NEUTRON AND X-RAY Grenoble Sciences Grenoble Sciences pursues a triple aim: ®to publish works responding to a clearly defined project, with no curriculum or vogue constraints, ®to guarantee the selected titles’ scientific and pedagogical qualities, ®to propose books at an affordable price to the widest scope of readers. Each project is selected with the help of anonymous referees, followed by a one- year (in average) interaction between the authors and a Readership Committee, whose members’ names figure in the front pages of the book. Publication is then confided to the most adequate publishing company by Grenoble Sciences. (Contact: Tél.: (33) 4 76 51 46 95 – Fax: (33) 4 76 51 45 79 E-mail: [email protected])

Scientific Director of Grenoble Sciences : Jean BORNAREL, Professor at the Joseph Fourier University, Grenoble 1, France

Grenoble Sciences is supported by the French Ministry of Education and Research and the “Région Rhône-Alpes”.

Front cover photo: harmonious composition based on figures extracted from the work: ®The body-centred tetragonal unit cell of the bilayer manganite by CURRAT (after ref. [38]) ®Methyl group in lithium acetate dihydrate and in aspirin by JOHNSON (from refs [19] and [49]) ®X-PEEM images of permalloy micro-structures and of carbon films by SCHNEIDER (from ref. [11] and taken from ref. [17]) ®Interference of outgoing and backscattered photoelectron wave by LENGELER. Neutron and X-ray Spectroscopy

Edited by

FRANÇOISE HIPPERT Institut National Physique, St. Martin d'Hères, France

ERIK GEISSLER Université J. Fourier de Grenoble, St. Martin d'Hères, France

JEAN LOUIS HODEAU Laboratoire de Cristallographie, CNRS, Grenoble, France

EDDY LELIÈVRE-BERNA Institut Laue Langevin, Grenoble, France and JEAN-RENÉ REGNARD Université J. Fourier de Grenoble, St. Martin d'Hères, France A C.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN-10 1-4020-3336-2 (HB) ISBN-13 978-1-4020-3336-0 (HB) ISBN-10 1-4020-3337-0 (e-book) ISBN-13 978-1-4020-3337-7 (e-book)

Published by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. www.springer.com

Printed on acid-free paper

All Rights Reserved © 2006 Springer No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

Printed in the Netherlands. TABLE OF CONTENTS

Contributors...... XV Foreword...... XIX Introduction ...... XXI

X--RAY SPECTROSCOPY

1 - Fundamentals of X-ray absorption and dichroism: the multiplet approach F. de Groot - J. Vogel ...... 3 1. Introduction...... 4 1.1. Interaction of X-rays with matter...... 4 1.2. Basics of XAFS spectroscopy...... 6 1.3. Experimental aspects...... 9 2. Multiplet effects ...... 11 2.1. Atomic multiplets...... 14 2.1.1. Term symbols...... 14 2.1.2. Matrix elements ...... 18 2.2. Atomic multiplet ground states of 3dn systems...... 20 2.3. j - j coupling...... 20 2.4. X-ray absorption spectra described with atomic multiplets...... 21 2.4.1. Transition metal LII-III edge...... 21 2.4.2. MIV-VV edges of rare earths...... 28 3. Crystal field theory...... 31 3.1. The crystal field multiplet Hamiltonian...... 32 3.2. Cubic crystal fields...... 33 3.3. The energies of the 3dn configurations...... 36 3.3.1. Symmetry effects in D4h symmetry ...... 40 3.3.2. The effect of the 3d spin-orbit coupling...... 40 3.3.3. The effects on the X-ray absorption calculations...... 42 3.3.4. 3d0 systems in octahedral symmetry...... 42 3.3.5. 3d0 systems in lower symmetries ...... 45 3.3.6. X-ray absorption spectra of 3dn systems ...... 46 4. Charge transfer effects...... 47 4.1. Initial state effects ...... 47 4.2. Final state effects...... 52 4.3. The X-ray absorption spectrum with charge transfer effects...... 53 5. X-ray linear and ...... 59 References...... 64

V VI NEUTRON AND X-RAY SPECTROSCOPY

2 - Multiple scattering theory applied to X-ray absorption near-edge structure P. Sainctavit - V. Briois...... 67 1. Introduction...... 67 2. MST for real space calculations...... 70 2.1. The absorption cross-section...... 71 2.2. Calculation of the cross-section for three different potentials...... 73 2.2.1. Free particle ...... 73 2.2.2. Particle in a spherical potential ...... 75 2.2.3. Particle in a "muffin-tin" potential...... 78 3. Construction of the potential ...... 83 3.1. X – a exchange potential...... 85 3.2. Dirac-Hara exchange potential ...... 85 3.3. Complex Hedin-Lundqvist exchange and correlation potential...... 87 4. Application of the multiple scattering theory ...... 89 4.1. Vanadium K edge cross-section for VO6 cluster...... 90 4.2. Iron K edge cross-section for FeO6 cluster...... 93 4.3. Cross-section calculations in distorted octahedra ...... 94 II 5. Spin transition in Fe (o-phenantroline)2 (NCS) 2 followed at the iron K edge...... 95 6. Conclusion ...... 97 ij ij Appendix - Expression for the propagators J LL' and H L L' ...... 97 Acknowledgments...... 99 References...... 100 3 - X-ray magnetic circular dichroism F. Baudelet...... 103 1. Introduction...... 103 2. Origins of magnetic circular dichroism...... 104 2.1. Theoretical aspects: role of the spin-orbit coupling...... 104 2.2. Light polarization and polarization-dependent selection rules...... 106 2.3. Origin of the XMCD signal ...... 108 2.3.1. Heuristic model: LIII - LIII edges of transition metals and rare earth elements...... 108 2.3.2. Calculation of Pe for the LIII - LIII edges: Pe(LIII ) and Pe(LIII ) ...... 109 2.3.3. Calculation of Pe for the K - LI edges: Pe(K) and Pe(LI )...... 112 2.4. Magnitude of the XMCD signal ...... 113 3. The sum rules...... 114 4. XMCD signal at the K edges (1s ö p)...... 117 5. XMCD and localized magnetism (multiplet approach)...... 118

6. LIII -IIII edges of rare earths...... 121 7.Magnetic EXAFS...... 125 8. Application to multilayers with GMR ...... 126 9. Study of highly correlated [Ce/La/Fe] and [La/Ce/Fe] multilayers...... 127 References...... 128 TABLE OF CONTENTS VII

4 - Extended X-ray absorption fine structure B. Lengeler...... 131 1. Introduction...... 131 2. X-ray absorption in isolated atoms ...... 133 3. X-ray absorption fine structure (XAFS) ...... 136 3.1. X-ray absorption fine structure: the basic formula ...... 136 3.2. Corrections to the basic EXAFS formula...... 141 3.2.1. Spherical photoelectron wave...... 142 3.2.2. Multiple scattering ...... 142 3.3. Set-up for measuring X-ray absorption ...... 143 3.4. XAFS data analysis ...... 145 3.5. Position and structure of the absorption edge ...... 149 4. Some applications of XAS ...... 153 4.1. Lattice distortion around impurities in dilute alloys ...... 154 4.2. Precipitation in immiscible giant magnetoresistance Ag1–x Nix alloys ...... 155 4.3. Lattice site location of very light elements in metals by XAFS...... 158 4.4. Valence of iridium in anodically oxidized iridium films...... 160 4.5. Copper-based methanol synthesis catalyst...... 163 4.6. Combined X-ray absorption and X-ray crystallography...... 165 References...... 167 5 - Inelastic X-ray scattering from collective atom dynamics F. Sette - M. Krisch ...... 169 1. Introduction...... 169 2. Scattering kinematics and inelastic X-ray scattering cross-section...... 170 3. Experimental apparatus ...... 175 4. The "fast sound" phenomenon in liquid water ...... 179 5. Determination of the longitudinal sound velocity in iron to 110 GPa ...... 182 6. Conclusions and outlook...... 186 References...... 187 6 - Photoelectron spectroscopy M. Grioni ...... 189 1. Introduction...... 189 2. What is photoemission? ...... 190 3. Photoemission in the single-particle limit - Band mapping ...... 193 4. Beyond the single-particle approximation ...... 201 5. Case studies ...... 209 5.1. Fermi liquid lineshape in a normal metal...... 209 5.2. Gap spectroscopy ...... 215 5.3. Electronic instabilities in low dimensions...... 220 5.4. Some recent developments...... 229 6. Conclusions...... 235 References...... 236 VIII NEUTRON AND X-RAY SPECTROSCOPY

7 - Anomalous scattering and diffraction anomalous fine structure J.L. Hodeau - H. Renevier...... 239 1. Anomalous scattering, absorption and refraction...... 240 2. Theoretical versus experimental determination of anomalous contribution...... 241 3. Applications of anomalous dispersion ...... 244 3.1. Structure factor phase solution (MAD method)...... 245 3.2. Element-selective diffraction (contrast method)...... 247 4. Diffraction anomalous fine structure data analysis ...... 249 4.1. DAFS and EDAFS formalism ...... 251 4.1.1. Single anomalous site analysis ...... 252 4.1.2. Multiple anomalous site analysis...... 253 4.2. DAFS and EDAFS determination ...... 255 4.3. DAFS and DANES valence determination ...... 258 4.4. Anisotropy of anomalous scattering...... 260 5.Requirements for anomalous diffraction experiments...... 262 6. Conclusion ...... 263 References...... 264 8 - Soft X-ray photoelectron emission microscopy (X-PEEM) C.M. Schneider...... 271 1.A "nanoscale" introduction ...... 271 2. Visualizing micro- and nanostructures...... 271 3. Technical aspects of an Electron Emission Microscope (EEM)...... 273 3.1. Electron-optical considerations...... 273 3.2. Transmission and lateral resolution...... 275 4. Non-magnetic image contrast in X-PEEM...... 276 4.1. Primary contrast mechanisms ...... 276 4.1.1. Work function contrast...... 276 4.1.2. Chemical contrast ...... 277 4.2. Secondary contrast mechanisms ...... 280 5. Magnetic contrast in X-PEEM ...... 281 5.1. Magnetic X-ray Circular Dichroism (XMCD)...... 282 5.1.1. Physics of the magnetic contrast mechanism...... 282 5.1.2. Contrast enhancement...... 284 5.1.3. Angular dependence of the image contrast...... 285 5.1.4. Magnetic domain walls ...... 286 5.1.5. Information depth...... 288 5.2. X-ray Magnetic Linear Dichroism (XMLD)...... 290 5.2.1. Properties of the contrast mechanism ...... 290 5.2.2. Imaging domains in antiferromagnets...... 291 6. Concluding remarks...... 292 References...... 293 TABLE OF CONTENTS IX

9 - X-ray intensity fluctuation spectroscopy M. Sutton ...... 297 1. Introduction...... 297 2. Mutual coherence functions ...... 301 3. Diffraction by partially coherent sources...... 304 4. Kinetics of materials...... 308 5. X-ray intensity fluctuation spectroscopy...... 310 6. Conclusions...... 316 References...... 317 10 - Vibrational spectroscopy at surfaces and interfaces using synchrotron sources and free electron lasers A. Tadjeddine - P. Dumas ...... 319 1. Introduction...... 319 2. Infrared synchrotron source and free electron lasers ...... 321 2.1. Infrared synchrotron sources...... 321 2.1.1. Principles...... 321 2.1.2. Extraction of the IR beam from the synchrotron ring...... 324 2.1.3. Fourier transform interferometer...... 324 2.2. Non linear surface spectroscopy with conventional and free electron lasers...... 325 2.2.1. Principle of SFG...... 326 2.2.2. Experimental SFG set-up...... 328 2.2.3. The CLIO-FEL infrared laser...... 328 3. Examples of applications in Surface Science ...... 331 3.1. Synchrotron at surfaces...... 331 3.1.1. Low frequency modes of adsorbed molecules and atoms...... 333 3.1.2. Vibrational dynamics of low frequency modes ...... 335 3.2. Sum Frequency and Difference Frequency Generation at interfaces...... 339 3.2.1. Identification of adsorbed intermediate of electrochemical reactions by SFG .....340 3.2.2. Vibrational spectroscopy of cyanide at metal-electrolyte interface...... 342 3.2.3. Vibrational spectroscopy of self-assembled monolayers on metal substrate...... 346 3.2.4. Vibrational spectroscopy of fullerenes C600, adsorbed on Ag(111), in UHV environment ...... 348 3.2.5. Adsorption of 4-cyanopyridine on Au(111) monitored by SFG ...... 352 4. Conclusion and outlook ...... 356 References...... 356 X NEUTRON AND X-RAY SPECTROSCOPY

NEUTRON SPECTROSCOPY

11 - Inelastic neutron scattering: introduction R. Scherm - B. Fåk ...... 361 1. Interaction of neutrons with matter...... 361 2.Kinematics...... 362 2.1. Energy and momentum conservation ...... 362 2.2. Scattering triangle...... 363 2.3. Parabolas...... 364 3. Master equation and S (Q, w) ...... 364 4. Correlation function...... 366 5. Coherent and incoherent scattering...... 367 6. General properties of S (Q, w)...... 369 6.1. Detailed balance ...... 369 6.2. Moments ...... 370 6.3. Total versus elastic scattering ...... 371 7. Magnetic scattering ...... 372 8. Response from simple systems...... 373 8.1. Examples...... 373 8.2. Response functions...... 375 9. Instrumentation ...... 376 9.1. TAS...... 378 9.2. TOF...... 379 10. How to beat statistics ...... 379 References...... 380 12 - Three-axis inelastic neutron scattering R. Currat...... 383 1. Principle of the technique ...... 383 2. The three-axis spectrometer ...... 386 3. The TOF versus TAS choice ...... 391 4. What determines the TAS count rate? ...... 394 5. What determines the size and shape of the resolution function? ...... 396 6. Decoupling energy and momentum resolutions: direct space focusing ...... 400 7.TAS multiplexing ...... 403 8. Phonon studies with TAS ...... 408 9. The INS versus IXS choice ...... 412 10. Magnetic excitation studies with TAS ...... 415 11. Practical aspects ...... 420 12. Summary and outlook ...... 422 References ...... 423 TABLE OF CONTENTS XI

13 - spectroscopy R. Cywinski ...... 427 1. Introduction ...... 427 2. Polarized neutron beams, Larmor precession and spin flippers ...... 428 3. Generalized neutron spin echo ...... 432 4. Polarization dependent scattering processes ...... 436 5. Practicalities: measurement of the spin echo signal ...... 440 6. Applications of neutron spin echo spectroscopy ...... 444 6.1. Soft condensed matter...... 444 6.2. Glassy dynamics...... 447 6.3. Spin relaxation in magnetic systems...... 449 7. Conclusions...... 453 Bibliography ...... 453 References ...... 454 14 - Time-of-flight inelastic scattering R. Eccleston ...... 457 1. Introduction ...... 457 2. Classes of TOF spectrometers ...... 458 2.1. Distance-time plots...... 458 2.2. Kinematic range ...... 460 3. Beamline components ...... 461 3.1. Choppers...... 461 3.1.1. Fermi choppers...... 462 3.1.2. Disk choppers...... 462 3.1.4. T = 0 choppers ...... 462 3.2. Monochromating and analyzing crystals...... 463 3.3. Filters ...... 463 3.4. Detectors...... 464 3.5. Neutron guides...... 464 3.6. Polarizers and polarization analysis...... 465 4. Direct geometry spectrometers ...... 465 4.1. Chopper spectrometer on a pulsed source...... 465 4.2. Chopper spectrometers on a steady state source ...... 466 4.3. Multi-chopper TOF spectrometers ...... 467 5. Resolution and spectrometer optimization ...... 469 6. Flux ...... 470 7. Resolution as a function of energy transfer and experimental considerations ...... 471 Single crystal experiments on a chopper spectrometer...... 472 8. Indirect geometry spectrometers ...... 474 8.1. The resolution of indirect geometry spectrometers...... 475 8.2. Backscattering spectrometer ...... 475 8.3. Crystal analyzer spectrometers ...... 477 XII NEUTRON AND X-RAY SPECTROSCOPY

8.4. Deep inelastic neutron scattering...... 478 8.5. Coherent excitations...... 479 9. Conclusions ...... 480 Further information ...... 481 References ...... 481 15 - Neutron backscattering spectroscopy B. Frick...... 483 1. Introduction ...... 483 2.Reflection from perfect crystals and its energy resolution ...... 485 3. Generic backscattering spectrometer concepts ...... 488 3.1. Neutron optics of the primary spectrometers of reactor-BS instruments...... 489 3.2. Neutron optics of the primary spectrometer of spallation source-BS instruments ...493 3.3. Secondary spectrometer ...... 493 4. Total energy resolution of the spectrometers ...... 494 5. How to do spectroscopy? ...... 494 5.1. Spectroscopy on reactor based instruments...... 494 5.2. Spectroscopy on spallation source-BS instruments ...... 497 6. More details on optical components ...... 498 6.1. BS monochromators and analyzers...... 498 6.2. How to obtain the best energy resolution in backscattering? ...... 499 6.3. Mosaic crystal deflectors and phase space transformer...... 500 6.4. Neutron guides...... 503 6.5. Higher order suppression ...... 503 6.6. Q-resolution...... 504 6.7. A second time through the sample?...... 504 7. Examples for backscattering instruments ...... 505 7.1. Reactor instruments...... 505 7.2. Spallation source instruments ...... 509 8. Data treatment ...... 511 9. Typical measuring methods and examples ...... 513 9.1. Fixed window scans ...... 513 9.2. Spectroscopy...... 516 References ...... 525 16 - Neutron inelastic scattering and molecular modelling M.R. Johnson - G.J. Kearley - H.P. Trommsdorfff...... 529 1. Introduction ...... 529 2. Theory framework for tunnelling, vibrations and total energy calculations ...... 532 2.1. Hamiltonians for quantum tunnelling...... 532 2.2. The dynamical matrix for molecular vibrations...... 536 2.3. Total energy calculations for determining PES and force constants...... 538 3. Experimental techniques for measuring rotational tunnelling and molecular vibrations ...... 540 TABLE OF CONTENTS XIII

4. Numerical simulations for understanding INS spectra ...... 544 4.1. SPM methyl group tunnelling...... 544 4.2. Multi-dimensional tunnelling dynamics of methyl groups...... 546 4.3. Vibrational spectroscopy of molecular crystals ...... 547 5. Discussion ...... 551 References ...... 553 Index...... 557 CONTRIBUTORS

F. BAUDELET Synchrotron SOLEIL L'Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette Cedex, France [email protected]

V. BRIOIS Synchrotron SOLEIL L'Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette Cedex, France [email protected]

D. CABARET Institut de Minéralogie et de Physique des Milieux Condensés Université P. et M. Curie, Case 115, 4 place Jussieu, 75252 Paris Cedex 05, France [email protected]

R. CURRAT Institut Laue-Langevin 6 Rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France [email protected]

R CYWINSKI School of Physics and Astronomy University of Leeds, Leeds LS2 9JT, UK [email protected]

P. DUMAS Synchrotron SOLEIL L'Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette Cedex, France [email protected]

R. ECCLESTON Sheffield Hallam University, Materials Engineering Research Institute Howard Street, Sheffield, S1 1WB, UK [email protected]

XV XVI NEUTRON AND X-RAY SPECTROSCOPY

B. FÅK Département de la Recherche Fondamentale sur la Matière Condensée, SPSMS CEA Grenoble,17 rue des Martyrs, 38054 Grenoble Cedex 9, France [email protected]

B. FRICK Institut Laue-Langevin 6 Rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France [email protected]

M. GRIONI Institut de Physique des Nanostructures Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland [email protected]

F. DE GROOT Department of Inorganic Chemistry and Catalysis Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, Netherlands [email protected]

J.L. HODEAU Laboratoire de Cristallographie CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France [email protected]

M.R. JOHNSON Institut Laue-Langevin 6 Rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France [email protected]

G.J. KEARLEY TU Delft, Interfacultair Reactor Institute Mekelweg, 15, 2629 JB Delft, Netherlands [email protected]

M. KRISCH European Synchrotron Radiation Facility 6 rue Horowitz, BP 220, 38043 Grenoble Cedex 9, France [email protected] CONTRIBUTORS XVII

B. LENGELER RWTH Aachen, II. Physikalisches Institut B Huyskensweg Turm 28, 52074 Aachen, Germany [email protected]

H. RENEVIER Département de la Recherche Fondamentale sur la Matière Condensée, SP2M/NRS CEA Grenoble, 17 Avenue des Martyrs, 38054, Grenoble Cedex 9, France [email protected]

P. SAINCTAVIT Institut de Minéralogie et de Physique des Milieux Condensés Université P. et M. Curie, Case 115, 4 place Jussieu, 75252 Paris Cedex 05, France [email protected]

R. SCHERM Physikalisch-Technische Bundesanstalt Bundesallee 100, 38116 Braunschweig, Germany [email protected]

C.M. SCHNEIDER Institute of Electronic Properties, Department of Solid State Research (IFF) Research Center Jülich, 52425 Jülich, Germany [email protected]

F. SETTE European Synchrotron Radiation Facility 6 rue Horowitz, BP 220, 38043 Grenoble Cedex 9, France [email protected]

M. SUTTON Center for the Physics of Materials Mac Gill University, Montreal, Québec, Canada, H3A 2T8 [email protected]

A. TADJEDDINE LURE Université Paris-Sud, Bât. 209D, BP 34, 91898 Orsay Cedex, France [email protected] XVIII CONTRIBUTORS

H.P. TROMMSDORFF Laboratoire de Spectrométrie Physique Domaine universitaire, BP 87, 38402 St-Martin-d’Hères, France [email protected]

J. VOGEL Laboratoire Louis Néel CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble, France [email protected] FOREWORD

This volume is devoted to the use of synchrotron radiation and neutron sources. It is based on a course delivered to young scientists in the HERCULES programme, organised by the Université Joseph Fourier and the Institut National Polytechnique de Grenoble. The success of such a course has been well demonstrated as more than 1000 researchers (about 900 from the European Union) have attended the sessions. The HERCULES programme i s supportedbythe European Commission, the Ministère de l'éducation nationale, de l'enseignement supérieur et de la recherche, the Centre National de la Recherche Scientifique (CNRS), the Commissariat à l'énergie atomique (CEA), the partner Large Infrastructures (ESRF, ILL, ELETTRA, LLB), the Synchrotron Radiation and Neutron European Integrated Infrastructure Initiatives (I3 and NMI3) and the Région Rhône-Alpes.

F. HIPPERT - E. GEISSLER - J.L. HODEAU - E. LELIÈVRE-BERNA - J.R. REGNARD

This volume is the fifth in a series. Previous volumes are: Neutrons and Synchrotron Radiation for Condensed Matter Studies: "Theory, instruments and methods", HERCULES Series, Editors: J. BARUCHEL, J.L. HODEAU, M.S. LEHMANN, J.R. REGNARD, C. SCHLENKER, Les Editions de Physique - Springer-Verlag, 1993. Neutrons and Synchrotron Radiation for Condensed Matter Studies: "Applications to solid state physics and chemistry", HERCULES Series, Editors: J. BARUCHEL, J.L. HODEAU, M.S. LEHMANN, J.R. REGNARD, C. SCHLENKER, Les Editions de Physique - Springer-Verlag, 1994. Neutrons and Synchrotron Radiation for Condensed Matter Studies: "Applications to soft condensed matter and biology", HERCULES Series, Editors: J. BARUCHEL, J.L. HODEAU, M.S. LEHMANN, J.R. REGNARD, C. SCHLENKER, Les Editions de Physique - Springer-Verlag, 1994. Neutrons and Synchrotron Radiation for Condensed Matter Studies: "Structure and dynamics of biomolecules", HERCULES Series, Editors: E. FANCHON, E. GEISSLER, J.L. HODEAU, J.R. REGNARD, P. TIMMINS, Oxford University Press, 2000.

XIX INTRODUCTION

Large research infrastructures such as synchrotrons for X-ray production and neutron sources have no doubt come to play an indispensable role in modern experimental research into the structure and dynamics of condensed matter and materials. We understand the terms condensed matter and materials to embrace all forms of matter that we encounter in our daily life or that can be produced in the laboratories on earth, including living matter and biological macromolecules. Synchrotron radiation offers intense and collimated photon beams with high peak brightness, short pulse time structure and easy wavelength tunability. Neutron methods provide exquisite sensitivity to magnetic moments, isotope labelling and collective fluctuations. These characteristics are exploited in fundamental research, such as the investigation of strongly correlated electrons or the determination of the structure of proteins; for more applied purposes such as the study of magnetic devices, the analysis of human tissues or the development of pharmaceutical products; and even for very down-to-earth industrial problems concerning e.g., construction or packaging materials. In spite of their large construction and operating costs (often beyond the resources of a single European country), and of the inconvenience and stress incurred by travelling to distant facilities and the need to complete the experiment in a few frantic days (and nights!), large research infrastructures are ever more popular. This is witnessed on the one hand by the coming on line of new European synchrotron sources and the opening of new beam lines at existing facilities, and on the other, by the start of the new German neutron source in Munich, together with the renewal of instrumentation at European neutron facilities. In order to exploit fully the exciting possibilities offered by synchrotrons and neutron sources to researchers from universities, academic and industrial laboratories, such an expansion must be accompanied by advanced training courses in the continuously evolving experimental methods associated with these facilities. The HERCULES programme, which started in 1989, has played a pivotal role by offering a series of lectures and practical "hands on" sessions on the use of photons and neutrons in science. The first volumes were dedicated to theoretical aspects and applications to particular fields such as condensed matter, biology and chemistry.

XXI XXII NEUTRON AND X-RAY SPECTROSCOPY

The current volume presents recent developments in spectroscopic methods involving neutrons or photons. Experimental developments often arise from concurrent progress in instrumentation and theoretical tools, and methods using photons or neutrons are no exception. In modern X-ray research, the so called "third generation" synchrotron sources provide undulator beams with unprecedented brilliance, tunable polarization properties and a substantial degree of spatial coherence. The first four chapters of this book, by F. DE GROOT and J. VOGEL, by P. SAINCTAVIT, V. BRIOIS and D. CABARET, by F. BAUDELET and by B. LENGELER, probe the breadth and depth of new possibilities that are offered to absorption-based spectroscopy by these features of modern sources. They also introduce the reader to the theoretical methods needed to understand the meaning and extract the maximum amount of information from the experiments. In particular, the use of polarization-dependent absorption– i.e., dichroism – in the study of magnetic materials, and the analysis of the spectra by sum rules and electronic structure calculations is emphasized.

The three following chapters, by F. SETTE and M. KRISCH, by M. GRIONI and by J.L. HODEAU and H. RENEVIER, discuss the new life of time-honoured techniques that have been undergoing a revolution because of the progress in sources and instrumentation. Inelastic scattering has proven invaluable in the study of the collective excitations of some classes of disordered systems, such as liquids and glasses, thanks to the high brilliance of the sources and the huge resolving power of the spectrometers. Photoemission has evolved into the primary tool for experimental investigations of electronic quasi-particles, band dispersion and Fermi surfaces in solids; and finally, anomalous scattering has been developing into an indispensable tool for the crystallographer, in particular for the characterization of the local environment of a specific atom.

The chapters by C.M. SCHNEIDER and by M. SUTTON illustrate the power of modern microscopic and coherent scattering techniques, respectively. Here the small source size of third generation sources is very important, as it allows a larger number of photons to be brought into a small area (typically micrometric or sub- micrometric). This is promoting the development of new techniques (PEEM microscopy) or the extension of visible laser techniques, such as intensity fluctuation spectroscopy, to the X-ray range.

The chapter by A. TADJEDDINE and P. DUMAS that follows reminds us of the incredible versatility of synchrotrons: here they are shown to be superior sources not just of VUV or X-ray radiation, but also of infrared photons. In the IR energy range the synchrotron brilliance is however surpassed by another accelerator-based INTRODUCTION XXIII source, the Free Electron Laser. Worldwide efforts to extend the spectral range of free electron lasers to ultraviolet and x-ray radiation are ushering in the next revolution, fourth generation light sources. A feature common to neutron and X-ray experiments is the increasing degree of complexity of data analysis. It has become obvious that a proper understanding of complex material systems, their static or dynamical properties, require modelling using ab initio or empirical methods. The beauty and the simplicity of neutron scattering cross-sections allow direct comparison between experimental data and computer modelling as demonstrated by M.R. JOHNSON, G.J. KEARLEY and H.P. TROMMSDORFF. Even if simulations do not replace experiments, these approaches will develop more widely: modelling gives a microscopic view and can be extended beyond the scope of the experimental measurements. Neutrons can tell where atoms or magnetic moments are, but more importantly, they can tell what they do. The dynamics of the constituent species of materials determine their characteristics and functions: neutron spectroscopy is a key tool to understand the properties of materials. Both reactor-based facilities and neutron spallation sources have developed new generations of neutron spectroscopy instruments that cover more efficiently large sections in energy transfer (or time windows) over large sections of reciprocal space (or spatial resolution). It is therefore timely to re-address the question of applications of inelastic neutron scattering in general. The review of inelastic processes by R. SCHERM and B. FÅK exposes all the beauty of inelastic neutron scattering and explains the simple connection between neutron data and theoretical quantities. The interplay between neutron spectroscopy measurements and theory is driving the development of many new concepts in science, especially solid state physics. Four different experimental methods are discussed that are related to the two categories of neutron production. Three-axis spectrometers (TAS) presented by R. CURRAT are perfectly suited to reactor-based installations with their high time average neutron fluxes; new developments to enhance data acquisition rates and performance are presented. Neutron TAS methods cover a large area in time and space but complementary techniques (X-ray inelastic scattering and other neutron methods) must be considered, depending on the field of applications. Neutron time-of-flight techniques (TOF) are used both at steady and at pulsed neutron sources. R. ECCLESTON and B. FRICK introduce the different geometries for spectrometers: space and time domains of application for each class of instruments are presented together with discussion on complementary use and possible overlap. Backscattering spectrometers at reactor and spallation sources are XXIV NEUTRON AND X-RAY SPECTROSCOPY exploited to investigate energy transfers down to the 100 neV range, corresponding to processes with characteristic times shorter than a nanosecond. Neutron spin echo methods occupy a very particular position because they give access to real time information in the nanosecond to microsecond range. As pointed out by R. CYWINSKI, they very nicely complement TAS and TOF neutron methods, offering gains in energy resolution by taking advantage of Larmor precession of spin-polarized neutrons. A vast domain of effects with slow dynamics up to the microsecond can be studied by these methods. This collection of chapters represents a useful support for all scientists interested in the study of condensed matter physics and chemistry. Spectroscopic methods using X-rays and neutrons are in fact exploited by a growing scientific community.

Massimo ALTARELLI Sincrotrone Trieste, Italy

Christian VETTIER Institut Laue-Langevin, Grenoble, France