo di governo dell'Ente, il Presidente ha costituito (luglio 1999) un Comitato di Coordinamento compo-

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

34

strumentale operante ad ISIS ed ha permesso anche ai ricercatori dell'ente la costruzione di due apparecchiature, per diffusione anelastica di neutroni, attualmente in funzione ad ISIS: PRISMA e TOSCA. Questo accordo, fin dalla sua stipula, rappresenta un impegno finanziario annuo di circa 2000 ML implica a cui deve aggiungere il finanziamento per la realizzazione dei due spettrometri, PRISMA e TOSCA, pari complessivamente a circa 7000 ML. Al fine di promuovere e coordinare l'utilizzo della spettroscopia neutronica anche in campi diversi da quelli più tradizionali della Fisica e della Chimica, nel 1985 è stata costituita una Commissione interdisciplinare di Spettroscopia Neutronica, che opera anche nel nuovo quadro organizzativo del CNR, come Commissione del Presidente.

Tra le attività di questa Commissione, costituita da esperti di varie discipline dell'ente e del mondo accademico, vale la pena ricordare che nel suo ambito sono stati individuati ed istruiti i progetti di fattibilità delle apparecchiature costruite dal CNR ad ISIS, dall'Istituto di Struttura della Materia di Frascati e dall'Istituto di Elettronica Quantistica di Firenze, rispettivamente lo spettrometro PRISMA e TO-

SCA, che essa ha coordinato le richieste di tempo macchina avanzate ad ISIS dalla comunità italian, ha svolto opera di informazione e sensibilizzazione presso la comunità scientifica italiana di discipline non fisiche e per ultimo si è occupata della definizione degli argomenti e dei contenuti scientifici della Scuola di Spettroscopia di Neutroni che si svolge con cadenza biennale a Palau (SS).

Più recentemente, nel corso dell'ultimo anno la Commissione, ha preso in esame la proposta avanzata in ambito europeo dal ESS R&D Council, organismo di cui fa parte anche il CNR, e che propone ai governi europei la costruzione in europa di una nuova sorgente a spallazione di neutroni, l'ESS, entro il 2010. Per tale iniziativa la Commissionee si è espressa molto favorevolmente auspicando una partecipazione dell'Ente anche al nuovo Council ESS, che verrà costituito il prossimo anno tra istitutizioni, laboratori ed enti europei, con l'obiettivo di predisporre il progetto realizzativo della sorgente entro il 2004.

sto dai Direttori degli Organi di Ricerca interessati al settore Luce di Sincrotrone. Ne fanno parte: Dr. Paolo PERFETTI Direttore Istituto di Struttura della Materia (ISM/RM) (Coordinatore) Dr. Silvio CERRINI Direttore Istituto di Strutturistica Chimica (ISC-Roma Montelibretti) Prof. Florestano EVANGELISTI Direttore Istituto di Elettronica Stato Solido (IESS/RM) Dr. Mario PAGANNONE Direttore Istituto di Metodologie Avanzate Inorganiche (IMAI-Roma) Dr. Tommaso PROSPERI Direttore Istituto di Chimica dei Materiali (ICMAT-Roma)

Dr. Carlo TALIANI Direttore Ist. Spettroscopia Molecolare ( ISM/BO ) con estensione a: Prof Silvano RIVA Direttore Istituto Genetica Biochimica ed Evoluzionistica (IGBE/PV) Prof. Settimio MOBILIO Responsabile Linea GILDA presso ESRF-Grenoble La Segreteria del Comitato è curata dalla Dr.ssa Paola Bosi.

•

Vol. 4 n. 2 Dicembre 1999

Dr.ssa Paola Bosi Segretaria Scientifica Commissione CNR di Spettroscopia Neutronica

Dr.ssa Paola Bosi Segreteria Scientifica Comitato di Coordinamento Luce di Sincrotrone

VARIE

TMR TMR

Training and Mobility of Researchers

Support for activities in the field of neutron scattering is available from the neutron round-table.

The neutron round-table is funded by the EC (DGXII) with approximately 100.000 Euro per year. The mission of the round-table is:

1.

To actively encourage co-

3.

5.

To support training of young scientists

The round-table consist of

ordination and collaboration

and other scientists, new to the

representatives from all major

between user facilities - such

field of neutron scattering about

European neutron user facilities,

that the European users will

the potential of the method.

from EC supported networks

benefit through a better quality

developing novel

access to the European neutron

4.

scattering facilities.

national access to summer

user representatives appointed

schools, workshops, training

by ENSA (European Neutron

To spread the

courses, co-ordination activities

Scattering Association). The

knowledge about the

etc. Detailed information on

name of all contact persons can

potential of neutron scattering,

how and when to apply for

be found on the web page

and support studies on future

support can be found on the

mentioned above. The present

prospects with neutron

round-table web page:

chairman/co-ordinator of the

scattering.

http://www.risoe.dk/fys/TMR.

round-table is Kurt Nørgaard

htm

Clausen, and can be contacted

and an increased quantity of

2.

The round-table

instrumentation and techniques

supports non-

for neutron scattering plus 5

as kurt.clausen@risoe.dk

Vol. 4 n. 2 Dicembre 1999

â&#x20AC;˘

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

35

SCUOLE E CONVEGNI

Italian participation at ECNS 99 This year the 2° European Conference of Neutron Scattering (ECNS 99) took place in Budapest, from the 1st to the 4th of September, and it was organized by the European Neutron Scattering Association in co-operation with the Budapest Neutron Centre. More than 500 participants were present, mostly from Eurasian countries; many were German researchers, but some also came from Japan, USA and few other countries in the world.

The conference organization ran smooth, with the usual parallel sessions and crowded poster sections, and all the topics regarding the use of neutrons were widely covered.

Also the extra-scientific activities (welcome party, social dinner, trip to

Balaton Lake) revealed to be quite interesting and were appreciated by the participants.

If one of the goals of the conference was to involve young participants, this was thoroughly reached. There was a massive participation of young researchers and PhD students, which surely contributed to the success of the meeting, and at the end of the conference a special session: "Young Scientists Panel", was entirely devoted to them. As it is usual at ECNS conferences, the ten best young scientists’ presentations were honoured by the "Young Scientists Award". Among the winners, we ought to mention the Italian Debora Berti (Università di Firenze) for her talk on: Micellar aggregates

from novel short-chain phospho-liponuclease, a SANS study.

At the conference it was also instituted the Walter Halg Prize, that should be renewed every two years.

The 1999 prize was given to F.Mezei, who on that occasion presented a personal overview on neutron scattering history: A Pilgrim’s progress (incidentally this is the title of John Bunyan’s work on the human quest for personal salvation written in 1678).

This conference gives us the chance to monitor the activity of the Italian neutrons users. To this end we can use, as an indicator, the number of contribution with a majority of Italian authors, that were presented in the various sessions of the conferen-

60.0

Figure 1

Italians abroad Italians in Italy

50.0 40.0 30.0 20.0 10.0

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

36

•

M ag ne St r. tis C m or r.

El ec t.

Q sy ua s. nt um sy M st at em .S s ci .In d.

Fr A om pp l.

Fu nd .t o Fu A pp nd l. .P ro p. of N eu .

B io lo gy

M et ho ds C he an m d .S In st tr . uc .a nd Ex So ci ft t.

C on d.

G M la at ss te es r an d Li qu id s

.0

Vol. 4 n. 2 Dicembre 1999

SCUOLE E CONVEGNI

ce, including invited, oral and posters. We recall that those sessions were: Neutron Methods and Instrumentation, Chemical structure and Excitations, Soft Condensed Matter, Glasses and Liquids, Biology, Magnetism, Strongly correlated electron system, Quantum systems, Material Science and Industrial applications, From Fundamentals to Applications, Fundamental properties of Neutrons.

Italian scientists participated with 65 contributions out of 642 (about 10%), that is not bad considering the lack of national neutrons sources. In figure 1 the percentage of Italian contributions in each session is presented, along with the contributions of young Italian researchers working

Ancona

tions by authors belonging to different centres, the score has been splitted accordingly. This graph clearly illustrates the type of research carried out in the various towns, by Universities and C.N.R. Most of the activities are in Glasses and Liquids, leaded by Firenze and in, Material Science and Industrial applications leaded by Ancona and ENEA. But a lot of work is also done in the field of Soft Condensed Matter, Biology and Neutron Methods and Instrumentation.

Ulderico Wanderlingh and Rita Giordano Dip. di Fisica and INFM UniversitĂ di Messina

12

Figure 2

Bari Bologna

at neutrons facilities as local contact or instrument staff/responsible and the like. We see that those young researchers working abroad are working hard and fruitfully exploiting the chances they have to increase their collaborations and publication list. It is also worth noticing that in some areas the Italian activity represent a quite substantial slice of the European research, i.e. in Biology (with a 36.7%) and in Glasses and Liquids (with a 25%); while in other areas Italian contributions are very modest or null.

In the second graph (fig.2) the same data are presented on an absolute scale and they are classified according to the centre in which the authors work. For those contribu-

10

ENEA Firenze Genova Messina Milano

8 6 4

Parma Palermo

2

Perugia B io lo gy M ag St ne r.

C tis or m r.

El ec Q t. ua sy nt s. um M s ys at .S te m ci s .In Fr d om .A pp Fu l. nd .t Fu o nd A pp .P l. ro p. of N eu .

0 M et ho C ds he an m .S d In tr uc st . .a nd So Ex ft ci C t. on G d. la M ss at es te r an d Li qu id s

Roma

Vol. 4 n. 2 Dicembre 1999

â&#x20AC;˘

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

37

SCUOLE E CONVEGNI

6th General Meeting of the European Spallation Source (ESS) The 6th General Meeting of the European Spallation Source (ESS) was held in Portonovo, near Ancona, Italy, from 20 to 23 September 1999.

ESS is the planned high power neutron source of the next generation, which will allow european scattering experiments with pulsed neutrons, being their peak-flux up to two orders of magnitudes higher than the values nowadays reachable.

The project is in a Research and Development (R&D) phase (1997-2001), and the outcoming results will give the possibility to complete the design phase within the end of 2003.

The works are performed in 14 european Laboratories and Universities and are coordinated by the ESS R&D Council. Most of the work will be performed by 3 german institutions, namely the Research Center Jülich (FZJ), the Institut of Applied Physics of the University of Frankfurt and the Hahn-Meitner Institut (HMI) in Berlin.

About 130 scientists did register for the Meeting, among which 14 and 10 from USA and Japan, respectively, where similar neutron sources are planned.

In the first plenary session, J.Kjems (RisØ, Denmark) reported about the status of the ESS project; his talk was followed by reports on the Scientific Case (A.Taylor, ISIS, UK), on the Instrumentations (M.Steiner, HMI, Berlin, Germany) and on the status of R&D of the Accelerator (G.H.Rees, ISIS, UK), Target (H.Ullmaier, FZJ, Germany) and Moderator (G.Bauer, PSI, Switzerland) working groups. In the second plenary session, G.Bauer (PSI, Switzerland) and U.Steigenberger (ISIS, UK) described the experiences at middle power spallation sources, while

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

38

T.Mason (SNS-ORNL, USA) and M.Furusaka (KEK, Japan) explained the status of their national projects, SNS and JSNS, respectively. Moreover, J.M.Lagniel (CEA, France) talked about the similarity between a plant for transmutating the radioactive waste and a spallation source; he drew the attention on the common problems, the solution of which should be helpful both for the spallation and the transmutation technique; the discussion was enriched by the contribution of G.Bauer.

In the middle part of the Meeting, three parallel sessions took place: Accelerator, Target and Instrumentation. In the 18 talks of the Target session, the actual status of the research in the frame of short pulsed high power systems was showed and discussed, with an emphasis to the ESS mercury target. The R&D plans of the european ESS, of the american SNS and of the japanese JSNS-JAERI targets were presented. SNS developed a mercury target system with 2 MW proton power, similar to the ESS one. JAERI is developing a mercury target as well. An alternative Pb-Bi target and an option for long pulses were also considered for SNS. Most of the talks dealt with theoretical and experimental studies of the pressure waves and fluid dynamics in high-current mercury targets. A 1:1 prototype of a mercury target loop is foreseen by SNS. A joint effort of FZJ and University of Latvia produced the first results on the measurements of the heat transfer in a mercury-steel system. The presentation of the irradiation program was impressive, and this has to be considered as very important for ESS; the lifetime and the availability of existing high power source are li-

•

Vol. 4 n. 2 Dicembre 1999

miting the investigation of the irradiation effects on structural materials. Results on the neutron multiplicity in target materials were also presented. New results of measurements of production cross sections in target and structural materials by charged particles showed the need of a theoretical interpretation. In the instrumentation sessions, the expectations from ESS to provide the users with a neutron flux of two orders of magnitude higher than the present sources, open new aspects and strategies for the instrumentation; this means that new moderators and neutron optic must be developed in order to optimize the new instruments. Within five sessions, problems on moderators, detectors, simulation software for instruments, neutron spectroscopy and instrument components were discussed.

Y.Kiyanagi (Hokkaido University, Japan) and N.Watanabe (JAERI, Japan) described the quality and the use of many moderator types together with their concepts: L.Charlton (ORNL, USA) presented those planned for SNS. Apart from the results shown during the sessions, the position of the moderators for the optimal use in a pulsed spallation source seemed to be one of the most important point. Many new moderators were discussed together with their use in ESS, such as a methane one with the multilayers option and a poisoned one. The need of more efficient detectors able to stand the higher neutron flux was pointed out by M.Johnson (ISIS, UK), who presented the work performed in the frame of the EU project TECHNI. The status of the development of large microstrip gas chamber (MSGC) and gas electron multiplier (GEM) detectors was described by B.Gebauer (HMI, Berlin) and R.Kreuger (TUDelft, Holland). The efficiency and the status of the simulation technique for neutron instruments was discussed: the simulation programs VITESS and McStas, developed in

SCUOLE E CONVEGNI

Participants to the 6° ESS General Meeting held in Portonovo, near Ancona, Italy, September 20-23, 1999

their own laboratories, were presented by D.Wechsler (HMI, Germany) and K.Clausen (RisØ, Denmark), respectively. R.McGreevy (Studsvik Neutron Research Laboratory, Sweden) underlined the need of a closer collaboration among theory and experiment in the development of new instruments and reported the work done within the EU project SCANS (Software for Computer Aided Neutron Scattering).

Simulation results for a specific problem of small angle neutron scattering at pulsed sources and of spectrometers with analyzator were described by F.Streffer (HMI, Germany) and G.Zsigmond (HMI, Germany), respectively. For what concerns the small angle scattering, it was shown a comparison between short and long pulses and it came out that there is a gain of intensity in the long pulsed option. The problem of the time-of-flight spin echo spectroscopy was discussed by B.Farago (ILL, France) by showing tests performed at IN15 at ILL. Of great importance was the description of the feasibility of the eV-neutron spectroscopy by C.Andreani (University of Rome2, Italy), as it is inve-

stigated at ISIS within the VESUVIO project and the use of a para hydrogen filter in inelastic neutron scattering by M.Zoppi (CNR, Italy). The study of a method for taking advantage of the Larmor-precession in time-of-flight powder diffraction, small angle scattering and inelastic scattering was explained by F.Mulder (TU-Delft, Holland). All the contributions aroused the interest of the participants. In conclusion, it was stated that the development of innovative and more powerful instruments must be concerted with new concepts in the target and accelerator and that the optimization of a spallation source requires that its design must consider the input from the new instruments as well. Workshop and meetings on these aspects of the instrumentation in a spallation environment are planned. In the third plenary session, the overview of the neutron scatterers in Europe was given by D.Richter (FZJ, Germany), as already discussed at the European Conference on Neutron Scattering (31.08-03.09.1999 Budapest, Hungary). F.Mezei compared the experimental conditions of the pulsed high power spallation source

Vol. 4 n. 2 Dicembre 1999

•

ESS with those at the ILL reactor in Grenoble, France. In the final talk, H.Rauch (Atominstitut Wien, Austria) pointed out the need, beyond a european high power spallation source, to build low-middle power spallation sources, as regional and national facilities. This event, where all the spallation people met together, was also used for discussing the results obtained in the frame of international collaborations, such as AGS Spallation Target Experiment (ASTE), Advanced COld Moderators (ACOM) and Jülich Experimental Spallation Target Set-up in Cosy Area (JESSICA). After the ESS General Meeting, each of the three international projects held its own meeting. Since the EU financed a TMR project for R&D of the ESS target system, the annual meeting took place meanwhile.

Updated information can be found in the ESS web site: (http://www.kfa-juelich.de/ess)

Flavio Carsughi Facoltà di Agraria Università degli Studi di Ancona

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

39

SCUOLE E CONVEGNI

Scuola di Luce di Sincrotrone di Pula La Società Italiana di Luce di Sincrotrone (SILS) ha recentemente organizzato la quinta edizione della Scuola Nazionale di Luce di Sincrotrone; la scuola si è svolta nell’arco di due settimane, dal 27 settembre all’8 ottobre nella sua ormai consolidata sede, presso l’Hotel Flamingo di Santa Margherita di Pula.

La Scuola ha ricevuto finanziamenti dall’Associazione Italiana di Cristallografia, dal CNR, dalla European Commission Round

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

40

Table for Synchrotron Radiation and FEL, dal Gruppo Nazionale di Struttura della Materia del CNR, dall’Istituto Nazionale di Fisica della Materia, dal Magnifico Rettore dell’Università di Cagliari e dalla Sincrotrone Trieste SCpA, che hanno permesso tra l’altro di sostenere parzialmente le spese di soggiorno per 26 studenti presenti. Va ricordato inoltre che tutti i docenti hanno sostenuto in proprio le spese di viaggio.

•

Vol. 4 n. 2 Dicembre 1999

Come per le quattro edizioni precedenti (1990, 1992, 1995 e 1997) la Scuola intendeva offrire a persone già operanti nel campo della Luce di Sincrotrone o interessate ad entrarvi una panoramica attuale delle caratteristiche e potenzialità dell'uso della stessa. Le possibilità di ricerca con L. S. sono state affrontate sia da un punto di vista teorico che sperimentale e viste nella loro connessione a varie discipline (chimica, fisica, biologia,

SCUOLE E CONVEGNI

scienze della terra) e a diversi tipi di materiali.

Hanno partecipato 36 studenti (21 di area fisica, 11 di provenienza chimica, 3 di area geomineralogica e un biologo), per la maggior parte iscritti a cicli di dottorato.

Il prolungamento di due giorni rispetto alle edizioni precedenti ha permesso di svolgere comples-sivamente circa 70 ore di lezione e di ampliare il programma; si sono inoltre introdotte alcune lezioni preliminari con lo scopo di fornire agli studenti gli elementi di conoscenza per poter seguire al meglio le lezioni più specialistiche successive.

Per entrare nello specifico, la scuola è stata così articolata: - introduzione alla Luce di Sincro-trone: sua generazione e proprietà; ottiche per raggi X da Luce di Sincrotrone: (4 ore) - interazione radiazione-materia: 2 ore di introduzione seguite da 2 ore di approfondimento; - diffrazione di raggi X: aspetti generali; diffrazione da polveri; diffrazione a basso angolo; diffrazione da superfici; tecniche DAFS e MAD; onde stazionarie; biocristallografia con luce di sincrotrone (14 ore); - assorbimento di raggi X: intro-

-

-

-

-

-

-

-

duzione generale alle spettroscopie EXAFS e XANES (XAS); lo scattering multiplo; applicazioni XAS alla scienza dei materiali e alla catalisi eterogenea (12 ore); spettroscopie di fotoemissione: introduzione generale; proprietà elettroniche e strutturali delle superfici; fotoemissione da livelli di core, da fase gassosa e in presenza di reazioni chimiche (8 ore); introduzione ai materiali magnetici; dicroismo magnetico e naturale; magnetismo e Luce di Sincrotrone (5 ore); tecniche di microscopia e di imaging (4 ore); proprietà dei liquidi; scattering inelastico ad altissima risoluzione (4 ore); spettroscopia IR con Luce di Sincrotrone; spettroscopia SNOM (3 ore); Luce di Sincrotrone e scienze della terra (2 ore) spettroscopie di emissione nel campo dei raggi X molli (2 ore) complementarietà e differenze tra neutroni e Luce di Sincrotrone (2 ore) presentazione delle facilities ELETTRA e ESRF ed attività degli enti di ricerca italiani (4 ore).

Vol. 4 n. 2 Dicembre 1999

•

Tra docenti e studenti si è creato ben presto un clima di cordialità e affiatamento, favorito dalle splendide spiagge e dall’ottimo clima; la cena sociale ha permesso a molti di scoprire la cucina ed il folklore sardi; è stata molto apprezzata una gita non program-mata al nuraghe di Barumini e alla Giara di Gesturi.

Visto l’alto livello delle dispense preparate dai docenti è attualmente allo studio la possibilità di stampare (in inglese) gli atti della Scuola, che possono costituire un valido supporto per vari corsi universitari e il materiale didattico per la prossima edizione della Scuola.

Arrivederci alla sesta edizione nel settembre del 2001!

Gilberto Vlaic Dip. di Chimica Università di Trieste e Sincrotrone di Trieste Settimio Mobilio Dip. di Fisica Università di Roma Tre e I.N.F.N. Lab. Naz. Frascati

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

41

CALENDARIO DOVE LUCE DI SINCROTRONE

23-25 gennaio 2000

LOS ALAMOS, NEW MEXICO

Fourth LANSCE User Group Meeting A.L. Archuleta, User Office, Los Alamos Neutron Science Center, MS H831, Los Alamos, NM 87545, USA. Tel: +1 505 665 1010; Fax: +1 505 667 8830. e-mail: lansce_users@lanl.gov http://lansce.lanl.gov/conferences/LUG4/index.htm.

13-17 marzo 2000

18th General Conference of Condensed Matter Division of the European Physical Society e-mail: eps-cmd18@psi.ch http://www.eps-cmd18.ch

20-24 marzo 2000 26-29 gennaio 2000

GRENOBLE, FRANCE

International Workshop on Dynamics in Confinement I. Volino, Institut Laue-Langevin, B.P. 156, F-38042, Grenoble Cedex 9, France Tel: +33 4 76207060; Fax: +33 4 76483906 e-mail: confit@ill.fr http://www.ill.fr/Events/confit.htlm

30 gennaio-5 febbraio 2000

FOLGARIA (TN), ITALY

SASP 2000: Symposium on Atomic and Surface Physics and Related Topics http://www.science.unitn.it/sasp/index.html

TRIESTE, ITALY

Joint INFM-the ABDUS SALAM ICTP School on Magnetic Properties of Condensed Matter Investigated

by Neutron Scattering and Synchrotron radiation techniques e-mail: smr1216@ictp.trieste.it http://www.ictp.trieste.it/

10-12 febbraio 2000

6-9 aprile 2000

Ten Years of HERCULES Workshop Secrétariat HERCULES X EuroConference, CNRS Laboratoire Louis Néel, BP 166, 38042 Grenoble Cedex 9, France.

Tel: +33 4 76889097; Fax: +33 4 76881191 e-mail: guerard@polycnrs-gre.fr http://www.polycnrs-gre.fr.hercules.html

GLOUCESTERSHIRE, U.K.

Materials Congress 2000 Mrs. M. Boyce, IOM e-mail: melanie boyce@materials.org.uk http://www.instmat.co.uk

24-28 aprile 2000

SAN FRANCISCO, CA, USA

Materials Research Society Spring Meeting http://dns.mrs.org

20-23 maggio 2000

GRENOBLE, FRANCE

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

•

BARCELLONA, SPAIN

7th European Powder Diffraction Conference (EPDIC7) e-mail: gcasanova@uex.es

Higher European Research Course for Users of Large Experimental Systems (HERCULES 2000) Secretariat HERCULES, CNRS - maison des Magis-teres, BP 166, 38042 Grenoble Cedex 9, France.

Tel: +33 4 76887986; Fax: +33 4 76887981 E-mail: simpson@polycnrs-gre.fr http://ww.polycnrs-gre.fr/hercules.html

42

GRENOBLE, FRANCE

GRENOBLE, FRANCE

ESRF Users’ Meeting http://www.esrf.fr

27 febbraio- 5 aprile 2000

MINNEAPOLIS, MN, USA

APS March Meeting http://www.aps.org

12-14 aprile 2000 1-11 febbraio 2000

MONTREUX, SWITZERLAND

Vol. 4 n. 2 Dicembre 1999

DOVE LUCE DI SINCROTRONE CALENDARIO

12-17 giugno 2000

JASZOWIEC, POLAND

5th International School and Symposium Synchrotron Radiation in Natural Sciences e-mail: synchron@castor.if.uj.edu.pl

18-23 giugno 2000

KRAKOW. POLAND

EDRXS-2000: European Conference on Dispersive X-ray Spectrometry 2000 http://www.ftj.agh.edu.pl/wfitj/conf/edxrs

20-25 giugno 2000

on

Energy

OXFORD, U.K.

The Sixth International Conference on Residual Stresses.

P. Farrelly, IoM Conferences & Events.

Tel: 44 171 4517391; Fax: 44 171 8392289 E-mail: Pauline_Farrelly@materials.org.uk

26-29 luglio 2000

HALLE/SAALE, GERMANY

Many Particle Spectroscopy of Atoms, Molecules and Surfaces e-mail: jber@mpi-halle.de

6-11 agosto 2000

MURCIA, SPAIN

European Conference on Iteration Theory Faculdad de Matematica, Campus de Espinardo Tel: 34 968 364176; Fax: 34 968 364182

2-6 ottobre 2000

CRIMEA, UKRAINE

NOLPC 2000 - 8th International Conference on Nonlinear Optics of Liquid and Photo Refractive Crystals http://www.isp.kiev.ua

ST. PETERSBURG, RUSSIA

3rd International Workshop on Polarized Neutrons for Condensed Matter Investigations (PNCMI 2000) Prof. A.I. Okorokov, Petersburg Nuclear Physics Inst., Russian Acad. of Sciences, 188350 Gatchina, St. Petersburg, Russia Tel: +7 81271 46023; Fax: +7 81271 39023 E-mail: okorokov@hep486.pnpi.spb.ru

10-12 luglio 2000

4-9 settembre 2000

1-4 novembre 2000

DENTON, USA

CAARI 2000: XVIth International Conference on the Application of Acceleratoes in Research and Industry http://www.phys.unt.edu/accelcon/

27 novembre-1dicembre 2000

BOSTON, MA, USA

MRS Fall Meeting http://dns.mrs.org

9-13 settembre 2001

MUNCHEN, GERMANY

International Conference on Neutron Scattering 2001 (ICNS 2001) Physik Dept. E13, Technische Univ. München , D-85747 Garching, Germany Tel: +49 89 28912452; Fax: +49 89 289 12473 e-mail: info@icns2001.de http://www.icns2001.de

ABERYSTWYTH, WALES, U.K.

NCM8, 8th International Conference on the Structure of Non-Crystalline Materials e-mail: ncm8@glass.demon.co.uk http://www.sgt.org

21-25 agosto 2000

BERLIN, GERMANY

7th International Conference on Synchrotron Radiation Instrumentation http://sri2000.tu-berlin.de

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

43

SCADENZE VARIE

Scadenze per richieste di tempo macchina presso alcuni laboratori di Neutroni

Scadenze per richieste di tempo macchina presso alcuni laboratori di Luce di Sincrotrone

ISIS

ALS

La scadenza per il prossimo call for proposals è il 16 aprile 2000 e il 16 ottobre 2000

Le prossime scadenze sono il 15 marzo 2000 (cristallografia macromolecolare) e il 1 giugno 2000 (fisica)

ILL BESSY

La scadenza per il prossimo call for proposals è il 15 febbraio 2000 e il 15 agosto 2000

Le prossime scadenze sono il 15 febbraio 2000 e il 4 agosto 2000

LLB-ORPHEE-SACLAY La scadenza per il prossimo call for proposals è il 1 ottobre 2000 per informazioni: Secrétariat Scientifique du Laboratoire Léon Brillouin, TMR programme, Attn. Mme C. Abraham, Laboratoire Léon Brillouin, CEA/SACLAY, F-91191 Gif-sur-Yvette, France.

Tel: 33(0)169086038; Fax: 33(0)169088261 e-mail: abraham@bali.saclay.cea.fr http://www-llb.cea.fr

DARESBURY La prossima scadenza è il 30 aprile 2000 e il 31 ottobre 2000

ELETTRA Le prossime scadenze sono il 28 febbraio 2000 e il 31 agosto 2000

ESRF BENSC La scadenza è il 15 marzo 2000 e il 15 settembre 2000

Le prossime scadenze sono il 1 marzo 2000 e il 1 settembre 2000

GILDA

RISØ E NFL La scadenza per il prossimo call for proposals è il 1 aprile 2000

(quota italiana) Le prossime scadenze sono il 1 maggio 2000 e il 1 novembre 2000

HASYLAB (nuovi progetti) Le prossime scadenze sono il 1 marzo 2000, il 1 settembre 2000 e il 1 dicembre 2000

LURE La prossima scadenza è il 30 ottobre 2000

MAX-LAB La scadenza è approssimativamente febbraio 2000

NSLS Le prossime scadenze sono il 31 gennaio 2000, il 31 maggio 2000 e il 30 settembre 2000

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

44

•

Vol. 4 n. 2 Dicembre 1999

FACILITIES

LUCE DI SINCROTRONE SYNCHROTRON SOURCES WWW SERVERS IN THE WORLD (http://www.esrf.fr/navigate/synchrotrons.html)

DAFNE INFN Laboratori Nazionali di Frascati, P.O. Box 13, I-00044 Frascati (Rome), Italy tel: +39 6 9403 1 fax: +39 6 9403304 http://www.lnf.infn.it/ Tipo:P Status: C

ALS Advanced Light Source MS46-161, 1 Cyclotron Rd Berkeley, CA 94720, USA tel:+1 510 486 4257 fax:+1 510 486 4873 http://www-als.lbl.gov/ Tipo: D Status: O AmPS Amsterdam Pulse Stretcher NIKEF-K, P.O. Box 41882, 1009 DB Amsterdam, NL tel: +31 20 5925000 fax: +31 20 5922165 Tipo: P Status: C

DELTA Universität Dortmund,Emil Figge Str 74b, 44221 Dortmund, Germany tel: +49 231 7555383 fax: +49 231 7555398 http://prian.physik.uni-dortmund.de/ Tipo: P Status: C

APS Advanced Photon Source Bldg 360, Argonne Nat. Lab. 9700 S. Cass Avenue, Argonne, Il 60439, USA tel:+1 708 252 5089 fax: +1 708 252 3222 http://epics.aps.anl.gov/welcome.html Tipo: D Status: C

ELETTRA Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy tel: +39 40 37581 fax: +39 40 226338 http://www.elettra.trieste.it Tipo: D Status: O

ASTRID ISA, Univ. of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark tel: +45 61 28899 fax: +45 61 20740 Tipo: PD Status: O

ELSA Electron Stretcher and Accelerator Nußalle 12, D-5300 Bonn-1, Germany tel:+49 288 732796 fax: +49 288 737869 http://elsar1.physik.uni-bonn.de/elsahome.html Tipo: PD Status: O

BESSY Berliner Elektronen-speicherring Gessell.für Synchrotron-strahlung mbH Lentzealle 100, D-1000 Berlin 33, Germany tel: +49 30 820040 fax: +49 30 82004103 http://www.bessy.de Tipo: D Status: O

ESRF European Synchrotron Radiation Lab. BP 220, F-38043 Grenoble, France tel: +33 476 882000 fax: +33 476 882020 http://www.esrf.fr/ Tipo: D Status: O

BSRL Beijing Synchrotron Radiation Lab. Inst. of High Energy Physics, 19 Yucuan Rd.PO Box 918, Beijing 100039, PR China tel: +86 1 8213344 fax: +86 1 8213374 http://solar.rtd.utk.edu/~china/ins/IHEP/bsrf/bsrf.html Tipo: PD Status: O CAMD Center Advanced Microstructures & Devices Lousiana State Univ., 3990 W Lakeshore, Baton Rouge, LA 70803, USA tel:+1 504 3888887 fax: +1 504 3888887 http://www.camd/lsu.edu/ Tipo: D Status: O CHESS Cornell High Energy Synchr. Radiation Source Wilson Lab., Cornell University Ithaca, NY 14853, USA tel: +1 607 255 7163 fax: +1 607 255 9001 http://www.tn.cornell.edu/ Tipo: PD Status: O

EUTERPE Cyclotron Lab.,Eindhoven Univ. of Technol, P.O.Box 513, 5600 MB Eindhoven, The Netherlands tel: +31 40 474048 fax: +31 40 438060 Tipo: PD Status: C HASYLAB Notkestrasse 85, D-2000, Hamburg 52, Germany tel: +49 40 89982304 fax: +49 40 89982787 http://www.desy.de/pub/hasylab/hasylab.html Tipo: D Status: O INDUS Center for Advanced Technology, Rajendra Nagar, Indore 452012, India tel: +91 731 64626 Tipo: D Status: C

Vol. 4 n. 2 Dicembre 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

45

DOVE LUCE DI SINCROTRONE FACILITIES

KEK Photon Factory Nat. Lab. for High Energy Physics, 1-1, Oho, Tsukuba-shi Ibaraki-ken, 305 Japan tel: +81 298 641171 fax: +81 298 642801 http://www.kek.jp/ Tipo: D Status: O Kurchatov Kurchatov Inst. of Atomic Energy, SR Center, Kurchatov Square, Moscow 123182, Russia tel: +7 95 1964546 Tipo: D Status:O/C

SOR-RING Inst. Solid State Physics S.R. Lab, Univ. of Tokyo, 3-2-1 Midori-cho Tanashi-shi, Tokyo 188, Japan tel: +81 424614131 ext 346 fax: +81 424615401 Tipo: D Status: O SRC Synchrotron Rad. Center Univ.of Wisconsin at Madison, 3731 Schneider DriveStoughton, WI 53589-3097 USA tel: +1 608 8737722 fax: +1 608 8737192 http://www.src.wisc.edu Tipo: D Status: O SRRC SR Research Center 1, R&D Road VI, Hsinchu Science, Industrial Parc, Hsinchu 30077 Taiwan, Republic of China tel: +886 35 780281 fax: +886 35 781881 http://www.srrc.gov.tw/ Tipo: D Status: O

LNLS Laboratorio Nacional Luz Sincrotron CP 6192, 13081 Campinas, SP Brazil tel: +55 192 542624 fax: +55 192 360202 Tipo: D Status: C LURE Bât 209-D, 91405 Orsay ,France tel: +33 1 64468014; fax: +33 1 64464148 E-mail: lemonze@lure.u-psud.fr http://www.lure.u-psud.fr Tipo: D Status: O

SSRL Stanford SR Laboratory MS 69, PO Box 4349 Stanford, CA 94309-0210, USA tel: +1 415 926 4000 fax: +1 415 926 4100 http://www-ssrl.slac.stanford.edu/welcome.html Tipo: D Status: O

MAX-Lab Box 118, University of Lund, S-22100 Lund, Sweden tel: +46 46 109697 fax: +46 46 104710 http://www.maxlab.lu.se/ Tipo: D Status: O NSLS National Synchrotron Light Source Bldg. 725, Brookhaven Nat. Lab., Upton, NY 11973, USA tel: +1 516 282 2297 fax: +1 516 282 4745 http://www.nsls.bnl.gov/ Tipo: D Status: O NSRL National Synchrotron Radiation Lab. USTC, Hefei, Anhui 230029, PR China tel:+86 551 3601989 fax:+86 551 5561078 Tipo: D Status: O Pohang Pohang Inst. for Science & Technol., P.O. Box 125 Pohang, Korea 790600 tel: +82 562 792696 f +82 562 794499 Tipo: D Status: C

SRS Daresbury SR Source SERC, Daresbury Lab, Warrington WA4 4AD, U.K. tel: +44 925 603000 fax: +44 925 603174 E-mail: srs-ulo@dl.ac.uk http://www.dl.ac.uk/home.html Tipo: D Status: O SURF B119, NIST, Gaithersburg, MD 20859, USA tel: +1 301 9753726 fax: +1 301 8697628 http://physics.nist.gov/MajResFac/surf/surf.html Tipo: D Status: O TERAS ElectroTechnical Lab. 1-1-4 Umezono, Tsukuba Ibaraki 305, Japan tel: 81 298 54 5541 fax: 81 298 55 6608 Tipo: D Status: O UVSOR Inst. for Molecular ScienceMyodaiji, Okazaki 444, Japan tel: +81 564 526101 fax: +81 564 547079 Tipo: D Status: O

Siberian SR Center Lavrentyev Ave 11, 630090 Novosibirsk, Russia tel: +7 383 2 356031 fax: +7 383 2 352163 Tipo: D Status: O SPring-8 2-28-8 Hon-komagome, Bunkyo-ku ,Tokyo 113, Japan tel: +81 03 9411140 fax: +81 03 9413169 Tipo: D Status: C

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

46

•

D = macchina dedicata; PD = parzialmente dedicata; P = in parassitaggio.

O= macchina funzionante; C=macchina in costruzione.

D = dedicated machine; PD = partially dedicated; P = parassitic.

O= operating machine; C= machine under construction.

Vol. 4 n. 2 Dicembre 1999

FACILITIES

NEUTRONI NEUTRON SCATTERING WWW SERVERS IN THE WORLD (http://www.isis.rl.ac.uk)

BENSC Berlin Neutron Scattering Center, Hahn-Meitner-Institut, Glienicker Str. 100, D- 14109 Berlin-Wannsee, Germany Rainer Michaelsen; tel: +49 30 8062 3043 fax: +49 30 8062 2523 E - Mail: michaelsen@hmi.de http://www.hmi.de BNL Brookhaven National Laboratory, Biology Department, Upton, NY 11973, USA Dieter Schneider; General Information: Rae Greenberg; tel: +1 516 282 5564 fax: +1 516 282 5888 http://neutron.chm.bnl.gov/HFBR/ GKSS Forschungszentrum Geesthacht, P.O.1160, W-2054 Geesthacht, Germany Reinhard Kampmann; tel: +49 4152 87 1316 fax: +49 4152 87 1338 E-mail: PWKAMPM@DGHGKSS4 Heinrich B. Stuhrmann; tel: +49 4152 87 1290 fax: +49 4152 87 2534 E-mail: WSSTUHR@DGHGKSS4 IFE Institut for Energiteknikk, P.O. Box40, N-2007 Kjeller, Norway Jon Samseth; tel: +47 6 806080 fax: +47 6 810920 telex: 74 573 energ n E-mail: Internet JON@BARNEY.IFE.NO ILL Institute Laue Langevin, BP 156, F-38042, Grenoble Cedex 9,France Herma Büttner; tel: +33 76207179 E-mail: sco@ill.fr fax: +33 76 48 39 06 http://www.ill.fr IPNS Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4814, USA P.Thiyagarajan,Bldg.200,RM. D125; tel :+1 708 9723593 E-mail: THIYAGA@ANLPNS Ernest Epperson, Bldg. 212; tel: +1 708 972 5701

fax: +1 708 972 4163 or + 1 708 972 4470 (Chemistry Div.) http://pnsjph.pns.anl.gov/ipns.html ISIS The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxfordshire OX11 0QX, UK Richard Heenan; tel +44 235 446744 E-mail: RKH@UK.AC.RUTHERFORD.DEC-E Steve King; tel: +44 235 446437 fax: +44 235 445720; Telex: 83 159 ruthlb g E-mail: SMK@UK.AC.RUTHERFORD.DEC-E http://www.isis.rl.ac.uk JAERI Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan.

Jun-ichi Suzuki (JAERI); Yuji Ito (ISSP, Univ. of Tokyo); fax: +81 292 82 59227 telex: JAERIJ24596 http:// neutron-www.kekjpl JINR Joint Institute for Nuclear Research, Laboratory for Neutron Physics, Head P.O.Box 79 Moscow, 141 980 Dubna, USSR A.M. Balagurov; E-mail: BALA@LNP04.JINR.DUBNA.SU Yurii M. Ostaneivich; E-mail: SANS@LNP07.JINR.DUBNA.SU fax: +7 095 200 22 83 telex: 911 621 DUBNA SU http://www.jinr.dubna.su KFA Forschungszentrum Jülich, Institut für Festkörperforschung, Postfach 1913, W-517 Jülich, Germany Dietmar Schwahn; tel: +49 2461 61 6661; E-mail: SCHWAHN@DJUKFA54.BITNET Gerd Maier; tel: +49 2461 61 3567; E-mail: MEIER@DJUKFA54.BITNET fax: +49 2461 61 2610 telex: 833556-0 kf d

Vol. 4 n. 1 Giugno 1999

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

47

DOVE LUCE DI SINCROTRONE FACILITIES

LLB Laboratoire Léon Brillouin, Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur-Yvette Cédex France J.P Cotton (LLB); tel: +33 1 69086460 fax: +33 1 69088261 telex: energ 690641 F LBS+ E-mail: COTTON@BALI.CEA.FR http://bali.saclay.cea.fr/bali.html NIST National Institute of Standards and TechnologyGaithersburg, Maryland 20899 USA C.J. Glinka; tel: + 301 975 6242 fax: +1 301 921 9847 E-mail: Bitnet: GLINKA@NBSENTH Internet: GLIMKA@ENH.NIST.GOV http://rrdjazz.nist.gov ORNL Oak Ridge National Laboratory Neutron Scattering Facilities, P.O. Box 2008, Oak Ridge TN 37831-6393 USA George D. Wignall, Small Angle Scattering Group Leader; tel: +1 423 574 5237 fax: +1 423 574 6268 E-mail: wignallgd@ornl.gov http://neutrons.ornl.gov PSI Paul Scherrer Institut Wurenlingen und Villingen CH-5232 Villingen PSI tel: +41 56 992111 fax: +41 56 982327 RISØ EC-Large Facility Programme, Physics Department, Risø National Lab.P.O. Box 49, DK-4000 Roskilde, Denmark K. Mortenses; tel: +45 4237 1212 fax: +45 42370115 E-mail: CLAUSEN@RISOE.DK or SANS@RISOE.DK NFL-Studsvik in Sweden E-mail: mcgreevy@studsvik.uu.se

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

48

•

Vol. 4 n. 2 Dicembre 1999