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Jeroen van Bokhoven has been an Associate Professor of Heterogeneous Catalysis in the Department of Chemistry and Applied Biology at ETH since 2010. He completed a degree in chemistry at Utrecht University in 1995 and went on to obtain a PhD in inorganic chemistry and catalysis in 2000. From 1999 until 2002 he was head of the XAS (X-ray absorption spectroscopy) users - support group at Utrecht University. In 2006 he obtained an SNF assistant professorship in the Department of Chemistry and Applied Biology. He was the 2008 recipient of the Swiss Chemical Society Werner Prize. Van Bokhoven works in the field of heterogeneous catalysis and (X-ray) spectroscopy. His main interests are heterogeneous catalysts and developing advanced tools in X-ray spectroscopy to study the catalyst structure under catalytic relevant conditions.
Carlo Lamberti achieved his degree in Physics in 1988 with a thesis in the field of many body Physics. From 1988 to 1993 he worked in the CSELT laboratories Torino, on the characterization of the interfaces of semiconductor heterostructures with high resolution XRD and X-ray absorption spectroscopies. He presented his PhD defense in solid state physics on this topic in Rome in 1993. He was appointed to the position of researcher in October 1998 at the Department of Inorganic, Physical and Materials Chemistry of the Torino University, and as Associate Professor in December 2006. In recent years he has become an expert in the techniques based on Synchrotron Radiation and Neutrons beams in the characterization of materials, performing more than 90 experiments approved by international committees on the following large scale facilities. He has authored and co-authored more than 200 research articles and five book chapters and two books. He is member of the PhD School in Material Science of the Torino University, and is the Italian coordinator of the MaMaSELF European Master in Materials Science.
VOLUME I
List of Contributors
Foreword
I INTRODUCTION: HISTORY, XAS, XES, AND THEIR IMPACT ON SCIENCE
1 Introduction: Historical Perspective on XAS
Jeroen A. van Bokhoven and Carlo Lamberti
1.1 Historical Overview of 100 Years of X-Ray Absorption: A Focus on the Pioneering 1913-1971 Period
1.2 About the Book: A Few Curiosities, Some Statistics, and a Brief OverviewII EXPERIMENTAL AND THEORY
2 From Synchrotrons to FELs: How Photons Are Produced; Beamline Optics and Beam Characteristics
Giorgio Margaritondo
2.1 Photon Emission by Accelerated Charges: from the Classical Case to the Relativistic Limit
2.2 Undulators, Wigglers, and Bending Magnets
2.2.1 Undulators
2.2.2 Wigglers
2.2.3 Bending magnets
2.2.4 High flux, high brightness
2.3 The Time Structure of Synchrotron Radiation
2.4 Elements of Beamline Optics
2.4.1 Focusing devices
2.4.2 Monochromators
2.4.3 Detectors
2.5 Free Electron Lasers
2.5.1 FEL optical amplification
2.5.2 Optical amplification in an X-FEL: details
2.5.3 Saturation
2.5.4 X-FEL time structure: new opportunities for spectroscopy
2.5.5 Time coherence and seeding
3 Real-Space Multiple-Scattering Theory of X-ray Spectra
Joshua J. Kas, Kevin Jorisson and John J. Rehr
3.1 Introduction
3.2 Theory
3.2.1 Independent-particle approximation
3.2.2 Real-space multiple-scattering theory
3.2.3 Many body effects in x-ray spectra
3.3 Applications
3.3.1 XAS, EXAFS, XANES
3.3.2 EELS
3.3.3 XES
3.3.4 XMCD
3.3.5 NRIXS
3.3.6 RIXS
3.3.7 Compton scattering
3.3.8 Optical constants
3.4 Conclusion
4 Theory of X-ray Absorption Near Edge Structure
Yves Joly and Stephane Grenier
4.1 Introduction
4.2 The x-ray Absorption Phenomena
4.2.1 Probing material
4.2.2 The different spectroscopies
4.3 X-ray Matter Interaction
4.3.1 Interaction Hamiltonian
4.3.2 Absorption cross-section for the transition between two states
4.3.3 State description
4.3.4 The transition matrix
4.4 XANES General Formulation
4.4.1 Interaction times and the multi-electronic problem
4.4.2 Absorption cross-section main equation
4.5 XANES Simulations in the Mono-Electronic Scheme
4.5.1 From multi- to mono-electronic
4.5.2 The different methods
4.5.3 The multiple scattering theory
4.6 Multiplet Ligand Field Theory
4.6.1 Atomic multiplets
4.6.2 The crystal field
4.7 Current Theoretical Developments
4.8 Tensorial Approaches
4.9 Conclusion
5 How to Start an XAS Experiment
Diego Gianolio
5.1 Introduction
5.2.1 Identify the scientific question
5.2.2 Can XAS solve the problem?
5.2.3 Select the best beamline and measurement mode
5.2.4 Write the proposal
5.3 Prepare the Experiment
5.3.1 Experimental design
5.3.2 Best sample conditions for data acquisition
5.3.3 Sample preparation
5.4 Perform the Experiment
5.4.1 Initial set-up and optimization of signal
5.4.2 Data acquisition
6 Hard X-ray Photon-in/Photon-out Spectroscopy: Instrumentation, Theory and Applications
Pieter Glatzel, Roberto Alonso-Mori, and Dimosthenis Sokaras
6.1 Introduction
6.2 History
6.3 Basic Theory of XES
6.3.1 One- and multi-electron description
6.3.2 X-ray Raman scattering spectroscopy
6.4 Chemical Sensitivity of x-ray Emission
6.4.1 Core-to-core transitions
6.4.2 Valence-to-core transitions
6.5 HERFD and RIXS
6.6 Experimental x-ray Emission Spectroscopy
6.6.1 Sources for x-ray emission spectroscopy
6.6.2 X-ray emission spectrometers
6.6.3 Detectors
6.7 Conclusion
7 QEXAFS: Techniques and Scientific Applications for Time-Resolved XAS
Maarten Nachtegaal, Oliver Muller, Christian Konig and Ronald Frahm
7.1 Introduction
7.2 History and Basics of QEXAFS
7.3 Monochromators and Beamlines for QEXAFS
7.3.1 QEXAFS with conventional monochromators
7.3.2 Piezo-QEXAFS for the millisecond time range
7.3.3 Dedicated oscillating monochromators for QEXAFS
7.4 Detectors and Readout Systems
7.4.1 Requirements for detectors
7.4.2 Gridded ionization chambers
7.4.3 Data acquisition
7.4.4 Angular encoder
7.5 Applications of QEXAFS in Chemistry
7.5.1 Following the fate of metal contaminants at the mineral-water interface
7.5.2 Identifying the catalytic active sites in gas phase reactions
7.5.4 Synthesis of nanoparticles
7.5.5 Identification of reaction intermediates: modulation excitation XAS
7.6 Conclusion
8 Time-Resolved XAS Using an Energy Dispersive Spectrometer: Techniques and Applications
Olivier Mathon, Innokenty Kantor and Sakura Pascarelli
8.1 Introduction
8.2 Energy Dispersive X-Ray Absorption Spectroscopy
8.2.1 Historical development of EDXAS and overview of existing facilities
8.2.2 Principles: source, optics, detection
8.2.3 Dispersive versus scanning spectrometer for time-resolved experiments
8.2.4 Description of the EDXAS beamline at ESRF
8.3 From the Minute Down to the Ms: Filming a Chemical Reaction in Situ
8.3.1 Technical aspects
8.3.2 First stages of nanoparticle formation
8.3.3 Working for cleaner cars: automotive exhaust catalyst
8.3.4 Reaction mechanisms and intermediates
8.3.5 High temperature oxidation of metallic iron
8.4 Down to the µs Regime: Matter under Extreme Conditions
8.4.1 Technical aspects
8.4.2 Melts at extreme pressure and temperature
8.4.3 Spin transitions at high magnetic field
8.4.4 Fast ohmic ramp excitation towards the warm dense matter regime
8.5 Playing with a 100 ps Single Bunch
8.5.1 Technical aspects
8.5.2 Detection and characterization of photo-excited states in Cu+ complexes
8.5.3 Opportunities for investigating laser-shocked matter
8.5.4 Non-synchrotron EDXAS
8.6 Conclusion
9 X-Ray Transient Absorption Spectroscopy
Lin X. Chen
9.1 Introduction
9.2 Pump-Probe Spectroscopy
9.2.1 Background
9.2.2 The basic set-up
9.3 Experimental Considerations
9.3.1 XTA at a synchrotron source
9.3.2 XTA at X-ray free electron laser sources
9.4 Transient Structural Information Investigated by XTA
9.4.1 Metal center oxidation state
9.4.2 Electron configuration and orbital energies of X-ray absorbing atoms
9.4.3 Transient coordination geometry of the metal center
9.5 X-Ray Pump-Probe Absorption Spectroscopy: Examples
9.5.1 Excited state dynamics of transition metal complexes (TMCs)
9.5.2 Interfacial charge transfer in hybrid systems
9.5.3 XTA studies of metal center active site structures in metalloproteins
9.5.4 XTA using the X-ray free electron lasers
9.5.5 Other XTA application examples
9.6 Perspective of Pump-Probe X-Ray Spectroscopy
10 Space-Resolved XAFS, Instrumentations and Applications
Yoshio Suzuki and Yasuko Terada
10.1 Space-Resolving Techniques for XAFS
10.2 Beam-Focusing Instrumentation for Microbeam Production
10.2.1 Total reflection mirror systems
10.2.2 Fresnel zone plate optics for x-ray microbeam
10.2.3 General issues of beam-focusing optics
10.2.4 Requirements on beam stability in microbeam XAFS experiments
10.3 Examples of Beam-Focusing Instrumentation
10.3.1 The total-reflection mirror system
10.3.2 Fresnel zone plate system
10.4 Examples of Applications of Microbeam-XAFS Technique to Biology and nenvironmental Science
10.4.1 Speciation of heavy metals in willow
10.4.2 Characterization of arsenic-accumulating mineral in a sedimentary iron deposit
10.4.3 Feasibility study for microbeam XAFS analysis using FZP optics
10.4.4 Micro-XAFS studies of plutonium sorbed on tuff
10.4.5 Micro-XANES analysis of vanadium accumulation in ascidian blood cell
10.5 Conclusion and Outlook
11 Quantitative EXAFS Analysis
Bruce Ravel
11.1 A Brief History of EXAFS Theory
11.1.1 The n-body decomposition in GNXAS
11.1.2 The exact curved wave theory in EXCURVE
11.1.3 The path expansion in FEFF
11.2 Theoretical Calculation of EXAFS Scattering Factors
11.2.1 The pathfinder
11.2.2 The fitting metric
11.2.3 Constraints on parameters of the fit
11.2.4 Fitting statistics
11.2.5 Extending the evaluation of ¿2
11.2.6 Other analytic methods
11.3 Practical Examples of EXAFS Analysis
11.3.1 Geometric constraints on bond lengths
11.3.2 Constraints on the coordination environment
11.3.3 Constraints and multiple data set analysis
11.4 Conclusion
12 XAS Spectroscopy: Related Techniques and Combination with Other Spectroscopic and Scattering Methods
Carlo Lamberti, Elisa Borfecchia, Jeroen A. van Bokhoven and Marcos Fernández-Garcia
12.1 Introduction
12.2 Atomic Pair Distribution Analysis of Total Scattering Data
12.2.1 Theoretical description
12.2.2 Examples of PDF analysis
12.3 Diffraction Anomalous Fine Structure (DAFS)
12.3.1 Theoretical description
12.3.2 Examples of DAFS
12.4 Inelastic Scattering Techniques
12.4.1 Extended energy-loss fine structure (EXELFS)
12.4.2 X-ray Raman scattering (XRS)
12.5 ß-Environmental Fine Structure (BEFS)
12.6 Combined Techniques
12.6.1 General considerations
12.6.2 Selected examples
12.7 Conclusion
VOLUME II
III APPLICATIONS: FROM SEMICONDUCTORS TO MEDICINE TO NUCLEAR MATERIALS
13 X-Ray Absorption and Emission Spectroscopy for Catalysis
13.1 Introduction
13.2 The Catalytic Process
13.2.1 From vacuum and single crystals to realistic pressure and relevant samples
13.2.2 From chemisorption to conversion and reaction kinetics
13.2.3 Structural differences within a single catalytic reactor
13.2.4 Determining the structure of the active site
13.3 Reaction Kinetics from Time-Resolved XAS
13.3.1 Oxygen storage materials
13.3.2 Selective propene oxidation over a-MoO3
13.3.3 Active sites of the dream reaction, the direct conversion of benzene to phenol
13.4 Sub-Micrometer Space Resolved Measurements
13.5 Emerging Methods
13.5.1 X-ray emission spectroscopy
13.5.2 Pump probe methods
13.6 Conclusion and outlook
14 High Pressure XAS, XMCD and IXS 383
Jean-Paul Itie, Francois Baudelet and Jean-Pascal Rueff
14.1 Introduction
14.1.1 Why pressure matters
14.1.2 High-pressure generation and measurements
14.1.3 Specific drawbacks of a high-pressure set-up
14.2 High Pressure EXAFS and XANES
14.2.1 Introduction
14.2.2 Local equation of state
14.2.3 Pressure-induced phase transitions
14.2.4 Glasses, amorphous materials, amorphization
14.2.5 Extension to low and high energy edges
14.3 High-Pressure Magnetism and XMCD
14.3.1 Introduction
14.3.2 Transition metal
14.3.3 Magnetic insulator
14.3.4 The rare earth system
14.4 High Pressure Inelastic X-Ray Scattering
14.4.1 Electronic structure
14.4.2 Magnetic transitions in 3d and 4f electron systems
14.4.3 Metal insulator transitions in correlated systems
14.4.4 Valence transition in mixed valent rare-earth compounds
14.4.5 Low-energy absorption edges: chemical bonding and orbital configuration
14.5 Conclusion
15 X-Ray Absorption and RIXS on Coordination Complexes
Thomas Kroll, Marcus Lundberg and Edward I. Solomon
15.1 Introduction
15.1.1 Geometric and electronic structure of coordination complexes
15.1.2 X-ray probes of coordination complexes
15.1.3 Extracting electronic structure from X-ray spectra
15.2 Metal K-Edges
15.2.1 The case of a single 3d hole: Cu(II)
15.2.2 Multiple 3d holes: Fe(III) and Fe(II)
15.3 Metal L-Edges
15.3.1 The case of a single 3d hole: Cu(II)
15.3.2 Multiple 3d holes: Fe(III) and Fe(II)
15.4 Resonant Inelastic X-Ray Scattering
15.4.1 Ferrous systems
15.4.2 Ferric systems
15.5 Conclusion
16 Semiconductors
Federico Boscherini
16.1 Introduction
16.2 XAS Instrumental Aspects
16.3 Applications
16.3.1 Dopants and defects
16.3.2 Thin films and heterostructures
16.3.3 Nanostructures
16.3.4 Dilute magnetic semiconductors
16.4 Conclusion
17 XAS Studies on Mixed Valence Oxides
Joaquýn Garcýa, Gloria Subýas and Javier Blasco
17.1 Introduction
17.1.1 X-ray absorption spectroscopy (XAS)
17.1.2 XES and XAS
17.1.3 Resonant x-ray scattering
17.2 Solid State Applications (Mixed Valence Oxides)
17.2.1 High tc superconductors
17.2.2 Manganites
17.2.3 Perovskite cobaltites
17.3 Conclusion
18 Novel XAS Techniques for Probing Fuel Cells and Batteries
David E. Ramaker
18.1 Introduction
18.2 XANES Techniques
18.2.1 Data analysis
18.2.2 Data collection
18.2.3 Comparison of techniques by examination of O(H)/Pt and CO/Pt
18.3 In Operando Measurements
18.3.1 Fuel cells
18.3.2 Batteries
18.4 Future Trends
18.5 Appendix
18.5.1 Details of the ¿µXANES analysis technique
18.5.2 FEFF8 theoretical calculations
19 X-ray Spectroscopy in Studies of the Nuclear Fuel Cycle
Melissa A. Denecke
19.1 Background
19.1.1 Introduction
19.1.2 Radioactive materials at synchrotron sources
19.2 Application Examples
19.2.1 Studies related to uranium mining
19.2.2 Studies related to fuel
19.2.3 Investigations of reactor components
19.2.4 Studies related to recycle and lanthanide/actinide separations
19.2.5 Studies concerning legacy remediation and waste disposal (waste forms, near-field and far-field)
19.3 Conclusion and Outlook
20 Planetary, Geological and Environmental Sciences
Francois Farges and Max Wilke
20.1 Introduction
20.2 Planetary and Endogenous Earth Sciences
20.2.1 Planetary materials and meteorites
20.2.2 Crystalline deep earth materials
20.2.3 Magmatic and volcanic processes
20.2.4 Element complexation in aqueous fluids at P and T
20.3 Environmental Geosciences
20.3.1 General trends
20.3.2 Environmentally relevant minerals and phases
20.3.3 Mechanisms and reactivity at the mineral-water interfaces
20.3.4 Some environmental applications of x-ray absorption spectroscopy
20.4 Conclusion
21 X-Ray Absorption Spectroscopy and Cultural Heritage: Highlights and Perspectives
François Farges and Marine Cotte
21.1 Introduction
21.2 Instrumentation: Standard and Recently Developed Approaches
21.2.1 From centimetric objects to micrometric cross-sections
21.2.2 Improving the spectral resolution of XRF detectors
21.2.3 From hard X-rays to soft X-rays
21.2.4 Spectro-imaging in the hard X-ray domain
21.3 Some Applications
21.3.1 Glasses
21.3.2 Ceramics
21.3.3 Pigments and Paintings
21.3.4 Inks
21.3.5 Woods: from historical to fossils
21.3.6 Bones and ivory
21.3.7 Metals
21.3.8 Rock-formed monuments
21.4 Conclusion
22 X-ray Spectroscopy at Free Electron Lasers
Wojciech Gawelda, Jakub Szlachetko and Christopher J. Milne
22.1 Introduction to X-ray Free Electron Lasers in Comparison to Synchrotrons
22.1.1 Overview of facilities
22.1.2 X-ray properties from an XFEL
22.1.3 Scanning the X-ray energy
22.1.4 Comparison with existing time-resolved techniques at synchrotrons
22.2 Current Implementations of X-Ray Spectroscopy Techniques at XFELs
22.2.1 X-ray absorption spectroscopy
22.2.2 X-ray emission spectroscopy
22.3 Examples of Time-Resolved X-Ray Spectroscopy at XFELs
22.3.1 Ultrafast spin-crossover excitation probed with X-ray absorption spectroscopy
22.3.2 Ultrafast spin cross-over excitation probed with X-ray emission spectroscopy
22.3.3 Simultaneous measurement of the structural and electronic changes in Photosystem II after photoexcitation
22.3.4 Investigating surface photochemistry
22.3.5 Soft X-ray emission spectroscopy measurements of dilute systems
22.4 Examples of Nonlinear X-Ray Spectroscopy at XFELs
22.4.1 X-ray-induced transparency
22.4.2 Sequential ionization and core-to-core resonances
22.4.3 Hollow atoms
22.4.4 Solid-density plasma
22.4.5 Two-photon absorption
22.5 Conclusion and Outlook
23 X-ray Magnetic Circular Dichroism
Andrei Rogalev, Katharina Ollefs and Fabrice Wilhelm
23.1 Historical Introduction
23.2 Physical Content of XMCD and the Sum Rules
23.3 Experimental Aspects and Data Analysis
23.3.1 Sources of circularly polarized x-rays
23.3.2 Sample environment
23.3.3 Detection modes
23.3.4 Standard analysis
23.4 Examples of Recent Research
23.4.1 Paramagnetism of pure metallic clusters
23.4.2 Magnetism in diluted magnetic semiconductors
23.4.3 Photomagnetic molecular magnets
23.5 Conclusion and Outlook
24 Industrial Applications
Simon R. Bare and Jeffrey Cutler
24.1 Introduction
24.2 The Patent Literature
24.2.1 Catalysts
24.2.2 Batteries
24.2.3 Other applications
24.3 The Open Literature
24.3.1 Semiconductors, thin films, and electronic materials
24.3.2 Fuel cells, batteries, and electrocatalysts
24.3.3 Metallurgy and tribology
24.3.4 Homogeneous and heterogeneous catalysts
24.3.5 Miscellaneous applications: from sludge to thermographic films
24.4 Examples of Applications from Light Sources
24.4.1 Introduction
24.4.2 Industrial science at the Canadian Light Source
24.4.3 Use of SOLEIL beamlines by industry
24.4.4 Industrial research enhancement program at NSLS
24.4.5 The Swiss Light Source: cutting-edge research facilities for industry
24.5 Examples of Applications from Companies
24.5.1 Introduction
24.5.2 Haldor Topsøe A/S
24.5.3 UOP LLC, a Honeywell Company
24.5.4 General Electric Company
24.5.5 IBM Research Center
24.6 Conducting Industrial Research at Light Sources
24.7 Conclusion and Outlook
25 XAS in Liquid Systems
Adriano Filipponi and Paola D'Angelo
25.1 The Liquid State of Matter
25.1.1 Thermodynamic considerations
25.1.2 Pair and higher order distribution functions
25.2 Computer Modelling of Liquid Structures
25.2.1 Molecular Dynamics simulations
25.2.2 Classical Molecular Dynamics
25.2.3 Born-Oppenheimer Molecular Dynamics
25.2.4 Car-Parrinello Molecular Dynamics
25.2.5 Monte Carlo simulation approaches
25.3 XAFS Calculations in Liquids/Disordered Systems
25.3.1 XAFS sensitivity and its specific role
25.3.2 XAFS signal decomposition
25.3.3 XAFS signal from the pair distribution
25.3.4 The triplet distribution case in elemental systems
25.4 Experimental and Data-Analysis Approaches
25.4.1 Sample confinement strategies and detection techniques
25.4.2 High pressure, temperature control, and XAS sensitivity to phase transitions
25.4.3 Traditional versus atomistic data-analysis approaches
25.5 Examples of Data Analysis Applications
25.5.1 Elemental systems: icosahedral order in metals
25.5.3 Transition metal aqua ions
25.5.4 Lanthanide aqua ions
25.5.5 Halide aqua ions: the bromide case
26 Surface Metal Complexes and Their Applications
Joseph D. Kistler, Pedro Serna, Kiyotaka Asakura and Bruce C. Gates
26.1 Introduction
26.1.1 Ligands other than supports
26.1.2 Supports
26.1.3 Techniques complementing x-ray absorption spectroscopy
26.1.4 Data-fitting techniques
26.2 Aim of the Chapter
26.3 Mononuclear Iridium Complexes Supported on Zeolite HSSZ-53: Illustration of EXAFS Data Fitting and Model Discrimination
26.4 Iridium Complexes Supported on MgO and on Zeolites: Precisely Synthesized Isostructural Metal Complexes on Supports with Contrasting Properties as Ligands
26.5 Supported Chromium Complex Catalysts for Ethylene Polymerization Characterization of Samples Resembling Industrial Catalysts
26.6 Copper Complexes on Titania: Insights Gained from Samples Incorporating Single-Crystal Supports
26.7 Gold Complexes Supported on Zeolite NaY: Determining Crystallographic Locations of Metal Complexes on a Support by Combining EXAFS Spectroscopy and TEM
26.8 Gold Supported on CeO2: Conversion of Gold Complexes into Clusters in a CO Oxidation Catalyst Characterized by Transient XAFS Spectroscopy
26.9 Mononuclear Rhodium Complexes and Dimers on MgO: Discovery of a Catalyst for Selective Hydrogenation of 1,3-Butadiene
26.10 Osmium Complexes Supported on MgO: Determining Structure of the Metal-Support Interface and the Importance of Support Surface Defect Sites
26.11 Conclusion
27 Nanostructured Materials
Alexander V. Soldatov and Kirill A. Lomachenko
27.1 Introduction
27.2 Small Nanoclusters
27.3 XAS and XES for the Study of Nanoparticles
27.4 Nanostructures and Defects in Solids
27.5 Conclusion and Outlook
Index
Carlo Lamberti1,2 and Jeroen A. van Bokhoven3,4
1Department of Chemistry, University of Turin, Italy
2Southern Federal University, Rostov-on-Don, Russia
3Swiss Light Source, Paul Scherrer Institute, Switzerland
4Institute for Chemical and Bioengineering, ETH Zurich, Switzerland
The x-ray absorption spectroscopy (XAS) adventure started about one hundred years ago and has come a long way since. The technique remained a curiosity for much of this time, representing a minor branch of science, developed by only a few highly motivated and enthusiastic scientists. without any apparent possibility of practical application and without a solid and comprehensive theory able to describe and predict the experimental observations done, on gases, liquids and solid (crystalline and amorphous) systems. In 1971, Sayers, Stern and Lytle made ground-breaking progress when they applied Fourier analysis to the point-scattering theory of x-ray absorption fine structure, so as to formally invert the experimental data (primarily collected in the photoelectron wave-vector space) into a radial distribution function. For the first time, they were able to quantitatively determine structural parameters, such as the bond distance, coordination number, as well as the thermal and disorder parameters [1]. In the 44 years following that key publication, the field developed exponentially. Nowadays it is impossible to imagine frontier research in materials science, solid state physics and chemistry, catalysis, chemistry, biology, medicine, earth science, environmental science, cultural heritage, nanoscience, etc. without the contribution of XAS and related techniques. In this introductory chapter we provide a brief sketch of the main events that have established XAS and related techniques as leading scientific characterization tools.
After the discovery of x-rays in 1895 by Röntgen [2, 3], it took a while before the first x-ray absorption spectrum was observed by de Broglie in 1913 [4]. De Broglie mounted a single crystal on the cylinder of a recording barometer, using a clockwork mechanism to rotate the crystal around its vertical axis at a constant angular speed. As the crystal rotated, the x-rays scattered at all angles between the incident beam and the diffraction planes hence, according to the Bragg law (2dhkl sin ? = ? = hc/E, with c being the speed of light, c = 2.9979 10+8 m/s, and h being the Planck constant, h = 6.626 × 10-34 J s [5], so that hc = 12.3984 Å keV) [6-8], changing the x-ray energy E. X-rays of varying intensities were recorded on a photographic plate. Two distinct discontinuities were observed on the film, which were found to be independent of the setting of the x-ray tube. These proved to be the K-edge absorption spectra of silver and bromine atoms contained in the photographic emulsion. As the spectrographic dispersion was poor at these short wavelengths, the spectra were of low energy resolution and the fine structure was not resolved. Successive work by de Broglie in this field proved remarkable [9, 10]. A posteriori, it is curious to note that de Broglie's famous intuition concerning the association of a wavelength (?) to any massive particle with momentum (p): ? = h/p [11], is actually the key to understanding the phenomenon related to the fine structure of the x-ray absorption spectra.
In 1913, Moseley published his empirical law describing the frequencies (energies, E = h?) of certain characteristic x-rays emitted from pure elements, named Ka and La lines according to the successive Siegbahn notation. Emission energies were found to be approximately proportional to the square of the element atomic number Z [12]. This finding supported Bohr's model of the atom [13-15] in which the atomic number corresponds to the positive charge of the nucleus of the atom measured in |e| units: |e| =1.602 10-19 C. Almost 50 years after Mendeleev's milestone work, Moseley's findings suggested that the atomic weight A was not a deciding player in the periodicity of physical and chemical properties of the elements within the periodic table. In contrast, the properties of the elements varied periodically with the atomic number Z. This x-ray emission study is historically important because it quantitatively justifies the nuclear model of the atom, where the atom's positive charge is located in the nucleus and associated on an integer basis with the atomic number. Until Moseley's work, the term "atomic number" was merely a label to identify the place of each element in the periodic table, and it was not known to be associated with any measurable physical quantity.
In 1916, in Lund in Sweden, Siegbahn and Stenström [16-18] developed the first vacuum x-ray spectrometer [19, 20] (Figure 1.1(a)), thereby taking a fundamental technological step in the progress of x-ray spectroscopy. With this kind of innovative technology, the fine structure beyond the absorption edges of selected atoms was - for the first time - experimentally observed by Fricke in 1920 [21] and by Hertz in 1921 [22]. Fricke detected the K-edges for the elements from magnesium (Z = 12, E0 = 1.3 keV) up to chromium (Z = 24, E0 = 6.0 keV) [21], whereas Hertz canvassed the L-edges of cesium (Z = 55, E0 = 5.0 keV) up to neodymium (Z = 60, E0 = 6.2 keV) [22]. In the period before World War II, other authors reported analogous behavior on several different absorption edges [20, 23-37].
Figure 1.1 (a) Scheme of the vapor cell and x-ray spectrometer used by Hanawalt [20] to measure the fine structures in K-edge x-ray absorption spectra of molecules in the vapor phase. The cell was composed of: two furnaces (B and A), the former to host the solid phase and control the vapor pressure via the temperature, the latter to prevent re-condensation of the evaporated phase; a long quartz tube hosting the vapor phase (V) equipped at the end with two concave windows (W) as thin as 3 µm so as to maintain a vacuum and remain sufficiently transparent to x-rays. The spectrometer consists of: an x-ray tube of the Siegbahn type (X); slits used to collimate the incoming x-rays (S); a calcite (CaCO3) crystal used as a monochromator (C); a quartz tube fluxed with H2 (H) and equipped with biological x-ray transparent membranes (G) acting as windows; and a photographic plate used as a detector (P). This spectrometer was able to cover the 4.9-12.4 keV spectral region, corresponding to 2.5 Å > ? > 1.0 Å and represents the historical prototype of a dispersive spectrometer (see [38], Chapter 8 in this volume). Depending on the absorbing gas, the length of time ranging from 4 to 30 h is needed to impress the photographic plate as shown in (b), where the energy increases from top to bottom. The absorption edge and the successive modulations are clearly visible in (b). The photographic images were then converted into absorption-energy plots such as those reported in (c) and (d) for AsCl3 (as K-edge at 11.8 keV) and Zn (K-edge at 9.6 keV), respectively. Because only I1 is measured in the set-up, the spectra appear inverted. The first resonance after the edge, representing a maximum in the absorption, was called the white line while the successive minimum in the absorption spectrum was called the black line. The former term is retained in the current terminology, whereas the latter has been lost. Adapted from Hanawalt, 1931, [20], with permission American Physical Society.
Hanawalt made remarkable observations in 1931 [20], observing that the chemical and physical state of the sample affects the fine structure of the corresponding XAS spectra. Using the experimental set-up reported in Figure 1.1(a), consisting of a quartz cell allowing the XAFS spectra of different molecules sublimated in the vapor phase to be acquired, and collecting XAFS spectra on a photographic plate (Figure 1.1(b)), he was able to make two empirical observations of fundamental importance. First, he proved that substances sublimating in the molecular form, such as arsenic (4Assolid (As4)gas) and AsCl3 (Figure 1.1(c)), are characterized by XAFS spectra exhibiting different fine structures above the edge when measured in the solid or in the vapor phase. Second, he observed that the monatomic vapors of zinc (Figure 1.1(d)), mercury, xenon and krypton elements exhibit no secondary structure. These incredibly advanced experiments already at this stage captured the main messages of EXAFS spectroscopy, but it took several years for the correct interpretation and decades before quantitative data could be extracted and the full potential of EXAFS exploited [1].
The first theoretical attempt to explain the fine structure in the XAS spectra was proposed in 1931 and 1932 by Kronig [39, 40], who developed a model based on the presence of long-range order in the probed system. The Kronig long-range order theory can be summarized in the following equation:
where Wn are the energy positions corresponding to the...
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