NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 3 n.2, 1998

NOTIZIARIO Neutroni e Luce di Sincrotrone Editoriale

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F.P. Ricci RASSEGNA SCIENTIFICA

The Use of Synchrotron Radiation in Protein Crystallography: Old Considerations and New Developments ......................................................................................................

G. Zanotti

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Orientational Correlations and Hydrogen Bonding in Liquid Hydrogen Chloride ..................................................................................................................................................

C. Andreani et Al.

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Development of Microfocus X-Ray Zone Plates M. Gentili and E. Di Fabrizio

DOVE LUCE DI SINCROTRONE

Infrared Synchrotron Radiation, a New Tool for the Italian Scientific Community ...........................................................................................................................................................................

P. Calvani

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SCUOLE E CONVEGNI

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CALENDARIO

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FACILITIES .............................................................................................................................................................

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SCADENZE

Cover photo: Molecular model of three molecules of the protein annexin IV arranged around the three-fold axis in the trigonal crystals (Zanotti et. al., Biochem. J., 329, 101-106, 1998). Annexins are a family of proteins able to bind to phospholipid membranes and consequently promote the formation of Ca2+ ion channels. Despite the fact the the protein is present in solution as a monomer, several experimental evidences suggest that the formation of the trimer shown in figure is relevant for the interaction with the membrane.

Vol. 3 n. 2 Dicembre 1998

EDITORIALE

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n questo ultimo mese vi sono stati due eventi di notevole importanza per lo sviluppo della nostra comunità neutronica. Alla fine di ottobre il professor Fontanesi, a nome del CNR, ha firmato a Madrid il “Memorandum of Understanding”. In questo modo il CNR è entrato ufficialmente a far parte dell’European Spallation Source R&D Council, l’organismo internazionale che ha come obiettivo la costruzione di una nuova sorgente di neutroni impulsati europea (ESS), che è bene ricordare, costituirà la sorgente di neutroni più intensa nell’immediato futuro (Vedi Notiziario Neutroni e Luce di Sincrotrone Vol. 3 n. 1 - 1998). Per la nostra comunità scientifica questo rappresenta uno stimolo per le attività di sviluppo di strumentazione nel campo, e una occasione per incrementare lo staff di giovani ricercatori operanti in spettroscopia neutronica. Inoltre questo impegno del CNR sarà per i ricercatori italiani occasione di scambio scientifico a carattere internazionale.

Il secondo evento è relativo alla visita del Presidente del CNR, prof. Lucio Bianco, recatosi al Rutherford Appleton Laboratory, presso la sorgente neutronica pulsata ISIS, per l’inaugurazione dello Spettrometro TOSCA (Vedi Notiziario Neutroni e Luce di Sincrotrone Vol. 3 n. 1 - 1998), l’1-2 dicembre. Oltre al Presidente, facevano parte della delegazione italiana, il prof.

Marcello Fontanesi, Presidente del Comitato Scienze Fisiche, la prof.ssa Carla Andreani, segretario scientifico del Comitato, il dott. Alberto Conti dell’ufficio rapporti internazionali del CNR ed il dott. Daniele Colognesi, il prof. Francesco Paolo Ricci e il dott. Marco Zoppi, in rappresentanza del gruppo che ha progettato e realizzato lo spettrometro TOSCA. Era presente anche l’ing. Checchi, in rappresentanza dell’industria fiorentina che ha costruito lo spettrometro. È rilevante che sotto la spinta del CNR un’industria italiana abbia realizzato questo strumento avviandosi quindi in un campo di elevata tecnologia il che permetterà al nostro Paese di concorrere con successo a ulteriori gare europee nel campo.

Il Presidente del CNR ha anche incontrato i ricercatori italiani, presenti ad ISIS in quei giorni per esperimenti, ed i componenti italiani nei panels di valutazione, il prof. Roberto Derenzi, la prof.ssa Maria Antonietta Ricci, ed il prof. Romano Rinaldi, i quali hanno sia illustrato le modalità di selezione delle proposte di esperimenti per ISIS sia sottolineato il buon valore scientifico delle proposte italiane.

Va sottolineata la lungimiranza scientifica del CNR nella scelta, che risale all’anno 1985, di partecipare alla sorgente neutronica pulsata ISIS. Infatti attualmente la comunità scientifica operante nel campo della spettroscopia neutronica nel mondo è orientata verso questo tipo di sorgenti, sia per le ampie possibilità scientifiche offerte sia per le garanzie ambientali.

Infatti al momento la costruzione di nuove sorgenti stazionarie incontra delle difficoltà. Basti ricordare il recente caso del progetto di costruzione del reattore di Monaco di Baviera che,

benché ormai quasi ultimato, ha subito un rallentamento.

Dal punto di vista della organizzazione della ricerca in luce di sincrotrone, durante i mesi scorsi vi sono state alcune importanti novità. Vi è stata l’assunzione di personale a tempo indeterminato (tecnico, tecnologo e ricercatore) presso i laboratori ELETTRA ed ESRF. Presso ESRF, l’INFM ha creato un gruppo che costituisce il primo nucleo stabile di supporto ai progetti ed agli utenti italiani di ESRF e della linea italiana GILDA. Nello stesso ambito opera personale a tempo determinato, parte del quale segue l’attività del CRG italiano ad ILL. Presso ELETTRA, l’INFM ha assunto personale che seguirà la progettazione e la costruzione di progetti INFM. Per quanto riguarda la situazione a Trieste è significativa e positiva la recente approvazione di quattro nuove beamline, tutte con finanziamento della Sincrotrone Trieste; sono le beamline dedicate a cristallografia di proteine (seconda linea), alla microfrabricazione, alla nanospectroscopia ed un laser ad elettroni liberi. Questi nuovi sviluppi consolidano in modo significativo l’attività italiana di luce di sincrotrone.

Formuliamo l’auspicio che la attuale cooperazione fra gli enti impegnati nella ricerca con luce di sincrotrone in Italia (CNR, INFM, INFN e Sincrotrone Trieste) possa non solo continuare nel futuro ma rafforzarsi.

La vitalità del campo è stata recentemente dimostrata dall’appena svolto Users’ Meeting di ELETTRA. Sono stati illustrati risultati in campi che vanno dalla spectromicroscopia delle interfacce, alle correlazioni angolari nella fotoemissione con tecniche di coincidenza, alla mammografia digitale. Durante lo Users’ Meeting il prof. Giorgio Margaritondo, al termine del suo mandato di Direttore Scientifico, ha eseguito una stimolante rassegna dei risultati scientifici più significativi ottenuti e degli obiettivi raggiunti per le linee e la macchina. Cogliamo l’occasione per fare gli auguri di buon lavoro al prof Massimo Altarelli, che dal 1° gennaio avrà la responsabilità di coordinare le attività scientifiche e complessive di ELETTRA. Ricordiamo che lo Users’ Meeting di ESRF viene svolto nel febbraio 1999; oltre alla giornata plenaria con quattro relazioni su invito, la relazione dei direttori e la premiazione del “Young Scientist” vi sono tre workshop, dedicati a dicroismo magnetico, scienza dei materiali e scattering a basso angolo di raggi X e neutroni. Gli argomenti delle relazioni ad invito dello Users’ Meeting di ESRF (rilevazione dell’ordine orbitale in ossidi, covalenza del legame idrogeno, nuove possibilità di radioterapia anti-tumorale e cristallografia di proteine) illustrano in modo chiaro l’importanza della luce di sincrotrone in numerosi campi della ricerca scientifica.

Infine, ricordiamo che a fine estate verrà svolta una nuova edizione della scuola di luce di sincrotone organizzata dal prof.

Settimio Mobilio e dal prof. Gilberto Vlaic presso S. Margherita di Pula (Cagliari). È auspicabile che si ripeta una numerosa

è pubblicato a cura del Gruppo Nazionale di Struttura della Materia del C.N.R. in collaborazione con il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

Vol. 3 n. 2 Dicembre 1998 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96

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EDITORIALE

partecipazione di giovani a questo importante appuntamento per la comunità della luce di sincrotrone.

F.P. Ricci

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ery recently there have been two important events for the Italian neutron community. At the end of October prof. Fontanesi, representing CNR, signed in Madrid a Memorandum of Understanding which will allow CNR to participate in the European Spallation Source R&D Council, the international group whose objective it is to build the new european pulsed neutron source (ESS); this source will be the most intense neutron source in the near future (as described in Notiziario Neutroni e Luce di Sincrotrone Vol. 3 n. 1 - 1998).

For our community this represents an opportunity for the development of neutron spectroscopy instrumentation and also to increase the number of young researchers working with neutrons. Moreover, this commitment of CNR increase international scientific exchange for Italian researchers.

The second event is the visit on 1st and 2nd of December of the President of CNR, prof. Lucio Bianco, to the ISIS pulsed neutron source (Rutherford Appleton Laboratory) for the inauguration of the TOSCA spectrometer (Notiziario Neutroni e Luce di Sincrotrone Vol. 3 n. 1 - 1998). With the President of CNR were prof. Marcello Fontanesi, President of the Physics Committee, prof. Carla Andreani, scientific secretary of the same committee, dr. Alberto Conti of the international relations office of CNR and dr. Daniele Colognesi, prof. Francesco Paolo Ricci and dr. Marco Zoppi, representing the group which designed and built TOSCA. Ing. Checchi, representing the florentine company which built the spectrometer was also present. It is important that thanks to the effort of CNR an Italian company has built this instrument, thus acquiring the know-how which will allow Italy to compete successfully in European tenders in this field.

The President of CNR also met Italian researchers involved in experiments at the time and the Italian members of the review panels: prof. Roberto Derenzi, prof. Maria Antonietta Ricci and prof. Romano Rinaldi, who explained the selection process for proposals at ISIS and stressed the good scientific level of Italian proposals.

We would like to stress the far-sighted policy of CNR in choosing, in 1985, to participate in the ISIS pulsed neutron source.

In fact, the international neutron community now prefers this kind of source because of the numerous scientific applications and also because of environmental concerns. Indeed, construction of new continuous sources is encountering problems, as demonstrated by the recent difficulties of the Munich reactor.

In the past few months there have been some important new

DIRETTORE RESPONSABILE:

F.P. Ricci

GRAFICA E STAMPA:

COMITATO DI REDAZIONE:

C. Andreani, F. Boscherini, R. Caciuffo, R. Camilloni

SEGRETERIA DI REDAZIONE:

D. Catena

HANNO COLLABORATO A QUESTO NUMERO:

events concerning the organization of Italian synchrotron radiation research. Permanent contract staff (technicians, development scientists and research scientists) has been hired in Trieste and Grenoble. At ESRF, INFM has established a group whose function it is to provide support to projects and Italian users of public beamlines and of the Italian CRG GILDA. Fixed term personnel is also involved in the Italian CRG at ILL. At ELETTRA, INFM has hired staff to work on the design and construction of projects and beamlines. Concerning ELETTRA, a significant and positive development is the recent approval of four new beamlines, all with Sincrotrone Trieste funding; they are dedicated to protein crystallography (second beamline), microfabrication, nanospectroscopy and an FEL. These new developments consolidate Italian activity in synchrotron radiation research. We hope that the cooperation which is presently taking place between the various institutes involved in synchrotron radiation research in Italy (CNR, INFM, INFN and Sincrotrone Trieste) will not only continue but will be further strengthened.

The vitality of the field was demonstrated by the recently held Users’ Meeting of ELETTRA. Results in fields ranging from spectromicroscopy of interfaces to angular correlations in photomemission with coincidence techniques to digital mammography were illustrated. During the Users’ Meeting prof.

Giorgio Margaritondo, at the end of his successful term as Scientific Director, reviewed the most significant scientific results obtained and the performance of the accelerators and of the beamlines. We take this opportunity to wish prof. Massimo Altarelli , who from January 1st takes global responsibility for activities at ELETTRA, every success in his new role.

The ESRF Users’ Meeting takes place in February 1999; along with the plenary session with four invited talks, the directors’ report and the Young Scientist Award, there are three workshops dedicated to magnetic scattering, materials science and neutron and x-ray small angle scattering. The topics of the invited talks clearly illustrate the importance of synchrotron radiation research in diverse areas of science; they deal with the detection of orbital ordering in oxides, of covalency in the hydrogen bond, of new possibilities of micro-beam radiation therapy and with protein crystallography.

Lastly, we remind readers that in the early fall a new edition of the synchrotron radiation school organized by prof. Gilberto Vlaic and prof. Settimio Mobilio will take place in S. Margherita di Pula (Sardinia). We hope that, as for previous editions, many researchers will participate to this important event for our community.

F.P. Ricci

C. Mariani, F. Boscherini, M. Catti

om grafica, via Fabrizio Luscino 73, Roma

Finito di stampare nel mese di Dicembre 1998

Per numeri arretrati: Grazia Ianni, GNSM-C.N.R., viale dell’Università 11, 00185 Roma. 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

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Articolo ricevuto in redazione nel mese di Settembre 1998

THE USE OF SYNCHROTRON RADIATION IN PROTEIN CRYSTALLOGRAPHY: OLD CONSIDERATIONS AND NEW DEVELOPMENTS G. Zanotti Dip. Chimica Organica dell’Università e Centro Studi sui Biopolimeri del CNR, via Marzolo 1, 35131 Padova, Italy

Summary The latest developments in the use of synchrotron radiation in macromolecular crystallography are briefly reviewed. In particular, the advantages in terms of increase of resolution and the possibility of measuring data on very small or poorly diffracting crystals are described. The advances in Multiple Anomalous Dispersion (MAD), direct methods and dynamic measurements are discussed, along with some possible future developments.

Introduction The determination of the three-dimensional structure of biological macromolecules by means of X-ray diffraction techniques has shown, since its birth in the ’50 th , a nearly-exponential growth: from about twenty-thirty

structures solved per year during the period 1974-80, to nearly 2000 structures which have been deposited at the Brookhaven Protein Data Bank1 in 1997 alone (Fig. 1).

Several different reasons are at the basis of this ’explosion’ of structural data, among them: a. the technological advancements in the field of computers and measuring devices (area detectors are commercially available since about fifteen years, graphic workstations for the display of electron density maps and model manipulation since the beginning of the ’80s); b. the improvement of the methodologies for structure solution, along with the parallel increase of the number of laboratories involved in protein crystallography; c. the progresses in molecular biology, that nowadays

Fig. 1. Number of macromolecular structures deposited at the Brookhaven Protein Data Bank (PDB) since 1974. PDB is the organization that takes care of the collection and free distribution of all macromolecular structures, solved either by X-ray diffraction or NMR. Of the total 8174 structures present in the data bank at the 1st of September, 1998, 6693 have been determined by X-ray diffraction, 1289 by NMR in solution and 192 are theoretical models.

The data for the graph were taken directly from PDB statistics.

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make available to the crystallographer relatively large amounts of proteins that are present in nature in quantity insufficient for crystallization trials; d. and finally, but not last for relevance, the availability of second and third generation synchrotrons: they have strongly contributed to accelerate the process of structure determination and made possible some types of measurements unattainable with conventional sources.

There are various reasons which stimulate the crystallographer to use the synchrotron radiation: the crystals are too small or do not diffract under conventional X-ray sources; the need for wavelengths different from the classical Cu Kα or Mo Kα; the need of a white-beam radiation for time-resolved studies; the presence of very large unit cells, as in the case of virus crystals, that requires a well collimated beam and the possibility of selecting the wavelength to separate the reflections; the possibility of attaining a definitely higher resolution with respect to a conventional source, either for a better definition of the molecular model or even to allow to solve the phase problem. All of these points will be briefly reviewed and their relative relevance and the theoretically obtainable results discussed.

It should be pointed out that the use of synchrotron radiation for a crystallographer consists very often in a routine measurement: data are collected with the same types of detectors as used on conventional sources and processed with the same kind of programs. The importance of the experiment can eventually reside in the biological or medical relevance of the results obtained.

Crystal size and diffracting power This point is apparently not worth a long discussion: if the crystal is too small or it diffracts only to low resolution under a rotating-anode X-ray source, the use of synchrotron radiation, well collimated and of high brilliance, often solves the problem. It is in fact true that this is for a protein crystallographer one of the main reasons for going to a synchrotron facility. Having only small crystals, in the past the crystallographer had no other choice than testing conditions for growing bigger and better crystals. Nowadays, the availability of a synchrotron source often allows the measurement from small crystals of diffraction data good enough for solving the structure. It must be pointed out that this behavior is not a result of laziness: the growth of crystals of appropriate size for X-ray analysis represents, in the generally long procedure of structural determination, the step that has obtained less improvements from the technical progresses of the last twenty years. Protein crystallization still represents a “trial and error” process, which requires months (or eventually years) of experiments, often unsuccessful. It has to be considered as one of the main reasons why only a few thousands of three-dimensional

structures have been determined, compared to the more than 200,000 protein sequences availablea,2.

One could ask what is the lower limit of the size of a protein crystal that could be utilized for the structural determination. Taking into account that the total energy of the diffracted beam from a family of planes (hkl) for an ideal crystal is inversely related to the cell volume: E(hkl) ∝ |F(hkl)|2 Vcryst/Vcell

(1)

where Vcryst and Vcell are the total volume of the crystal and of the unit cell, respectively, it appears evident how the diffracting power is dependent on the number of cells present in the crystal and, finally, on the size of the cell itself. In practice, for crystals with very small cells, like those of minerals, structural determinations have been successfully undertaken with samples of few microns of linear size: for a cell with a volume of 1000 Å3, in a crystal with edges of 10 µm there are 1012 cells, still a quite high number. In the case of a macromolecule, whose crystal cells range usually from 100,000 to some millions of Å3, a crystal of the same size as the previous one contains a number of cells that ranges from 1010 to 109. That corresponds to a mean decrease of the diffracted intensities of at least two-three orders of magnitude.

Besides, protein crystals are essentially composed of light atoms and they also contain a lot of solvent, which decreases the order of the crystal lattice and consequently the diffracting power. It is not possible to state what is exactly the minimum size useful for a data collection of a protein crystal, since it depends on many different conditions, but we could cautiously state that crystals with a total volume of 105 ÷ 106 µm3 (i.e. 50-100 µm of linear size in all directions) can be measured on a synchrotron source. Much smaller volumes are nevertheless possible, depending on the crystal quality and the beam microfocusing: diffraction patterns that extend to about 2.5 Å resolution have been obtained3 for bacteriorhodopsin crystals with a total volume of about 5·103 µm3.

All the previous considerations have a relevant practical aspect. It is in fact relatively easy to obtain very small crystals of a globular protein. What is often considered an “amorphous” precipitate is composed, if observed with the appropriate microscope, of crystals of very small size. It is much more difficult to grow these small crystals to a size suitable for a diffraction experiment.

Lowering the threshold to which crystals are suitable for X-ray diffraction experiments shortens, in practice, the process of crystal growth and, finally, of the structural determination.

Resolution Crystals of globular proteins present a very high solvent content, generally from 40 % to 60 %, but cases are known where the estimated solvent is much larger. This

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fact, coupled with the size and the intrinsic flexibility of the macromolecule itself, strongly reduces the order of its crystal lattice. It is not comparable with that of a ionic crystal and not even with that of a molecular one. Under conventional sources, a ‘normal’ protein crystal diffracts at about 2 Å resolution, whilst quite good crystals can reach 1.5 Å. But often, particularly for very large macromolecules, the crystallographer is happy enough in obtaining a spectrum at 2.5 Å or 3 Å resolution.

Owing to its high brightness and brillianceb, synchrotron radiation generally gives rise to a significant increase in resolution: it is impossible to quantify a priori this improvement, but I will illustrate this point with a few examples from my personal experience. In the cases described, data were measured at the ELETTRA facility in Trieste, a third generation synchrotron, with a MAR Research imaging plate as a detector. The laboratory source was a M18XHF-SRA rotating anode, and the detector a multiwire proportional counter HIGH STAR (Brücker).

It must also be taken into account that rotating anode data were collected at room temperature and synchrotron data at 100°K (there was no special reason for that, but simply the lack of the cooling device in our laboratory. On the other hand, data at ELETTRA cannot be measured without freezing the crystal, since the very high intensity of the beam drastically reduces the crystal life-time).

The first example is represented by the enzyme rhodanese, whose crystals diffract to a maximum of 1.8-1.9 Å resolution on a rotating anode source4. Data could be collected at a resolution of 1.36 Å with synchrotron radiation, with an increase of the number of independent re-

flections from 21000 to 56000 5. This has allowed a much better definition of the molecular structure and the positioning of 407 ordered solvent molecules (Fig. 2).

Crystals of annexin IV, despite their reasonable size (Fig. 3), are very unstable and they last for only few minutes under a rotating anode source. Frozen crystals at ELETTRA show a diffraction pattern that extends to at least 3 Å. The measurement of a native data set has allowed to solve the structure using the molecular replacement technique6.

As a final example, the diffraction spectrum of crystals of a mutated B-subunit of heat-labile toxin from E. coli can be hardly observed at 6-7 Å on a rotating anode, owing to the exceptionally high solvent content of the crystal, more than 75% of the total volume (Fig. 4). Data could be measured in Trieste at 3 Å and the crystal structure solved and refined7.

It must be stressed that, whilst in the first example described the synchrotron radiation simply allowed to extend significantly the resolution and to have a better defined model, in the other two cases it was absolutely essential, since without its use data could not have been measured at all. Since two other similar situations happened to the author in the last two years, it is easy to extrapolate that the use of synchrotron can often be determinant in structural biology.

The phase problem and the multiple anomalous scattering (MAD) MAD represents the technique where synchrotron radiation has given the more crucial contribution to the resolution of the phase problem in biocrystallography: about hundred structures has been solved up to the 1997 using it, 80 of which only in the years 1995-19978.

The so called “anomalous scattering” is based on the following principle: when the energy of the X-ray radiation that interacts with the matter is close to the value of the electronic transition from a bonding atomic orbital, a resonance condition takes place and the classical Thomson scattering is perturbed9. At the wavelengths used for the normal diffraction experiments on single crystal this component is totally negligible for light atoms, like H, C, N, O and S. It becomes on the contrary significant for heavy atoms, i.e. for atoms with many electrons. For the latter, the atomic scattering factor, f, can be separated in two components, one, f0, purely real and totally independent from the wavelength and a second, f∆ , complex and dependent on λ. We can consequently write: f = f0+ f∆ = f0+ f’(λ) + if”(λ)

Fig. 2. Cartoon showing the structure of enzyme rhodanese. Arrows represent β-strands, helical ribbons α-helices, the two black spheres the sulfur atoms of the catalytic cystein 247, small crosses the ordered solvent molecules surrounding the protein in the crystal (ref. 5).

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(2) c

An example of the trend of the two components of f∆, real and imaginary, with the wavelength is illustrated in Fig. 5 for selenium. It must be remembered that both f’ and f” depend on the bonding situation of the heavy-

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Fig. 3. Trigonal crystals of bovine annexin IV, obtained in the absence of Calcium ion (ref. 6). Crystals are approximately 0.1 mm on all sides.

atom itself in the macromolecule: this makes necessary their experimental determination from fluorescence measurements of atomic absorption of the sample under study.

However, as a consequence of this resonance effect, there is a wavelength (λ1 in Fig. 5) for which f” is maximum and one (λ2) for which f’ is minimum: in both cases the value of the structure factor will be different from that measured at a wavelength (λ3) where both effects are negligible. When the imaginary component f” is not negligible, the Friedel law, that states that | F(hkl) | = | F(-h-k-l) |

(3)

does not hold: the two reflections, formerly equivalent, are now called a “Bijvoet pair” and their difference is somehow related to the position of the anomalous scatterer, i.e. the heavy-atom. Moreover, the value of | F(hkl) | measured at λ1 will be different from that at λ2. Consequently, the measurement of each structure factor at the three different wavelengths will be virtually equivalent to that of diffraction data of a native data set and two heavy-atom derivatives. The processing of MAD data can consequently be turned into that of the multiple isomorphous replacement (MIR), well known and documented in protein crystallography10. The disadvantage arising from the fact that the anomalous effect is relatively small, particularly if compared to that of isomorphous replacement, is largely compensated by the total absence of the problems deriving from the lack of isomorphism, largely present in the latter.

Anomalous scattering measurements are anyhow not so

simple: despite of being routinely performed at many synchrotron facilities, great care must be taken to minimize systematic and statistical errors (counting errors, crystal absorption and decay). The Bijvoet pairs should be measured on the same frame or eventually in quite close time intervals.

Despite the practical problems during data collection, the need of a synchrotron for performing the measurements and of a heavy-atom in the macromolecule, the MAD technique has become a nearly routine approach to the solution of the phase problem. This is due to the fact that nowadays, thanks to the developments of molecular biology, within recombinant proteins it is possible to substitute the methionine, an amino acid containing sulfur, with seleno-methionine or, less frequently, telluro-methionine, i.e. methionines where the S atom has been substituted by Se or Te11. The protein so modified behaves, from the conformational point of view, as the native protein and can generally be crystallized isomorphously to it. In this way one or more anomalous scatterers have been introduced inside the macromolecule without perturbing its three-dimensional structure and a MAD experiment can be easily performed.

The phase problem and direct methods ‘Direct methods’ in crystallography refers to the technique that allows the determination of the phases of structure factors only from the knowledge of their moduli.

They are based on probabilistic relationships, that take into account the fact that relationships among intensities make some phase values for a single structure factor more or less likely. Their physical validity stands on the quite obvious assumptions of the positivity of the electron density and of the ‘atomicity’ of it. But whilst the former is always necessary true, the latter is unfortunately valid only for diffraction data at atomic resolution.

The probabilistic formulas derived from them allow anyhow the solution of the phase problem in a nearly automatic way, for the so called ‘small molecules’ crystals, which includes those with a maximum of about one hundred non-hydrogen atoms in the asymmetric unitd.

One of the fundamental formulas of direct methods gives the probability that the phase of the triplet φ-h+φk+φh-k assumes the value Φhk and can be written as: P(Φhk) = (1/L)exp [(2/√N) |EhEkEh-k| cos(Φhk)]

(4)

In (4) E are normalized structure factors, L is a normalization term and N the number of atoms in the unit celle.

Relationship (4) estimates the probability distribution of the phase Φ based on the values of the moduli of three reflections. A more general relationship, called “tangent formula”, estimates the phase from the moduli of all the

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reflections (for a general description, see, for example, ref. 12). But the important point, evident from (4), is that the probability depends on 1/√N and, since N can be very large for a macromolecule, the probability distribution becomes very flat. It is also true that for a macromolecule, at a given resolution, there are many more reflections with respect to a small molecule crystal and this should compensate the reduced relevance of each relationship. Unfortunately, the diffracting power of macromolecular crystals only exceptionally reaches atomic re-

Fig. 4 Schematic drawing of the crystal packing of the subunit B mutant of heat-labile enterotoxin from E. coli. In the drawing, the cell is seen projected along the c axis and only Cα atoms are represented. Each protein molecule is an omo-pentamer, made by five polypeptide chains of 111 amino acids each. The space group is P41212 and there is only one pentameric molecule in the asymmetric unit, which accounts for a total of 76% of the crystal volume being approximately occupied by solvent and only 24% by protein atoms. The macromolecules, packed in the crystal around the four-fold axes in super-helical structures, form the large solvent channels that can be observed in the drawing. (The figure was produced by Dr Dubravka Matkovic-Calogovic with coordinates taken from ref. 7).

solution. If this condition is not fulfilled, the classical relationships of direct methods do not work, and other techniques (MIR, MAD etc.) have to be used to solve the phase problem. The use of synchrotron radiation in this respect can be fundamental: the increase of diffracting power, for example, from 1.5 Å to about 1.2 Å, can allow the use of direct methods. Unfortunately this represents for the moment an exception rather than the rule and it has been successfully applied only in a very limited number of test cases13,14.

Nevertheless, it is likely that the use of more powerful phase relationships and the improvements of the mathematical tools can decrease the limit of resolution necessary to reach the structure solution. In this respect,

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the combination of them with the high resolution data measured at a synchrotron could lead to the determination of macromolecular structures without the need to resort to the cumbersome experimental methods used in these days.

Time-resolved measurements: The Laue method As described previously, protein crystals contain a lot of solvent. A large portion of it is disordered or, in any case, is in equilibrium with the external solution where the crystal is kept. This fact allows the soaking of the crystal in a solution that contains ‘small’ molecules, which can diffuse in the channels formed by macromolecules packed in the crystal. This is in fact the basis of the isomorphous replacement technique, which historically has represented the first method of determination of a protein structure and it is still used to solve new structures.

It consists in diffusing compounds containing ‘heavy’ atoms, i.e. atoms with many electrons if compared to N, C, O and S, inside the crystal. From their positions it is possible to calculate the phases of the structure factors of the native crystal.

Moreover, the presence of the solvent can allow the diffusion in the crystal of molecules that bind to the macromolecule, like substrates or inhibitors in the case of enzymes. Enzymes very often maintain their activity in the solid state and ‘static’ studies, that is the comparison between the structure of the enzyme with and without ligands (inhibitors, partial substrates or activators) can give indirect information about the mechanism of action of the enzyme itself. It is obvious that these kind of information can, in principle, be obtained also from dynamic measurements, i.e. measurements taken during the enzymatic reaction. This can be done with the methodology known as ‘Laue’. It originates from Max von Laue, that used it at the dawning of crystallography (1912) to demonstrate the diffraction of X-rays from a crystal of copper sulfate. The method was very rapidly ousted by monochromatic measurements, much simpler to carry out. Besides, polychromatic techniques are very hard to perform when X-rays are produced from a conventional source, whose spectrum is characterized by a few sharp peaks and a low, continuos background.

On the contrary, synchrotron radiation is very well suited for this kind of measurements and Laue techniques were consequently rediscovered in recent years, along with the diffusion of synchrotron facilities. After the determination of a small molecule structure in 198315, the first electron density map of a protein was obtained for the enzyme glycogen phosphorylase b16.

In this modern version of Laue technique, an entire diffraction data set can be measured in a very short time, from few ms to some ps: with a stationary crystal and white radiation, a lot of lattice planes satisfy the Bragg condition and so most of the spectrum can be measured

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without rotating the specimen. The larger the interval of wavelengths used, the more are the spots accessible to this geometry17. But, of course, with increasing ∆λ, the bigger is the probability that spots can overlap. Best suited crystals for this kind of measurements are those with very high symmetry, that allow the collection of nearly the entire set of data with the stationary crystal.

The very brilliant beam of the third generation synchrotrons, coupled with the advent of fast detectors, allows in principle the examination of very fast reactions. The

Fig. 5. f’ and f” components of the scattering of Se as a function of the Xray energy. Continuos lines represent the theoretical values calculated according to the Cromer/Liberman theory, dashed lines are “simulated” experimental ones for Se in a macromolecule. The shift of the absorption edge is significant.

basis of the method is the following: the crystal is positioned in a flow-cell, which allows the exchange of the mother liquid inside the crystal with a solution from the outside. When the substrate is injected, data sets are measured at given intervals. If the time necessary for a measurement of a set of data is sensibly shorter than the reaction time, in principle we can take pictures of the course of the chemical reaction. Time intervals obviously depend from the reaction rate: fortunately, the speed of biochemical reactions can be often controlled by the pH and the solvent medium and so reactions can be made not too fast.

Time-resolved studies present, as can be easily imagined, some very serious difficulties, either practical or theoretical. One of the former is represented by the speed of substrate diffusion inside the crystal: since diffusion is a relatively slow process, only chemical reactions much slower than it could be studied. To overcome this problem, it is possible to introduce the substrate, in an inactive form, in advance inside the crystal: the substrate is suddenly activated, through a photochemical reaction, generally with a pulse of a laser light18. This solves the diffusion problem and allows a precise control of the reaction initiation. Unfortunately, photochemical

reactions are not always available for all kinds of possible substrates of enzymes and this partially limits the use of the technique.

Nevertheless, the major hindrance to the method is a theoretical one. What we observe in a diffraction experiment is the mean, over time and space, of all the molecules in the crystal. It is therefore necessary that a stable intermediate accumulates during the reaction, and it has to do so in a large fraction of molecules of the crystal at the same time. It represents possibly the reason for which, despite the many promises and some very brilliant results, it has not yet given very outstanding contributions to the understanding of biological mechanisms.

Only advantages?

It is reasonable to ask if no drawbacks are present when using synchrotron radiation. In fact, at least one physical and some practical disadvantages can be listed. The former is particularly true for very brilliant sources, where the crystal has to be frozen in order to limit the drastic crystal decay due to the very high photon flux. Crystals frozen in the liquid nitrogen stream, usually at 100°K, last for several hours (or eventually forever, if kept cooled), generally enough for at least an entire data collection. The freezing process is anyhow not a simple task, since protein crystals contain mother liquid which is mainly composed of water, that can crystallize by itself when cooled, cracking the crystal and giving rise to a powder diffraction spectrum of ice (along with that of other salts, if present in the crystallization medium). The mother liquid must be therefore substituted by an appropriate solution, called cryo-protectant, containing substances with a low freezing-point, like glycerol.

Unfortunately, the cryo-protectant solution often increases the mosaicity of the protein crystal, in some cases significantly and so diminish the gains obtained by the intense, collimated source.

Practical problems in the use of synchrotrons arise instead from the need of moving to a distant place to perform the measurements: the transport of unstable samples is not so easy, particularly by plane, and, when possible, crystallographers tend to take their measurements at home. Synchrotron measurements have to be a real necessity.

Future developments It is always risky to foretell the future developments in science, particularly in a field of rapid growth. In the case of synchrotron radiation, it is quite easy to predict that its use by the crystallographic community will grow: at present, despite the large number of beamlines in the world which are dedicated to single-crystal diffraction, the demand largely exceeds the total time available to usersf. Moreover, the continuos increase of the

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number of research groups in the world engaged in protein crystallography, along with the reasons listed in the previous paragraphs, all drive towards a massive use of synchrotron radiation. But here I want to discuss about another development that is at the moment far from being utilizable in the field (and perhaps will never be), but that could eventually change, if successful, our experimental approach to structure determination: X-ray holography.

Holographic methods offer a mean of extracting both intensity and phase information from a spectrum, letting the scattered waves interfere with a single wave, taken as reference19. Holographic images using visible light are very common, but their resolution is limited by the wavelength. To be useful for structural applications, holography must be performed with a wavelength sufficiently small to resolve atomic details, i. e. X-rays, or electrons with the appropriate energy. The latter is in fact used, but, owing to the limited penetration of electrons, only for imaging of surfaces 20. X-rays present however an apparently insuperable problem: what can be used as the reference wavelength? Two years ago it has been demonstrated21 that atomic resolution is attainable in special cases: in a single crystal of SrTiO3, fluorescent X-rays emitted from the strontium atoms were used as the reference wave and interference with diffracted beams used for reconstructing the three-dimensional image. The Sr atoms in the crystal lattice can consequently be directly visualized. Despite the limited information (only the Sr atoms, heavier than Ti and O, can be seen), the importance of the experiment stands in the fact that a molecular structure has been visualized directly, without deriving the phase information from some other sourceg. Can this be used for direct imaging of macromolecules? For the moment the answer is no: eventually, some heavy atoms in the macromolecule could be seenh. But we cannot exclude that further advances in the technique will allow the direct visualization of macromolecules in the future.

Acknowledgments I would like to thank Dubravka Matkovic-Calogovic, Roberto Battistutta and Anna Bassetto for reading the manuscript and for the helpful comments.

References 1. Bernstein, F. C., Koetzle, T. F., Williams, G. J. B., Meyer, Jr., E. F., Brice, M. D., Rodgers, J. R., Kennard, O., Shimanouchi, T. and Tasumi, M. “The Protein Data Bank: a Computer-based Archival File for Macromolecular Structures” J. Mol. Biol. 112, 535-542 (1977) 2. Bairoch A. and Apweiler R. “The SWISS-PROT protein sequence data bank and its supplement TrEMBL in 1998” Nucleic Acids Res. 26, 38-42 (1998) 3. Pebay-Peyroula, E., Rummel, G., Rosenbusch, J.P. and Landau, E.M. “X-ray structure of Bacteriorhodospin at 2.5 Å from microcrystals

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grown in lipid cubic phases” Science 277, 676-681 (1977) 4. Gliubich, F., Gazerro, M.L., Zanotti, G., Delbono, S., Bombieri G. and Berni, R. “Active site structural features for chemically modified forms of rhodanese” J. Biol. Chem. 271, 21054-21061 (1996) 5. Gliubich, F., Berni, R., Colapietro, M., Barba, L. and Zanotti, G. “Structure of sulfur-substituted rhodanese at 1.36 Å resolution” Acta Cryst. D54, 481-486 (1998) 6. Zanotti, G. Malpeli, F., Gliubich, F., Folli, C., Stoppini, M., Olivi, L., Savoia A. and Berni, R. “Structure of the trigonal crystal form of bovine annexin IV” Biochem. J. 329, 101-106 (1998) 7. Matkovic-Calogovic, D., Loreggian, A., D’Acunto, M.R., Battistutta, R., Palù, G. and Zanotti, G. “Structures of two mutants of B subunit of heat-labile enterotoxin from E. coli”, in preparation 8. Hogata, C.M. “MAD phasing grows up” Nature Struct. Biol., 5, 638643 (1998) 9. Hendrickson, W.A. and Hogata, C.M. “Phase determination from multiwavelength anomalous diffraction measurements” in Methods in Enzymology, Vol. 276, Part A, Academic Press, N.Y., pp.494-523 (1997) 10. Terwilliger, T.C. “MAD Phasing: treatment of dispersive differences as isomorphous replacement information” Acta Cryst. D50, 17-23 (1994) 11. Doublié, S. “Preparation of selenomethionyl proteins for phase determination” in Methods in Enzymology, Vol. 276, Part A, Academic Press, N.Y., pp. 523-530 (1997) 12. Giacovazzo, C., Monaco, H.L., Viterbo, D., Scordari, F., Gilli, G., Zanotti, G. and Catti, M. “Fundamentals of Crystallography” Oxford University Press, Oxford, (1992) 13. Smith, G.D., Blessing, R.H., Ealick, S.E., Fontecilla-Camps, J.C., Hauptman, H.A., Housset, D., Langs, D.A. and Miller, R. “Ab initio structure determination and refinement of a scorpion protein toxin” Acta Cryst. D53, 551-557 (1997) 14. Frazão, C., Carrondo, M.A. and Sheldrick, G.M. “Ab initio determination of the crystal structure of cytochrome C6. Comparison with plastocyanin” Structure 3, 1159-1169 (1995) 15. Wood, I.G., Thompson, P. and Matthewman, J.C. “Crystal structure refinement from Laue photographs taken with synchrotron radiation” Acta Cryst. B39, 543-547 (1983) 16. Hajdu, J., Machin, P., Campbell, J.W., Greenhough, T.J., Clifton, I., Zurek, S., Gover, S. Johnson, L.N., and Elder, M. “Millisecond X-ray diffraction and the first electron density map from Laue photographs of a protein crystal” Nature 329, 178-181 (1987) 17. Helliwell, J.R. “Macromolecular crystallography with synchrotron radiation” Cambridge University Press, Cambridge (1992) 18. Moffat, K., Bilderback, D. and Schildkamp, W. “Laue photography from protein crystals” in Synchrotron Radiation in Structural Biology, Sweet, R.M. and Woodhead, A.D. eds, Plenum Press, N.Y., pp. 325330 (1989) 19.Gabor, D. Nature, 161, 777-778 (1948) 20. Harp, G.R., Saldin, D.K. and Tonner, B.P., Phys. Rev. Lett. 65, 10121015 (1990) 21. Tzege, M. and Faigel, G. “X-ray holography with atomic resolution” Nature, 380, 49-51 (1996) 22. Szöke, A. “Holographic methods in X-ray crystallography. II. Detailed theory and connections to other methods of crystallography” Acta Cryst. A49, 853-866 (1993) 23. Xu, G. “Solving the phase problem of X-ray diffraction using atomic resolution X-ray holograms” Acta Cryst. A53, 236-241 (1997)

Notes a. Protein sequences are available through specialized data bank, like SWISS PROT (ref. 2) b. Brightness and brilliance are defined as: brightness = photons/sec/0.1%δλ/λ/mrad2 brilliance = photons/sec/0.1%δλ/λ/mrad2/mm2

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the former defines a well collimated source, the latter a source of small size and well collimated. c. Decreasing the wavelength, for example around 5-6 Å, anomalous dispersion can be observed for S and P atoms. Experiments made by the group of Prof. Sthurman in Hamburg have shown that these kind of measurements are possible. The operating experimental conditions are nevertheless quite hard to manage: at these wavelengths, for example, the absorption of X-rays by the air is determinant and it is necessary to operate in the vacuum. At present the approach is still under development. d. This number is definitely underestimated, since most of the directmethods programs are at present able to solve structures with a bigger number of atoms, not necessarily in a fully automatic way. As a matter of fact, crystals with more than about 100 atoms in the asymmetric unit are not common, since medium-size molecules are difficult to crystallize. e. Relathionship (4) is valid for N equal atoms in the unit cell. If the atoms are different, as normally happens, (4) takes a slightly more complicated form, but our considerations are not affected.

f. The previous statement is true for ESRF (Grenoble) and ELETTRA (Trieste) and it is reasonable to assume that the situation cannot be too different in other facilities around the world. g. As pointed out by Szöke (ref. 22), the crystallographic technique can be considered a sort of holography, for example making the assumption that, if a portion of the electron density in the cell is known, the complex amplitude of the diffracted wave can be considered as the reference wave and used to reconstruct the hologram. This is in fact what happens in real diffraction experiments. Otherwise, in a holographic experiment it is assumed that phases are recovered directly from the experiment itself. h. An holographic approach for the visualization of the entire molecule has been recently proposed, but not yet applied in practice (ref. 23).

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Articolo ricevuto in redazione nel mese di Ottobre 1998

DEVELOPMENT OF MICROFOCUS X-RAY ZONE PLATES M. Gentili, E. Di Fabrizio Istituto di Elettronica dello Stato Solido - CNR, Via Cineto Romano 42, I-00156 Roma, Italy

Conventional electron microscopes cannot be used to investigate biological samples under their natural conditions, due to the need for a vacuum environment. X-ray microscopy, instead, provide a different way of studying biological specimens giving in principle the possibility of observing living cells or similar materials at atmospheric pressure. On the other hand, x-ray focusing is a formidable challenge, since the refractive indexes of all materials is close to unit at the used wavelength.

A different way to focus x-rays is to use a Zone Plate, which is essentially a circular diffraction grating made of alternate concentric rings of absorbing material and transparent ones. Zone Plates combined to high brightness X-ray sources, such as those provided by third generation synchrotrons, open a new path for the investigation of samples by means of micro-focused xrays. This paper describes the Zone Plate development activity carried out at the Solid State Electronics Institute of the CNR in Rome, Italy. It will be demonstrated that by making use of Electron Beam Lithography it is possible to design and fabricate soft X-ray zone Plates whose minimum feature size is well below 100 nm.

Introduction X-ray microscopy is living a period of fast development, which is having strong impact on many domains of science and technology. Thanks to the new possibilities offered by microfocusing techniques, X-rays are finding application in new fields, and are opening new avenues in disciplines that have been using them for a long time.

This rapid progress was triggered by two combined factors: on one hand the recent technological developments in the fabrication of microfocusing optical elements for X-rays, which have pushed their resolution limit to the nanometer scale; on the other, the advent of third generation synchrotron radiation sources, which provide enough brightness to make efficient use of this high spatial resolution. In this report the main aspects involved in manufacturing of high resolution Zone Plates (ZPs) for soft X-ray focusing are described. In particular, we show how direct patterning by means of electron beam lithography is used to draw on thin membranes ZPs with critical dimensions down to 70 nm. These features of nanometer size are successfully transferred into gold or nickel by plating, producing efficient absorbing media for the desired wavelength. The precision reached in the

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control of feature size and aspect ratio, line-to-space ratio and external diameter makes this patterning approach particularly suitable to cover a wide range of microfocusing applications and illumination wavelengths. The developed process allows reproducible results with sub100 nm resolution absorbers made of gold or nickel.

Fresnel ZPs It is well known that focusing of X-rays is a very difficult task. The refractive index of most of the materials at such a wavelength is close to unity so that X-rays are inherently difficult to bend by means of conventional optics such as refractive lens [1]. A conventional refractive Xray lens would have an impractical focal length when designed to operate with a refractive index close to the unity, as well as its depth of focus would be extremely limited [2]. On the other hand, reflection focusing with a refractive index close to unity implies that conventional mirrors would be operated at a very small X-ray beam angle of incidence [3]. Normal reflection focus can be achieved by exploiting Bragg reflection from crystal planes or by making use of ultra-precise and ultra-smooth multilevel X-ray mirrors; however this choice is particularly challenging due to the difficult in manufacturing them [4,5,6]. Diffraction can, however, be used to focus efficiently X-rays. A Zone Plate (ZP) is essentially a circular diffraction grating made of alternating concentric rings of absorbing and transmitting materials, where the spatial modulation of the single ring follows a precise law: r2n= nfλ+(nλ)2/4

(1)

where r 2n is the radius of the nth zone, f is the focal length and λ is the wavelength with which the ZP is to be illuminated [7]. Figure 1 shows a typical ZP layout where the black zones represents the absorbing region and the white zones the transmitting one.

ZPs are not ideal optical elements; in fact they suffer from chromatic aberration and, since they are essentially diffraction gratings, also produce several diffractive focusing orders, which result in many coaxial foci to form.

In addition, ZPs working in amplitude mode, where alternate zones are blocking completely the destructive in-

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ped at room temperature in a mixture of one part of Methil-Isobutil-Ketone (MIBK) and three part of Iso-Propil-Alcohol (IPA) for two minutes.

The following relation relates the resolution of any ZP to the outermost zone width: R=1.22 drn

Fig. 1 Typical Zone Plate layout.

terference can produce focusing efficiency not higher than 10%. High resolution ZPs can be fabricated using different pattering techniques including, electron and Xray lithography [8,9], holography [10], and slicing technique [11]. Other fabrication methods, such as those relaying on scanning probes have still to prove effectiveness and reliability for ZP patterning. So far the best results in terms of efficiency and resolution are achieved by electron beam lithography patterning followed by a suitable absorber manufacturing technique. This report describes the fabrication process for soft X-ray ZPs developed at the Solid State Electronics Institute of the National Research Council in Rome (Italy).

(2)

where R is the focusing resolution and drn is the outermost zone width. Therefore, any nanofabrication procedure must comply with the requirements of having adequate resolution whilst maintaining flexibility for the generation and the placement of the many circular rings composing the ZP itself. The electron beam lithography machine used for ZP exposure is a Leica Microsystems Lithography EBMF 10 working at accelerating voltage up to 50 kV, and equipped with an exaboride lanthanum electron emitter. Since most of commercial e-beam systems do not have a specific polar pattern generator, a software capable to approximate the circular feature requested in ZP patterning, with the available set of feature primitives which include rectangles and polygons, had to be developed [12]. This software also compensates for both forward and backward scattering effects in resist and substrate. This correction, which in most of the high resolution application is mandatory, is called proximity effect compensation [13]. The developed algorithm assigns the single zone exposure dose as a result of all neighbored zone-zone interactions and thus avera-

Description of the fabrication process • Substrate Preparation and Patterning by Electron Beam Lithography The standard substrate used are silicon nitride (SN) membranes 100 nm thick, made by means of standard wet etching on silicon wafers with chemically vapor deposited layers of SN. The dimension of membrane windows depends on the desired ZP diameter, and in our case ranged from 100 to 500 micron. Substrates were first covered with a double layer of chromium (7 nm) and gold (7nm) which was evaporated directly onto the surface making it electrically conductive as required by the subsequent absorber formation electroplating process.

Figure 2 shows schematically the various fabrication steps involved in ZP manufacturing. The recording media used for the e-beam exposure (resist) was Poly-MethilMethacrylate (PMMA) having molecular weight of 950 K, which was spun on the substrate for a thickness of 200-300 nm. After exposure, the samples were develo-

Fig. 2 Fabrication steps involved in Zone Plate manufacturing.

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Fig. 3 Monte Carlo calculated absorbed dose after proximity effect compensation.

ging locally the resist absorbed dose. Otherwise, when passing from the center of the ZP to its outermost part the electron scattering effects would result very different, leading to a non-uniform absorbed dose. The effectiveness of the correction is shown in Figure 3. Figure 3 shows for a 40 kV exposure and an e-beam spot size of 50 nm the assigned dose as a function of the single exposure pixel (upper curve) and the resulting convolution on the actual 150 micron diameter ZP (lower curve). As it can be seen, after the correction the dose is correctly equalized to the unit (resist threshold or clearing dose).

This guarantees, in principle, that the ZP feature size would result all correctly exposed.

• Metallization After the development, the substrate was electrically contacted by selectively removing a small PMMA region; this was performed with the aid of oxygen reactive plasma and by protecting the ZP pattern with a metallic mask. Plating was carried out by a commercial apparatus; a typical growth rate of 100 nm/min ensured a deposited metallic film which exhibits a nanometer grain size.

• Inspection and Zone Control Patterning of ZP with outermost zone widths below 100 nm and external diameter exceeding 100 µm introduces a great challenge in e-beam exposure. First, resolution requirements are demanded for external zones where, ebeam in-field distortions are the highest; secondly, large diameter ZPs force the use of adequately large field sizes, which in turn, always result in a less dense e-beam addressing grid. This latter fact is of paramount importance in ZP patterning; the approximation of circular features with rectangular primitive shapes, always demands a dense beam addressing grid, which gets less dense when the deflection field size is progressively increased. As general rule, the greater the field size, the larger the distortion errors. In order to minimize e-beam scale errors, which could be introduced if the writing

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plane is put higher than the calibration one, special calibration routines were used by making use of a geometrically regular (square) high contrast (200 nm thick gold) marker fabricated on the substrate itself. Under these conditions, and for the used writing fields ( less than 500 µm), it is expected that our e-beam machine has a pattern placement accuracy which is a small fraction of the minimum line-width and close to the laser interferometer intrinsic precision (4.6 nm). However, the exposure and the resist development influence the trade-off between resolution and ZP diameter. Errors in the ZP pattern can be introduced during the fabrication process; non-circular zone pattern or misplacement of zones are accountable errors in manufacturing, and introduce aberrations. For amplitude ZPs, partial transmission of the absorbing media, irregularities in line-width of zones and in line-to-space ratio as well as deviation from a perfectly rectangular zone profile, also affect the efficiency.

• Resolution, line-to-space ratio and resist profile Resolution in electron beam lithography is a strong function of both forward and backward scattering effects in the resist and the substrate [14]. Due to the very thin substrate (100 nm silicon nitride), most of the incoming electrons pass through the membrane, and therefore a weak effect of electron backscattering from the substrate is expected. Monte Carlo electron scattering simulation was carried out to evaluate quantitatively this amount [15]; only 2 % of electrons are reflected back and their contribute is negligible in terms of a proximity effect [16]. More important is the spreading of the electron beam passing through the resist; i.e. forward scattering effect (FS). At 40 kV, Monte Carlo simulation indicates that beam spreading caused by FS is about 50 nm for a 250 nm thick resist. Figure 4 shows the Monte Carlo calculated point spread function (proximity function) along with its analytical approximation by means of a linear combination of gaussian functions. The point spread function represents the behavior of the normalized expo-

Fig. 4 Monte Carlo proximity function and multi-gaussian fit.

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tioned effects, also the resist development time, the exposure dose or a very aggressive developers can alter significantly the targeted line-to-space ratio, and therefore the diffraction ZP efficiencies. Nominal line-width values have to be normally smaller than the final pattern, because FS and spot size are finite; this procedure, which consists in altering (reducing) the dimension of the patterns to be written, is called “biasing”. The typical amount of bias introduced in our process is in the range of 30-50 nm.

Fig. 5 Effect of e-beam spot size variation on Monte Carlo calculated absorbed dose.

sure dose for an infinitely small beam spot size incoming on a particular experimental system (substrate and resist). The most intense and collimated peak accounts mainly for the electron forward scattering effect into the resist, whereas the broader distribution is related to the electron backscattering arising from the substrate. It is noticeable the “noisily” character of the backscattering distribution; this is caused by the relatively low amount of electrons coming from the substrate. This distribution extends for only 1.3-1.4 µm and is several orders of magnitude less intense than the collimated part accounting the forward scattering Therefore, it can be concluded that the attainable ultimate resolution in this experimental system is related to the e-beam spot size, FS effect and degree of approximation of the circular features. Figure 5 shows the effect of e-beam spot size variation on the modulation of the absorbed dose for an high resolution ZP. A 30 nm beam spot size increase results in a change of about 40 % of the absorbed dose modulation (peak-to-valley distance). In addition to the above men-

• Aspect ratio Aspect ratio is the ratio between height and width of a given feature. It is known that high aspect ratio feature can collapse due to the lack of mechanical strength [17].

In the case of soft X-ray ZPs, in order to have in half contrast and hence good efficiency too, gold plated structures should be 120-300 nm thick. This introduces a practi-

Fig. 7 SEM micrograph showing a 70 micron diameter gold Zone Plate.

cal limit in the resist thickness, which in turn has to be as thick as 350 nm. Under these conditions a 70 nm resolution feature exhibits an aspect ratio that can exceeds four. In addition, we have observed more pronounced mechanical instability in case of ZPs rather than in fabricated conventional one-dimensional grating. The resist mechanical stability problem at sub-100 nm level is still an unknown process and more investigations are needed to clarify its behavior. We observed that best results can be achieved by making use of fresh developer and by stirring the sample during its development.

Fig. 6 SEM micrograph showing a 150 micron diameter gold Zone Plate.

Examples of fabricated Zone Plates In this section a few examples of fabricated ZPs will be presented. They are all made of gold on silicon nitride and most of them are in use or have been used for spectro-microscopy applications at several synchrotron radiation facilities world-wide, including ELETTRA in

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ge on the 70 nm resolution outermost zone. Microscopy test carried out on some of the fabricated ZP have confirmed the excellent quality of these devices that have performed high resolution focusing with efficiency very close to 10% (theoretical limit) [18].

Fig. 8 Close up of the outermost zones of a sub-100 nm resolution Zone Plate.

Trieste, Italy and the Advance Light Source in Berkley, CA, USA. Figure 6 shows a scanning electron micrograph taken at low magnification on a finished 150 micron gold ZP made on silicon nitride. The central bright spot is the so-called apodization region, that is a thicker gold pad (about 1 micron thick) necessary to stop the unwanted zero diffraction order. The dark square in the center is the thin silicon nitride membrane supporting the ZP itself. The three small bright features visible at the corner of the pattern are the calibration marker used to locate and align the exposure field to the pattering

Fig. 9 Detail of 70 nm resolution gold absorbers.

area. Figure 7 is a close-up showing a 70 micron diameter ZP, the interference patterns visible on the picture are the so-called Moiré figures resulting from the superposition of the scanning grid of the electron microscope used to take the image and the circular gratings composing the ZP. Figure 8 is a close up of a group of outermost zones, whereas Figure 9 shows a high-resolution ima-

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Conclusions A fabrication process for sub-100 nm resolution, large diameter, amplitude soft x-ray ZP has been developed. It makes use of electron beam nanolithography and gold plating. In particular the issues of data preparation and optimization, resolution, control of line-to-mark ratio, have been addressed in detail by means of Monte Carlo electron scattering modeling, proximity effect correction and experiments. Under optimized conditions, 70 nm resolution. Measured efficiency for amplitude ZP was 9.5 % very close to the theoretical limit of 10%.

Acknowledgments The authors wish to thank colleagues who contributed during the past years to the development of this technology, in particular we wish to acknowledge Dr Marco Baciocchi, Dr Luca Grella, Mr Luigi Mastrogiacomo and Mr Romano Maggiora.

References 1. E. Spiller, in Handbook of Synchrotron Radiation, Vol. 1, E.E. Koch Ed., North Holland Amsterdam 1983.

2. J.H. Underwood and D.T. Attwood, Phys. Today, 37 (4), 44, (1984) 3. A. Franks, “X-ray Optics”, Sci. Prog. 64, 371, (1977) 4. T. W. Barbee, in X-ray Microscopy, 114, ( Springer, Berlin 1984), 5. J. Kirz and H. Rarback, Rev. Sci. Instruments, 56 (1), 1, (January 1985) 6. H. A. Padmore, G. Ackerman, R. Celestre, C-H Chang, K. Franck, M. Howells, Z. Hussain, S. Irick, S. Locklin, A.A. McDowell, J.R. Patel, S.Y. Rah, T.R. Renner and R. Sandler, Synchrotron Radiation News, Vol. 10, No. 6, (1997).

7. J. Soret, Arch. Sci. Phys. Nat. 52, 320, (1875) 8. Y. Vladimirski, D. Kern, T.H.P. Chang, D. Attwood, H. Ade, J. Kirz, I. McNutty, H. Rarback amd D. Shu, J. Vac. Scie. Technol., B 6 (1), (Jan/Feb 1988) 9. D. C. Shaver, D.C. Flanders, N.M. Ceglio, H.I. Smith, J. Vac. Scie. Technol. 16 (6), 1626, (Nov/Dec 1979) 10. C. David, in Springer Series in Optical Science, Vol. 67, X-ray Microscopy III, eds.: A. Michette, G. Morrison and C. Buckely, Springer Verlag, Berlin Heidelberg, 87, (1992) 11. D. Rudholp, S. Niemann and G. Schmahl, High Resolution X-ray Optics, Proc. SPIE 316,103, (1981) 12. E. Di Fabrizio, L. Grella, M. Baciocchi, M. Gentili, D. Peschiaroli, L. Mastrogiacomo and R. Maggiora, Jpn. J. App. Phys. Vol. 35, 2855, (1996) 13. T.H.P. Chang, J. Vac. Scie. Technol. 12, 1271, (1975).

J.S. Greenich, Electron Beam Processes, in Electron Beam Technology in Microelectronic Fabrication, Edited by G.R. Brewer, Academic Press, 1980 D.F. Kyser and R. Pyle, IBM J. Res. Develop. Vol. 24, No.4, (July 1980).

M. Gentili , A. Lucchesini , P. Lugli , G. Messina , A. Paoletti , S. Santangelo, A. Tucciarone and G. Petrocco, J. Vac. Scie. Techol. B 7 (6),1586, (1989) Tanaka, M. Morigami and N. Atoda, Jpn. J. App. Phys., 32 , 6059, (1993) Morris, M. Gentili, M. Baciocchi, S. Contarini, P. De Gasperis, C. Gariazzo, M. Kiskinova, R. Maggiora, P. Melpignano, N. Minnaja, M. Musicanti, P. Nataletti and R. Rosei, Proceedings of the XIII International Congress on X-ray Optics and Microanalysis, 539-542, August 1992 Manchester U.K.

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Articolo ricevuto in redazione nel mese di Novembre 1998

ORIENTATIONAL CORRELATIONS AND HYDROGEN BONDING IN LIQUID HYDROGEN CHLORIDE C. Andreani Dip. Fisica, Università degli Studi di Roma Tor Vergata, Istituto Naz. per la Fisica della Materia, Unità di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy

A.K. Soper ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K, and Dept. of Physics and Astronomy, University College London, Gower Street. London WC1E 6BT, U.K.

M.A. Ricci, M. Nardone, F.P. Ricci Dip. Fisica “E. Amaldi”, Università degli Studi di Roma Tre, Istituto Naz. per la Fisica della Materia, Unità di Roma Tre. Via della Vasca Navale 84, 00146 Roma, Italy.

Neutron diffraction studies aimed to look at orientational correlations among molecules in a liquid have always been limited by the intrinsic difficulty that the response of the system to the probe is a weighted sum of all the independent site-site correlation functions.1 It is nevertheless now well established that for hydrogen containing liquids one can exploit the large change in the coherent neutron scattering length existing between H and D isotopes by performing neutron diffraction experiments with Isotopic H/D Substitution (NDIS)2 and thus extract three independent site-site correlation functions. In simple molecular fluids, such as the hydrogen halides, this technique allows the extraction of the whole set of independent functions.3,4 In these molecular liquids the availability of the whole set of sitesite correlation functions is ‘’per se’’ of great help in understanding the orientational correlations, although a detailed knowledge of the entire angular pair correlation function cannot be achieved. As a matter of fact the angular pair correlation function contains more information than contained in the set of site-site correlation functions, since the latter are averages of the angular pair correlation function over the molecular orientations, keeping the site-site distance fixed.5 Deeper insight into the orientational correlations can be obtained only by interpreting the experimental data along with appropriate computer simulations, such as Reverse Monte Carlo6, Molecular Dynamics7 (MD) or the recently developed Empirical Potential Structural Refinement8 (EPSR).

Molecular liquids composed of hydrogen halides have two features in common, which can be of help in understanding the orientational correlations. First the centre of mass is approximately coincident with the halide atom, which means that the halide-halide correlation function extracted from the diffraction experiment coincides with the centres of mass radial distribution function to a good approximation. Secondly the hard core part of the intermolecular potential is almost spherically symmetric: therefore the orientational correlations can be interpreted in

terms of electrostatic multipolar interactions and hydrogen bond formation. In particular the anisotropy of the intermolecular potential is strongly varying within the halide series, since the dipolar contribution decreases when going from hydrogen fluoride to hydrogen iodide, while the quadrupole contribution increases.5,9 Previous neutron diffraction studies10-12 and MD simulations13-15 suggest that hydrogen chloride is likely to form weak hydrogen bonds. To better investigate this point and to complete our study on the orientational correlations in hydrogen halides3-4,9 we have performed a detailed NDIS study on liquid hydrogen chloride at two thermodynamic states (in the vicinity of the melting point and of the critical point respectively) and applied the EPSR method to produce three-dimensional configurations of HCl molecules compatible with the experimental site-site radial distribution functions. From these configurations selected angular pair correlation functions have been calculated.

Figure 1 shows the probability, g(r,θL), that, given a molecule in the origin, oriented along the z axis, another molecule, however oriented, is found at a distance r in the direction θL at the lowest temperature state. Two shells of neighbors are identified in Figure 1: the first at r= 35nm and the second at r= 65nm. Similar maps although with broader peaks are obtained at the other state point. The function g(r,θL) shows a strong dependence on θL at r values close to the first neighboring shell, indicating, contrary to what would be expected for an uncorrelated model liquid, a clear preferred orientation of the intermolecular axis within a cone of half width θL =50°. In other words, while the preferred distance of the first neighboring molecules around an oriented molecule is the same in all θL directions, they are not isotropically distributed, since they prefer to lie in the axial direction. It is worthwhile noticing that, since the Cl site is close enough to the molecular center of mass, deviations from the uncorrelated model results, and in particular the existence of a hydrogen bond peak in the Cl-H radial distribution function, depend on the

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Fig. 1 Map of the of the probability, g(r,θL), that, given a central HCl molecule in the origin, oriented along the z axis, another molecule, however oriented, is found at a distance r in the direction θL, at T=194 K.

strong directionality observed in the first neighbors distributions.

From the EPSR simulation we have also calculated the probability that, given a central HCl molecule in the origin, oriented along the z axis, another molecule lies at a distance r, with orientation defined by the polar axis θM, , at nine selected θL values, g(r, θM,θL=costant). This analysis shows that at both state points the majority of the first neighboring molecules, which lie in a region where 0< θL <20° or 160°< θL <180°, have their dipoles almost aligned. Very few L-shaped configurations are found at θL =90°, where however a preferred relative orientation of molecules is hardly detectable. This analysis suggest the existence of strong correlations between the intermolecular vector and the molecular axis. A closer look at the preferred orientations at θL =0°, is given in Figure 2, where the function g(r, θM,θL=0) at both state points is reported. In the vicinity of the melting point, although first neighboring molecules seen along the positive z direction strongly prefer to align their dipole parallel to the dipole of the molecule at the origin,

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significant density of molecules is also found at θM=70°, while no antiparallel dipole orientations are found. The angular distribution sharpens on going towards the critical point, indicating a better alignment of the dipole moments at the lower density state, while the intensity of the shoulder at θM=70° decreases, and only a few antiparallel configurations appear.

The EPSR analysis of the HCl data gives strongly different results in comparison with those found by the spherical harmonics analysis of the HI16 and HBr4 data. In those liquids the correlation between θL and the molecular orientation was found to be very low, with the majority of the molecules forming an angle θM=45° with the z direction and almost the same number of parallel and antiparallel dipoles. These differences may be due to the different dipole and quadrupole moments associated with the three molecules or to the presence in liquid HCl of hydrogen bonds, which implies stronger directionality in the local coordinations.

Because the Cl-H bond direction coincides with the HCl dipole moment direction, a suitable geometric definition

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of the H-bond in this system is not available at the present time, unlike the case of liquid water where the O-H bond direction is distinct from the water molecule’s dipole moment direction. What is clear already however is that there appear to be competing dipolar and hydrogen bonding interactions in HCl, and as the temperature is raised and the density is lowered the dipolar interactions tend to dominate the structure.

In summary this study has shown that although strong orientational correlations between HCl molecules are present at both states investigated, the radial distribution function of molecular centres is similar to that found in so-called ‘’simple liquids’’; thus implying that the short range electronic overlap forces in liquid HCl are spherically isotropic.

Electrostatic and hydrogen bonding forces must therefore be primarily responsible for the observed orientational correlations. These correlations are characterized by a strongly anisotropic angular distribution of first neighboring molecules, which is markedly peaked around θL=0°. First neighboring molecules along this direction prefer to align their dipoles parallel to the molecule at the origin, although, at the lowest temperature also a relevant fraction of molecules forming an angle θM=70° with the molecule at the origin is found. We believe that these molecules are those engaged in hydrogen bonds.

Going towards the critical point the number of these molecules decreases in favor of those with parallel orientation. This assignment is supported by the evidence for zig-zag structures with θM=87° in the crystalline forms of HCl17 and by its dependence on the thermodynamic state. The presence of hydrogen bonds is traditionally associated with the presence of a peak at r=24nm in the Cl-H radial distribution function: this peak is indeed present in our data. The behavior of its intensity is consistent with that of the number of first neighboring mole-

cules around θL=0° forming an angle θM=70°.

We stress that liquid HCl looks different from the other hydrogen halides studied so far, because in the other two systems no evidence was found for the presence of hydrogen bonds and the correlations between the intermolecular vector and the molecular orientations were also very weak.3,4,16 References 1. S. W. Lovesey, “Theory of Neutron Scattering from Condensed Matter”, vol.1 (Clarendon Press, Oxford, 1984).

2. A. K. Soper, in “Methods in the Determination of Partial Structure Factors”, edited by J.B.Suck, D. Raoux, P.Chieux and C. Riekel, (World Scientific Publishing, London, 1993), p.58.

3. C. Andreani, M. Nardone, F. P. Ricci, and A. K. Soper, Phys.Rev.A 46, 4709 (1992).

4. C. Andreani, F. Menzinger, M. A. Ricci, A. K. Soper, and J. Dreyer, Phys.Rev.B 49, 3811 (1994).

5. C. G. Gray and K. E. Gubbins, “Fundamentals, Theory of Molecular Liquids”, vol.1 (Oxford University Press, New York, 1984).

6. R. L. McGreevy, and M. A. Howe, Annual Review of Material Science 22, 217 (1992).

7. M. P. Allen, and D. J. Tildesley, “Computer Simulation of Liquids”, (Oxford University Press, Oxford, 1987).

8. A. K. Soper, Chem. Phys. 202, 295 (1996).

9. C. Andreani, F. Menzinger, M. Nardone, F. P. Ricci, M. A. Ricci, and A. K. Soper, in “Hydrogen Bond Networks”, edited by M. C. Bellissent-Funel and J. Dore, NATO ASI Series, Series C, 435, 113, (Kluwer Academic Plublishers, Dordrecht, 1993).

10. J. G. Powles, E. K. Osae, J. C. Dore, and P. Chieux, Mol. Phys. 43, 1051 (1981).

11. A. K. Soper, and P. A. Egelstaff, Mol. Phys. 42, 399 (1981).

12. T. Bausenwein, H. Bertagnolli, K. Todheide, and P. Chieux, Ber. Bunsenges. Phys. Chem. 95, 577 (1991).

13. J. G. Powles, W. A. B. Evans, E. McGrath, K. E. Gubbins, and S. Murad, Mol. Phys. 38, 893 (1979).

14. M. L. Klein, and I. R. McDonald, Mol. Phys. 42, 243 (1981).

15. D. Gutwerk, T. Bausenwein, and H. Bertagnolli, Ber. Bunsenges.

Phys. Chem. 98, 920 (1994).

16. A. K. Soper, C. Andreani, and M. Nardone, Phys.Rev.E 47, 2598 (1993).

17. “Comprehensive Inorganic Chemistry”, edited by J.C. Bailar, H.J.Emeleus, Sir Ronald Nyholm, and A. F. Trotman-Dickenson (Pergamon Press, Oxford, 1974).

Fig. 2 Map of the probability, g(r, θM,θL=0°), that, given a central HCl molecule in the origin, oriented along the z axis, another molecule lies at a distance r, in the direction θL=0°, with orientation defined by θM : a) T=193 K; b) T=313 K.

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Articolo ricevuto in redazione nel mese di Settembre 1998

INFRARED SYNCHROTRON RADIATION, A NEW TOOL FOR THE ITALIAN SCIENTIFIC COMMUNITY P. Calvani Istituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Università La Sapienza, P.le A. Moro, 2, 00185 Roma, Italy

After a couple of decades of attempts, the aim of extending the unique performances of the synchrotron source to the infrared domain is achieved by about ten dedicated beamlines in different countries. With their high-brilliance, broad-band radiation one may perform experiments that are out of the range of conventional black-body sources. The first Italian infrared beamline, SINBAD, is presently under construction on the new double-ring DAΦNE of Laboratori Nazionali INFN di Frascati.

Introduction Infrared spectroscopy probes the rotations and the vibrations of molecules (their “finger-print” spectral region), the low-energy excitations of solids (phonons, excitons, polarons, etc.), the forces between a surface and an adsorbate, and many other low-energy phenomena of basic importance for condensed matter physics, chemistry, biophysics, and materials science.

A giant step in the detection of infrared spectra was made in the Sixties with the introduction of Michelson interferometers coupled to Fourier-transforming computers. On the other hand, until mid-eighties the only broad-band radiation sources available for this technique were those of the early experiments, namely globars or mercury lamps. The emittivity of a globar, which can be roughly approximated by that of a black body heated at 1500 or 2000 K, is peaked at λ≈5 µm, and decreases rapidly for λ>20 µm, the crucial region of the far infrared. A mercury lamp has slightly better performances at large λ’s. Moreover, just a small fraction of the power emitted by these sources on a 4π solid angle can be focused on the sample.

The spectra collected with conventional sources are then affected by a poor signal-to-noise ratio whenever the experiment is made under non- standard conditions. Modern spectroscopy may demand a high source stability, like in differential spectroscopy, or a large angle of incidence, as in surface science, or a small spot, as in infrared microspectroscopy and in experiments at high-pressure. As, moreover, Fourier-transform spectroscopy requires a broad-band source, the answer to these requirements is the extension to the infrared of the use of synchrotron radiation, with its high brilliance over a “white” spectrum and its absence of thermal fluctuations.

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A short history The development of InfraRed Synchrotron Radiation (IRSR) has been less rapid than one may expect on the basis of its present success. This has been partly due to technical reasons, partly to the spectacular development of SR in the UV and X-ray domain, that has heavily engaged the scientific and budgetary effort of the research institutions in the last three decades.

The first attempts aimed at extending the use of the synchrotron source to the infrared date back to the Seventies, when pioneering observations were made in Stoughton (USA) by Stevenson and coworkers1 and in Orsay (France) by Lagarde and coworkers.2 At the beginning of the 80’s, a port dedicated to the extraction of IRSR was built on the ring of Daresbury (GB) by Yarwood.3 This project was interrupted for a few years but a member of the Daresbury group, Takao Nanba,

Fig. 1 The ultra-high-vacuum section of SINBAD under test in summer 1998. The 3m-long front-end which connects the beamline to DAΦNE through a series of rapidly-swithcing valves can be seen on the left. The two chambers in the middle contain the extraction mirror and the focusing ellipsoid, together with their motors.

built in Japan on UVSOR, in 1985, the first IRSR beamline routinely open to users.4 In 1987 Gwyn Williams inaugurated the first American infrared beamline at Brookhaven,5 which triggered the rapid development of the IRSR in the USA. The first efficient European beamlines were realized in the early 90’s at Lund (Sweden) by Bengt Nelander,6 at Orsay by Pascale Roy

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and coworkers,7 at Daresbury by the group of Michael Chesters.8 The realization in Frascati of an IRSR beamline for the Italian spectroscopic community was first proposed by A. Marcelli and P.C. in 1993.9 The old ring ADONE was about to be dismanteled, for leaving place to the “Φfactory” collider DAΦNE, at that time already designed and funded. The high current (2A) and the low energy (0.51 GeV) foreseen for DAΦNE made this double-ring an ideal source of infrared radiation. The optical layout of SINBAD (Synchrotron INfrared Beamline At DAΦ− NE) was calculated by Augusto Marcelli of INFN and Alessandro Nucara of INFM, who first applied ray-tracing simulation techniques to the infrared domain.10 The project was presented in 1996 by Emilio Burattini to Istituto Nazionale di Fisica Nucleare, together with that for two beamlines in the soft X-ray domain, and rapidly approved. INFN started the construction of SINBAD and of its laboratory in 1997 under the direction of Burattini and Marcelli. The measuring apparatus, which includes a Michelson interferometer suitably modified for high-vacuum operation, an infrared microscope, high-pressure cells, and several detectors, has been instead prepared at the Dept. of Physics of University La Sapienza. Other groups of users already involved in the project come from the Dept. of Earth Sciences of Università di Roma III (A. Mottana), the Dept. of Earth Sciences of La Sapienza (A. Maras), the Dept. of Physics of Università dell’Aquila (U. Buontem-

Fig. 3 Evoultion with time of the number of infrared beamlines in the world. The extrapolation is made by considering the beamlines under construction.

growing interest of industrial users for infrared microscopy. In Europe, as already mentioned, the development of IRSR has proceeded at a slower rate, in spite of the pioneering experiments made in France, Britain and Germany in the Seventies. In order to fill the gap with USA and Japan in this field, in 1995 the European Union has funded a network that has been coordinated by the Department of Physics of University La Sapienza. The network was aimed at providing a forum for the European groups involved in the realization and the exploitation of IRSR sources. It has greatly helped to realize the beamline of Frascati and to better exploit those of Orsay and Daresbury. Moreover, the network has organized the first international meetings in this field (University of Rome III, 1995; LNF-INFN Frascati, 1996, LURE-Orsay, 1997) and has published the first book entirely devoted to IRSR.[11] Characteristics of synchrotron radiation in the infrared

Fig. 2 The DAΦNE double-ring assembled in the experimental hall, end 1997.

po). After a series of successful tests on the vacuum chambers (Fig. 1), presently the beamline SINBAD is being assembled in the DAΦNE hall (Fig. 2) .

Nowadays there are fifteen IRSR beamlines in the world (see Fig. 3), if one includes SINBAD and the others under construction. Most of them have been installed in the USA (up to six at Brookhaven, in addition to those of Berkeley and Stoughton), in a context of

• Brilliance The gain in brilliance of infrared synchrotron radiation with respect to a conventional source may attain two orders of magnitude or more. This quantity is calculated in Fig. 4 for the radiation emitted by a bending magnet of DAΦNE as a function of the wavelength λ and for different angles of acceptance. The angle of acceptance is defined by the dimension of the port placed in front of the magnet. In turn, this selects the arc of electron trajectory which contributes to the emission. At long wavelengths the brilliance gain is of 103 or even more, a performance which can be surpassed only by a monoch-

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may produce serious problems in several applications, as microspectroscopy. At the NSLS of Brookhaven, this problem has been solved by a feedback which automatically corrects the electron orbit by acting on the electromagnetic fields along the ring. An economic solution may consist in transfering the infrared beam by a cylindrical waveguide, instead of using complicated optics.

The multiple reflections along the pipe should make the infrared beam intrisically stable against small displacements of the source, while the transmittance of the waveguide is comparable with that of standard mirror-optics beamlines.13

Fig. 4 The calculated gain in brilliance of SINBAD with respect to an ideal black body at 2000 K, in the infrared spectral region, for different angles of acceptance.

romatic source like a free-electron laser. The reflectivity of the diamond window, the absorption from the residual gas, and other effects which limit the optical transmittance of the beamline may reduce the gain on the black body, once measured at the sample position.

However one easily obtains for this latter, in the far infrared, values which range from 10 to 100 for samples smaller than ≈0.5x0.5 mm.

• Pulsed structure and polarization Other features of potential importance are the pulsed structure of IRSR and its polarization. The former will be fully exploited when fast detectors will be developed also for the far infrared. The typical cut-off frequency of a liquid-helium-cooled bolometer is presently 300 Hz, while the period of a radiation pulse sequence in singlebunch operation is shorter than 1 µs.

The polarization properties of infrared synchrotron radiation are more promising for immediate applications.

IRSR is linearly polarized in the plane of the electron orbit if collected on the plane itself, circularly polarized clockwise or anticlockwise if observed above or below the plane. By placing a slit on the exit port one can then

• Divergence The complicated behavior of the brilliance in Fig. 4 is due to the combined effect of the port size and of the natural divergence θnat of the synchrotron radiation. In the infrared domain this latter is much larger than in the UV or X region and depends on the photon wavelength λ through:12 θnat = 1.66 (λ/ρ)1/3

(1)

where ρ is the radius of curvature of the electron trajectory. As it will be described below, the large divergence predicted by Eq. (1) implies the use of focusing optics of large size for transfering the infrared beam to the detection apparatus.

• Stability Another remarkable advantage of IRSR with respect to thermal sources, whose intensity varies at random, is that the power delivered is intrinsically stable in time, and directly proportional to the current circulating in the beam. After correcting for the slow variation of this latter, that can be monitored in real time, IRSR provides the spectroscopist with an absolute radiation source.

This is of basic importance, for instance, in differential spectroscopy.

On the other hand, the IRSR spot at the sample may fluctuate due to spatial instabilities of the electron beam. These are amplified by the collecting optics and

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Fig. 5 Extraction system of the infrared radiation from a bending magnet at the port U4IR at Brookhaven (from Ref. 19). A flat mirror at 45° deflects the radiation on an aspherical mirror, often an ellipsoid, which focuses the radiation on a diamond window (not shown in the Figure). Here the selected angle is 90x90 mrad.

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select the desired degree of polarization. In the case of SINBAD, nearly 100% circularly polarized light can be obtained by a slit which selects 10% of the total flux available.14 Some interesting applications have been already explored successfully, like the extension to the infrared of studies of circular dichroism in magnetic materials.15 Sources and beamlines The most common sources of IRSR are bending magnets.

Nonetheless one beamline, at Orsay, extracts radiation from a wiggler while two others, those in Stoughton and Karlsruhe, will exploit the strong emission from the edge of a magnet, where the field rapidly drops.

In the case of a bending magnet, the large intrinsic divergence of IRSR in the infrared and the considerable size of the source require large aspherical mirrors which may transfer the radiation to the interferometer and focus the beam on its entrance pupil. The standard extraction system of IRSR from the bending magnet of a storage ring is shown in Fig. 5. A flat mirror, placed in front of the port at 45° with respect to the electron orbit, deflects the radiation cone on a focusing mirror, often an ellipsoid. In the case of the Figure, which refers to the port U4IR at Brookhaven, the angle of extraction is 90x90 mrad. If the beam energy E is high, much power W from “hard” radiation will also hit the extraction mirror, which has then to be water-cooled. Sometimes the first mirror has even to be partially shielded by an absorber as in Daresbury, where the beam energy is 2 GeV. The second mirror, typically an ellipsoid, focuses the radiation on a diamond window (DW). This isolates the ultra-high-vacuum section of the beamline, directly open on the ring, from the remaining part. The latter is kept under a high or low vacuum (depending on its length) for reducing the infrared absorption from air (both water vapour and carbon dioxide are strong IR absorbers due to their permanent dipole moments). In spite of its high cost, diamond is chosen for its excellent hardness and chemical stability, and for its “flat” transmittance in the whole infrared domain (except for an absorption band at ≈5µm). Moreover, diamond can be brazed on conflat flanges.

The first beamlines were equipped with natural diamonds, 2 cm2 or more in size and ≈0.2 mm thick. Cutting such windows could take several months. In a few cases, the window was placed at a Brewster angle with respect to the incident radiation, in order to use the polarization of this latter for reducing the interference effects due to multiple reflections. At SINBAD, a more efficient solution at a lower cost has been obtained with a CVD diamond film, with a wedging angle of ≈1°, realized under a collaboration with De Beers Industrial Diamond Division of England.16 At SINBAD the second focus is located at the entrance pupil of the Michelson interferometer, the same detec-

Fig.6 Intensity distribution at the last focus of SINBAD, placed at the entrance of the interferometer, as obtained by ray-tracing simulation. One may remark the small size of the image of the bending-magnet source.

tion device that one uses with conventional sources.

Even if the radiation source is a 2m-long bending magnet, at SINBAD its image on the pupil (Fig. 6) has a size of a few mm2 even at the longest wavelengths. The

Fig. 8 Spatial distribution of the intensity emitted at λ = 100 µm by (a) the edge of the bending magnet A2 of the ring SuperAco at Orsay, (b) the wiggler SU3 of the same ring with K = 5.38 (from Ref. 17).

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whole optical layout of SINBAD, one of the longest beamlines due to the constraints of the DAΦNE hall, is shown in Fig. 7.

As already mentioned, bending magnets are not the only sources of infrared radiation. Fig. 8 shows the spatial distribution of the radiation emitted at 100 µm by two unconventional sources: the edge of a magnet (top), where the field experiences rapidly drops to zero, and a wiggler or an undulator (bottom). Unlike for a uniform dipole (see Fig. 6), a characteristic minimum appears at the center of the emission pattern in both figures, due to coherence effects. One may also notice that most radiation provided by the wiggler is concentrated within a small angle.

Both the edge effect and the wiggler emission contribute to the radiation collected by the SIRLOIN beamline of LURE, at Orsay.7 Its peculiar extraction system has indeed provided first evidence for the infrared emission from the edge of a bending magnet.17 For λ>>d/γ2, where d is the length required to deflect an electron through an angle of order 1/γ, the edge radiation may be even brighter than that emitted from the uniform magnetic field region.18 Similar effects are found at both edges of an undulator.19 A beamline projected by R.A. Bosch and coworkers to exploit the edge effect is starting its opera-

ting life at the Aladdin storage ring of Stoughton (USA).

A similar one is being built by Y-L. Mathis at ANKA in Karlsruhe (Germany). The edge effect is instead expected to be negligible at SINBAD. Spectroscopy with infrared synchrotron radiation The unique features of IRSR allow the spectroscopist to collect higher quality data in a number of experiments usually performed with conventional sources. In a few cases, IRSR has opened to routinely infrared spectroscopy new fields of application, where too severe experimental conditions prevented the use of black body sources.

• Physics and chemistry of surfaces One of the earliest applications of IRSR has been the study of molecular adsorbates on metallic surfaces. The observation of their vibrational lines and of the shifts with respect to those of isolated molecule provides information on the molecule-surface interaction, on the eventual ordering of the adsorbate, on the amount of stress on molecular bonds. Such results are also of great interest for applied research, for instance in electrochemistry. As one is interested to monitor the intramolecular vibrations of molecules which are generally ali-

Fig. 7 Optical layout of SINBAD, the infrared beamline at Laboratori Nazionali INFN di Frascati.

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made at Okasaki (Japan) in the Eighties, at pressures on the order of 10 Gpa. Nowadays, infrared spectra at pressures even higher than 200 GPa are collected at Brookhaven by use of infrared microscopes.21 Among the basic results obtained by this technique one may cite the determination of the phase diagram of solid hydrogen up to 200 Gpa, by Hemley and collaborators. Spectra like that of Figure 10 (where the sample is solid deuterium) have excluded any metallization of hydrogen up to those pressures, imposing entirely new theoretical models for this system.

Fig. 9 The change in reflectance of a Cu(100) single-crystal surface induced by the adsorption of 0.5 monolayers of CO, as measured at the U4IR beamline of Brookhaven at grazing incidence and shown for two different resolutions (from Ref. 20) . The negative peak (with respect to the broad reflectivity background) monitors the carbon-metal vibrational mode, the positive peak corresponds to the hindered rotational mode.

gned orthogonally to the surface, the electric field of the infrared radiation should also be perpendicular to the surface. This implies that the angle of incidence should be as large as possible. Performing such experiments at grazing incidence with a conventional source, when moreover the adsorbate is a monolayer, is a very hard task. By use of IRSR one has: i) the high brilliance necessary to get a small and intense spot on the sample even at grazing angles; ii) the high stability needed to perform differential spectroscopy. One may thus obtain excellent signal-to-noise levels even in such severe conditions, as shown in Figure 9 for the far infrared absorption from a half-monolayer of CO adsorbed on copper.

• High-pressure studies in diamond anvil cells Infrared spectroscopy by use of the synchrotron source is presently considered as a powerful tool for investigating the properties of matter at high pressure. The high brilliance of IRSR is needed for obtaining appreciable results in diamond anvil cells, where the size of the windows is a few tenths of mm or smaller. The gain in sensitivity with respect to conventional sources may reach three orders of magnitude when using a Fourier transform interferometer, five orders when using a grating monochromator.20 Pioneering measurements by Nanba where

• Microspectroscopy The gain in brilliance of IRSR with respect to black bodies is also remarkable at shorter wavelengths, where an infrared microscope can work without experiencing major diffraction problems. The use of infrared microscopes coupled to synchrotron radiation sources has started recently, but has rapidly met with outstanding success. Figure 11 shows a spectrum taken on the inner part of the cross-section of a polymer laminate, commonly used as packaging material. The sheet comes out from the industrial process in form of a sandwich, where the recipe of the 10 µm-thick filling is unknown. The spectrum of such a thin inner layer, if taken by a microscope using a conventional source (Fig. 10a) does not help to solve the problem due to its poor contrast. The IRSR spectrum obtained by the group of M. Chesters at Daresbury (Fig. 10b) shows instead the “fingerprints” of the filling, which identify it as an ethylene/vinyl acetate copolymer.22

Fig. 10 Evidence that solid deuterium persists in its insulating phase up to 170 Gpa, provided by the absorption of infrared synchrotron radiation through a diamond anvil cell at Brookhaven (from. Ref. 21). The solid line is the observed absorption spectrum. The dashed lines are the Drude profiles expected for a metallic sample, for different possible values of the plasma frequency.

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Fig. 11 Infrared microspectrum of the central section (10 µm thick) of a polyethilene sheet with a conventional source (a) and with the Daresbury synchrotron source (b) (from Ref. 22).

An example taken from geophysical research is instead presented in Fig. 12. The visible-light objective of the microscope has selected in a rock sample an area which contains a small fluid inclusion. After switching the microscope to the infrared, one may collect the spectrum of the inclusion. The spectrum displays the fingerprint of an oil thus providing detailed information on its composition and quality. The possibility of exploiting that rock as an oil source can then be evaluated on a sound basis.

Another application of infrared microspectroscopy by synchrotron radiation to geophysical research is the

study of meteoritic fragments recently performed under a collaboration between University La Sapienza and the SERC of Daresbury. These samples, collected by the Italian expedition to the Antarctic, are strongly inhomogeneous. They show insulating zones made of different silicates, a few tens of microns in size, embedded into a metallic matrix. By use of the IRSR microscope of Daresbury, detailed spectra of those silicates have been collected, and characteristic lineshifts with respect to the laboratory spectra of the same substances have been recorded. These data will provide exhaustive information on the composition of the meteorite and on the conditions under which it was formed.23 Acknowledgments I wish to thank Alessandro Nucara and Augusto Marcelli for useful suggestions and for providing part of the illustrations.

References 1. J.R. Stevenson, H. Ellis, and R. Bartlett, Appl. Optics 12, 2884 (1973).

2. P. Meyer and P. Lagarde, J. Phys. (Paris) 37, 1387 (1976).

3. J. Yarwood, T. Shuttleworth, J.B. Hasted, and T. Nanba, Nature 317, 743 (1984).

Fig. 12 Infrared microspectroscopy of a tiny oil inclusion in a rock (from Ref. 23). The spectrum at the center displays the “fingerprint” of the oil.

The spatial distributions of different chemical species, together with the visible image of the inclusion (top left), are also shown.

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4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

T. Nanba, Rev. Sci. Instrum. 60, 1680 (1989).

G.P. Williams, Nucl. Instr. & Meth. A291, 8 (1990).

B. Nelander, Vibr. Spectrosc. 9, 29 (1990).

P. Roy, Y.L. Mathis, A. Gerschel, J.P. Marx, J. Michaut, B. Lagarde, and P. Calvani, Nucl. Instrum. & Meth. A325, 568 (1993).

D.A. Slater, P. Hollins, M.A. Chesters, J. Pritchard, D.H. Martin, M. Surman, D.A. Shaw and I. Munro, Rev. Sci. Instrum. 63, 1547 (1992).

A. Marcelli and P. Calvani, LNF-INFN report 93/027 (1993).

A. Nucara, P. Calvani, A. Marcelli, and M. Sanchez del Rio, Rev. Sci. Instrum. 66, 1934 (1995).

Infrared Synchrotron Radiation, Ed. by P. Calvani and P. Roy, Ed. Compositori, Bologna, 1998.

W.D. Duncan and G.P. Williams, Appl. Optics 22, 2914 (1983).

A. Nucara, P. Dore, P. Calvani, D. Cannavò, and A. Marcelli, ibid., 527 (1998).

A. Marcelli, E. Burattini, A. Nucara, P. Calvani, G. Cinque, C. Mencuccini, S. Lupi, F. Monti, and M. Sanchez del Rio, ibid., 463 (1998).

S. Kimura, UVSOR Activity Report 1997, BL6A1.

P. Dore, A. Nucara, D. Cannavò, G. De Marzi, P. Calvani, A. Marcelli, R. Sussmann, A.J. Whitehead, C.N. Dodge, A.J. Krehan, and H.H. Peters, Applied Optics 37, 5731 (1998).

17. Y-L. Mathis, P. Roy, B. Tremblay, A. Nucara, S. Lupi, P. Calvani, and A. Gerschel, Phys. Rev. Lett 80, 1220 (1998).

18. R.A. Bosch, Nucl. Instr. & Meth. A386, 525 (1997); Nuovo Cimento D 20, 483 (1998).

19. M. Castellano, Nucl. Instr. & Meth. A391, 375 (1997) 20. L. Carr, P. Dumas, C.J. Hirschmugl, and G.P. Williams, Nuovo Cimento D, 20, 375 (1998).

21. R.J. Hemley, H.K. Mao, A.F. Goncharov, M. Hamfland, and V.V. Struzhkin, Phys. Rev. Lett. 76, 1667 (1996).

22. M.A. Chesters, E.C. Hargreaves, M. Earson, P. Hollins, D.A. Slater, J.M. Chammers, B. Ruzicka, M. Surman, and M.J. Tobin, Nuovo Cimento D, 20, 439 (1998).

23. N. Guilhaumou, P. Dumas, G.L.Carr and G.P.Williams, Applied Spectroscopy 52, 1029 (1998).

24. A. Maras, S. Lupi, P. Calvani, B. Ruzicka, M. Tobin, and M. Chesters, to be published.

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VI CONVEGNO NAZIONALE DELLA SILS Il Sesto Convegno Nazionale della SILS, tenuto nella magnifica sede del Palazzo Bo dell’Università di Padova in giugno, è stato organizzato in quattro sessioni orali ed una sessione poster aperta per tutta la durata del convegno. Le sessioni orali sono state aperte o chiuse da sei relatori invitati (Nordgreen, Itié, Wulff, Goulon, Sauvage e Fadley).

La conferenza d’apertura nel primo pomeriggio di giovedì 18 giugno è stata tenuta da J. Nordgren, che ha mostrato le notevoli applicazioni della spettroscopia risonante di fluorescenza a raggi X molli, grazie alle nuove sorgenti di luce di sincrotrone ad alta brillanza. In particolare si è soffermato su alcuni esempi di studio della struttura elettronica di interfacce sepolte e di sistemi con adsorbati (sfruttando la selettività chimica e di simmetria), di superconduttori ad alta temperatura critica, e dell’influenza dell’intrappolamento di idrogeno nei metalli, nonché dello studio di sistemi altamente correlati quali i materiali magnetici. Il secondo oratore del pomeriggio, A. Martorana, ha presentato studi di caratterizzazione strutturale di catalizzatori eterogeni supportati su pomice, con tecniche EXAFS (eseguita presso la beamline GILDA ad ESRF), ed XPS e XAS (eseguite in laboratorio): con opportune tecniche di sottrazione del segnale si sono determinate le fasi strutturali dei metalli supportati, sono state inoltre messe in luce le prospettive di correlazione fra i difetti di impilamento (stacking faults) e la attività catalitica, ottenibili con tecniche di diffusione anomala. F. D’Acapito ha mostrato interessanti studi su agglomerati di Xe ad alta pressione impiantati in cristalli di silicio, studiandone la dinamica di crescita che mostra una lunghezza di correlazione Xe-Xe consistente con lo Xe liquido, e studi di interfacce sepolte di metalli nobili in SiO2; le tecniche utilizzate sono la diffrazione radente di raggi X (eseguita sulla beamline ID9 di ESRF) e la fotoemissione (presso SRRC di Taiwan). G. Piazzesi ha presentato i più recenti studi condotti su singole fibre muscolari, analizzando la diffrazione di raggi X da molecole di miosina (esperimenti condotti presso la beamline ID2 di ESRF); l’analisi delle strutture regolari nelle diverse fasi di attivazione muscolare permette di approfondire le conoscenze in questo campo. G. Wulff ha chiuso la prima sessione orale, presentando un interessantissimo studio dell’evoluzione temporale delle molecole di mioglobina (Mb), seguendo la dissociazione del CO dal Fe

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nella MbCO, utilizzando impulsi laser da 100 fs per provocare la dissociazione e "fotografie" di diffrazione X ogni 3 ns, grazie alla struttura temporale del fascio di ESRF. La possibilità di studiare la dinamica di azione delle proteine apre scenari impressionanti per l’uso di tecniche di luce di sincrotrone nella biologia.

La mattina di venerdì 19 giugno si apre con l’intervento di J.P. Itié, che ha presentato una panoramica sullo studio strutturale di materiali sotto condizioni estreme di pressione e di temperatura, studi condotti a Grenoble, con l’uso di celle appropriate che possono raggiungere i 300 GPa, pressione vicina a quella presente nel nucleo terrestre (~365 GPa); ha quindi mostrato transizioni di fase di semiconduttori, di metalli e trasformazioni di fase magnetiche in leghe PtFe ad alte pressioni. Il secondo intervento, di L. Alagna, è stato incentrato sullo studio di strutture ordinate eteroepitassiali di InGaP/GaAs con tecniche di struttura fine da diffrazione anomala, mostrando l’influenza positiva del leggero disorientamento rispetto alle direzioni di massima simmetria per ottenere larghi domini regolari. A. Pavese ha presentato le tecniche di cristallografia ad altissime pressioni ed alta temperatura, riprendendo i temi trattati da Itié, a pressioni vicine a quelle presenti nel nucleo terrestre; ha mostrato quindi alcuni risultati su fasi tipo-mica, interpretati con l’ausilio di tecniche computazionali. F. Rocca ha parlato dell’influenza dell’ordine locale nel processo di emissione di luce del Si poroso, con la formazione della porosità strettamente legata al processo elettrochimico di preparazione; gli studi sono stati condotti con tecniche di luminescenza ottica di emissione di raggi X (XEOL) ed EXAFS. P. Ghigna ha presentato studi strutturali EXAFS su campioni di Zr1-xFexZrO2, con la messa in evidenza di legami diretti Fe-Zr, quando i campioni sono preparati per sintesi di combustione. Dopo la pausa per il caffè J. Goulon ha presentato le nuove prospettive aperte ad ESRF per le tecniche di dicroismo di raggi X circolare magnetico e naturale, applicato ai solidi; il dicroismo naturale si osserva in cristalli uni- o biassiali ed in soluti chirali in cristalli liquidi allineati. L’uso di nuovi ondulatori (come Helios II) con variazione di gap e fase, permette di cambiare polarizzazione, conseguendo l’opportunità di eseguire studi di EXAFS con polarizzazione di spin. L. Gregoratti ha presentato applicazioni avanzate della tecnica di spettromicroscopia eseguite ad ELETTRA, con lo studio della composizione chimica e distribuzione spaziale laterale di strati bidimensionali (2D) e

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3D di fasi NiSi cresciute su Si; questa rivisitazione dei silicuri di Ni ha mostrato come la microcaratterizzazione chimica di queste interfacce sia necessaria data la natura morfologica complessa con coesistenza di diverse fasi. V. Carravetta, con l’uso di fotoemissione risonante con raggi X ad alta risoluzione (RPE), ha studiato la dipendenza energetica della forma di riga spettrale del CO vicino all’eccitazione C 1s-pi*; gli esperimenti sono stati interpretati con l’ausilio di simulazioni numeriche e modelli di riga Fano.

L’intervento successivo di A. Bravin ha riguardato la radiografia con luce di sincrotrone presso la linea SYRMEP di ELETTRA, in particolare sfruttando la visualizzazione con diffrazione e contrasto di fase, grazie alla alta coerenza spaziale della sorgente; sono stati presentati gli aspetti tecnici ed alcuni esempi di mammografia su campioni modello, discutendo le prospettive positive di tali tecniche in diagnostica medica a confronto con la radiografia convenzionale.

Il primo intervento di sabato 20 giugno è di M. Sauvage, incentrato sulla presentazione di vari esempi di allontanamento dalle strutture a periodicità 2D ideale presenti su superfici ed interfacce di materiali cristallini. Gli esperimenti sono stati condotti al LURE con diffrazione di raggi X ad incidenza radente, in apparati con sistemi di crescita epitassiale in situ; in particolare Sauvage ha presentato diversi esempi su composti III-V, Si e Bi/Si, con identificazione di domini 2D e con la identificazione di confini in antifase fra i domini. E seguito l’intervento di S. Turchini, che ha presentato studi di dicroismo X circolare naturale su cristalli complessi composti con Nd, eseguiti presso ESRF. L’effetto di asimmetria fra radiazione polarizzata destrogira o levogira è debole in questi sistemi naturali, ma osservabile grazie all’alta brillanza della sorgente; i risultati sperimentali sono confrontati con calcoli di teoria a diffusione multipla. V. Corradini ha presentato uno studio di diverse fasi 2D ordinate di Bi cresciuto su Si(100); grazie all’alta risoluzione energetica della linea UV di ELETTRA, l’analisi dei livelli profondi di Bi e Si è stata correlata ai siti di adsorbimento, e quindi alla struttura 2D ordinata, nelle diverse fasi di formazione dell’interfaccia.

La conferenza di chiusura è stata di C.S. Fadley, che ha mostrato i nuovi sviluppi nella fotoemissione risonante e risolta temporalmente, possibili presso le sorgenti di terza generazione. Ha presentato alcuni esperimenti eseguiti ad ALS, fra i quali lo studio degli ossidi metallici (quali MnO), evidenziando il grande effetto di risonanza in funzione dell’energia dei fotoni, dovuti ad un effetto interatomico; un altro esempio legato alla velocità di raccolta dei dati è stato lo studio dell’ossidazione del W(110), con la possibilità di seguire temporalmente la reazione chimica.

Nel tardo pomeriggio della seconda giornata si è svolta l’Assemblea annuale dei soci. In chiusura del convegno sono stati premiati i migliori poster, fra i circa 40 esposti,

presentati dai seguenti giovani: Chiara Maurizio, Manuel a Panzalorto, Mauro Sambi e Massimo Tormen.

(C. Mariani)

CONFERENZA XAFS-X Nei giorni 10 - 14 agosto 1998 si è svolta la decima edizione della conferenza XAFS presso l’Illinois Institute of Technology, Chicago. Chicago è stata selezionata come sede del congresso per la vicinanza della Advanced Photon Source, in linea con la scelta di svolgere le conferenze XAFS di questi anni presso le nuove sorgenti di terza generazione.

La conferenza ha previsto sessioni plenarie, contributi orali e varie sessioni poster; sono stati esposti risultati recenti nei molteplici campi di applicazione dello XAFS e delle tecniche simili e sono stati illustrati nuovi metodi sperimentali, con particolare enfasi nei riguardi di quelli resi possibili dalle nuove sorgenti. Nella impossibilità di descrivere tutti gli interventi riporterò una una scelta personale dei risultati di maggior rilievo.

Durante il primo giorno J. Goulon (ESRF) ha descritto la prima osservazione chiara del dicroismo naturale nei raggi X. Egli ha dimostrato, con l’ausilio di calcoli di diffusione multipla, che questo effetto è dovuto alla interferenza tra termine di dipolo e di quadrupolo elettrico e che esso è sensibile al grado di mescolamento di stati finali di parità opposta; è prevedibile che questo importante risultato apra la strada ad una nuova tecnica di indagine a carattere generale.

Il giorno seguente A. Manceau (CNRS, Grenoble) ha illustrato in modo particolarmente efficace come lo XAFS possa fornire delle informazioni uniche sulla speciazione di metalli in ambito geologico/ambientale; molti dei problemi descritti hanno delle implicazioni importanti nell’ambito dell’inquinamento e della politica del territorio. Questa relazione ha mostrato come si stia colmando la distanza tra studio di sistemi modello e sistemi reali; ulteriori importanti avanzamenti in questo campo si possono ottenere con l’applicazione delle tecniche di XAFS risolto spazialmente, come illustrato da P. Bertsch (Georgia). A. DiCicco (Camerino) ha fatto una rassegna dello studio della struttura locale in solidi e liquidi con l’ausilio di codici di calcolo utilizzanti la diffusione multipla (GNXAS). La mattinata del martedi si è conclusa con la relazione di H. Renevier (CNRS, Grenoble), la quale ha illustrato i principi base ed alcuni risultati recenti ottenuti con la tecnica DAFS. Il mercoledì K. Baberscke (Berlin) ha effettuato una ampia rassegna degli studi sul magnetismo con l’assorbimento di raggi X. Egli ha posto l’accento sulla possibilità di determinare la temperatura di Curie con sensibilità atomica (per esempio nei multilayer), sulle applicazioni dell’EXAFS magnetico e su varie tecniche complementari quali il dicroismo in microscopia e lo scattering diffu-

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so magnetico. I lavori del giovedì sono stati aperti da una relazione a carattere storico di uno dei fondatori dell’era moderna dello XAFS, F. Lytle. Egli ha fornito una appassionata ricostruzione dello sviluppo della spettroscopia di assorbimento di raggi X dal 1913 fino ai primi anni settanta. Tra l’altro ha ricordato un importante finanziamento al suo gruppo da parte della Montedison nel periodo in cui vi era un forte interesse di tale azienda italiana allo sviluppo dello XAFS. Durante l’ultimo giorno dei lavori S. Pascarelli (ESRF) ha illustrato le possibilità offerte dalla tecnica di acquisizione dispersiva presso una sorgente di terza generazione, riportando esempi di studi nel campo del dicroismo magnetico (utilizzando una lamina a quarto d’onda) e nel campo dello studio delle cinetiche (XAFS risolto in tempo). La conferenza si è conclusa con una visita alla Advanced Photon Source.

(F. Boscherini)

IV SCUOLA DI SPETTROSCOPIA NEUTRONICA «DIFFUSIONE DEI NEUTRONI DALLA MATERIA DURA» Hotel Capo d’Orso, Palau (Sassari) 26 settembre - 4 ottobre 1998 Direttori: M. Catti (Milano) e F. Sacchetti (Perugia) Segreteria: G. Ianni (GNSM, Roma) La Scuola si è tenuta con notevole successo, a due anni di distanza dalla III edizione, nuovamente all’Hotel Capo d’Orso di Cala Capra (Palau). La sede si è confermata come ben attrezzata logisticamente e particolarmente gradevole, per cui si spera possa essere adottata anche per la prossima Scuola. Vi sono stati 22 partecipanti, giovani laureati di formazione prevalentemente fisica o chimica di cui molti già con esperienza di attività di ricerca legata alla spettroscopia neutronica. Gli argomenti delle lezioni hanno toccato sia i fondamenti metodologici della spettroscopia neutronica, nei sui aspetti tanto teorici quanto sperimentali, sia un buon numero di applicazioni a problematiche scientifiche specifiche. Queste sono state incentrate sulla cosiddetta “materia dura”, per alternanza con i contenuti della passata edizione che era stata dedicata principalmente ai materiali d’interesse biologico. Possiamo quindi riassumere le tematiche trattate come segue: generalità sullo scattering dei neutroni; sorgenti e strumentazione; diffrazione; diffusione anelastica e quasi-elastica; tecniche a basso angolo; applicazioni a: materiali superconduttori ad alta Tc, metalli e leghe, conduttori ionici, catalizzatori, materiali polimerici, minerali. I docenti che hanno collaborato sono: A. Albinati (Milano), C. Andreani (Roma), V. Arrighi (Edimburgh), G. Artioli (Milano), E. Caponetti (Palermo), F. Carsughi (Ancona), C.J. Carlile (ISIS, Oxford), M. Catti (Milano), B. Dorner (ILL, Grenoble), J. Eckert (Los Alamos, USA), S. Enzo (Sassari), M. Marezio (Parma), S.V. Meille (Mila-

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no), R. Rinaldi (Perugia), F. Sacchetti (Perugia). L’obiettivo della Scuola era presentare una panoramica della neutronica per lo studio della materia condensata, e fornire le metodologie per un corretto uso delle tecniche sperimentali relative, portando gli studenti ad un adeguato livello di conoscenza per l’effettuazione di esperimenti di base, dalla progettazione dell’esperimento alla elaborazione dei dati. Uno spazio adeguato è stato riservato all’illustrazione di numerosi esempi di applicazioni alle tematiche di fisico-chimica dei materiali e di scienze della terra, nella convinzione che il carattere interdisciplinare della scuola sia di grande giovamento alla formazione scientifica e all’apertura culturale dei giovani ricercatori. Le conoscenze acquisite consentiranno loro, inoltre, di utilizzare le potenzialita delle sorgenti neutroniche presso centri di ricerca europei a carattere multinazionale, in cui l’Italia ha tra l’altro investito in tempi recenti risorse non indifferenti.

Un aspetto importante dell’organizzazione didattica, che si è cercato di curare in modo particolare in questa edizione della Scuola è stato l’impegno richiesto agli studenti di partecipare in modo attivo 1) ad esercitazioni al calcolatore per l’elaborazione di dati sperimentali, e 2) ad una esercitazione di “progettazione” di un esperimento per la risoluzione di un problema specifico assegnato. Gli studenti hanno quindi tenuto seminari per illustrare i risultati del loro lavoro, impegnandosi con entusiasmo in un lavoro intensivo e certamente non facile.

Si può valutare in modo estremamente positivo la risposta ottenuta, in termini sia di interesse dimostrato sia di profitto conseguito nell’apprendimento.

Si ringraziano il CNR e l’INFM per il sostegno finanziario che ha reso possibile la realizzazione della Scuola.

(M. Catti)

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CALENDARIO

CALENDARIO

11-13 febbraio 1999 ESRF Users’ Meeting http://www.esrf.fr

GRENOBLE, FRANCE

21 febbraio-1 aprile 1999 GRENOBLE, FRANCIA HERCULES, Higher European Research Course for Users of Large Experimental Systems http://www.polycnrs-gre.fr/hercules 5-9 aprile 1999 SAN FRANCISCO, USA Materials Research Society Spring http://www.mrs.orgr 17-20 maggio 1999 UPTON, NY, USA SAS-99: XIth Internat. Conf. on Small-Angle Scattering Ann Emrick, Biology 463, Brookhaven National laboratory, Upton, NY 11973 Tel: +1 516 344 5756; Fax: +1 516 344 6398 E-mail: emrick@bnl.gov http://sas99.bnl.gov/sas99 22-27 maggio 1999 BUFFALO, NY, USA ACA '99 E-mail: aca@hwi.buffalo.edu http://www.hwi.buffalo.edu/ACA 14-18 giugno 1999 INFMeeting http://www.infm.it

CATANIA, ITALY

3-7 luglio 1999 GRANADA, SPAIN IV Liquid Matter Conference Prof. Dr. Roque Hidalgo Elvarez, Depart. de Fisica Aplicada, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva, E-18071 Granada (Spagna) Tel: +34 958 243213; Fax: +34 958 243214 E- mail: liquid99@ugr.es http://www.ugr.es/~liquid99 4-13 agosto 1999 GLASGOW, SCOTLAND 18th IUCr Gen. Assembly and International Congress of Crystallography http://www.chem.gla.ac.uk/iucr99

23-27 agosto 1999 CHICAGO, USA X-99 18th International conference on X-ray and inner shell processes http://www.phy.anl.gov/X99 1-4 settembre 1999 BUDAPEST, HUNGARY Second European Conference on Neutron Scattering (ECNS’99) Dr. Tamos Grûsz, Neutron Physics Laboratory Research Institute for Solid State Physics and Optics, H-1525 Budapest, P.O.B. 49, KFKI, Bldg.10, Hungary Tel: +36 1 395 9220/1738; Fax: +36 1 395 9165 E-mail: ECNS99@sunserv.kfki.hu http://www.kfki.hu/ECNS99/ 5-7 settembre 1999 SCHWAEBISCH GMUND, GERMANIA International Conference on Solid State Spectroscopy http://cardix.mpi-stuttgart.mpg.de/icss/ 20-24 settembre 1999 STUTTGART, GERMANIA 7th International Conference on Quasicrystals ICQ7'99, Institut für Theoretisch und Angewandte Physik, Universitat Stuttgart Tel: +49 711 685 5253/5254; Fax: +49 711 685 5271 icq7@itap.physik.uni-stuttgart.de 21-24 settembre 1999 VIENNA, AUSTRIA ECOSS-18, 18th European Conference on Surface Science Institut für Allgemeine Physik, U Wien, Wiedner Hauptstr. 8-10/134 E-mail: ecoss18-secretary@iap.tuwien.ac.at 29 settembre-2 ottobre 1999 DUBNA, RUSSIA 2nd International Seminar on Neutron Scattering at High Pressure (NSHP-II) D.P. Kozlenko, Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980 Dubna, Moscow Reg., Russia Tel: +7 09621 65644; Fax: +7 09621 65882 E-mail: denk@nf.jinr.ru http://nfdfn.jinr.ru/~denk/NSHPII/

Vol. 3 n. 2 Dicembre 1998 • NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

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SCADENZE

SCADENZE

SCADENZE PER RICHIESTE DI TEMPO MACCHINA PRESSO ALCUNI LABORATORI DI NEUTRONI ISIS

La scadenza per il prossimo ‘call for proposals’ è il 16 aprile e 16 ottobre 1999

ILL

La scadenza per il prossimo ‘call for proposals’ è il 1 marzo 1999

LLB-SACLAY

La scadenza per il prossimo ‘call for proposals’ è il 1 ottobre 1999

BENSC

La scadenza è il 15 marzo e 15 settembre 1999

RISØ E NFL

La scadenza per il prossimo ‘call for proposals’ è il 1 aprile 1999

SCADENZE PER RICHIESTE DI TEMPO MACCHINA PRESSO ALCUNI LABORATORI DI LUCE DI SINCROTRONE

32

ALS

Le prossime scadenze sono il 1 giugno e il 1 dicembre 1999

BESSY

Le prossime scadenze sono il 15 febbraio 1999 e il 15 agosto 1999

DARESBURY

La prossima scadenza è il 24 novembre 1999

ELETTRA

Le prossime scadenze sono il 28 febbraio 1999 e il 31 agosto 1999

ESRF

Le prossime scadenze sono il 1 marzo 1999 e il 1 settembre 1999

GILDA

(quota italiana) Le prossime scadenze sono il 1 maggio e il 1 novembre 1999

HASYLAB

(Nuovi progetti) Le prossime scadenze sono: 1 marzo, 1 settembre e 1 dicembre 1999

LURE

La prossima scadenza è il 30 ottobre 1999

MAX-LAB

La scadenza è approssimativamente febbraio 1999

NSLS

Le prossime scadenze sono il 31 gennaio, 31 maggio e 30 settembre 1999

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE • Vol. 3 n. 2 Dicembre 1998

FACILITIES

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

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

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 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 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 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 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 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

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

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

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

INDUS Center for Advanced Technology, Rajendra Nagar, Indore 452012, India tel: +91 731 64626 Tipo: D Status: C

Vol. 3 n. 2 Dicembre 1998 • NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

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FACILITIES

KEK Photon Factory Nat. Lab. for High Energy Physics, 1-1, Oho,Tsukubashi 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 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 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

SPring-8 2-28-8 Hon-komagome, Bunkyo-ku ,Tokyo 113, Japan tel: +81 03 9411140 fax: +81 03 9413169 Tipo: D Status: 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 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 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

NSRL National Synchrotron Radiation Lab. USTC, Hefei, Anhui 230029, PR China tel:+86 551 3601989 fax:+86 551 5561078 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

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

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

Siberian SR Center Lavrentyev Ave 11, 630090 Novosibirsk, Russia tel: +7 383 2 356031 fax: +7 383 2 352163 Tipo: D Status: O

UVSOR Inst. for Molecular ScienceMyodaiji, Okazaki 444, Japan tel: +81 564 526101 fax: +81 564 547079 Tipo: D Status: O

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.

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NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE • Vol. 3 n. 2 Dicembre 1998

FACILITIES

NEUTRONI NEUTRON SCATTERING WWW SERVERS IN THE WORLD (http://www.ISIS.RL.AC.UK/ISISpublic/neutron_sities.html)

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 Peter Timmins; tel: +33 76207263 E-mail: TIMMINS@FR ILL Peter Linder; tel: +33 76207068; E-mail: LINDER@FR ILL Roland P.May;tel:+3376207047; E-mail: MAY@FRILL fax: +33 76 48 39 06 telex: ILL 320-621 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.nd.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. 3 n. 2 Dicembre 1998 • NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

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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

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

ORNL Oak Ridge National Laboratory Neutron Scattering Facilities, P.O. Box 2008, Oak Ridge TN 37831-6393 USA G.D. Wignall; tel: +1 615 574 5237 fax: +1 615 576 2912 E-mail: GDW@ORNLSTC http://www.ornl.gov/hfir/hfirhome.html

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NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE • Vol. 3 n. 2 Dicembre 1998