X-ray Absorption Spectroscopy

Targeted for chemists, the current textbook outlines the principles, experimental methods and data analysis in X-Ray Absorption Spectroscopy (XAS). The authors introduce EXAFS, Near-Edge XAS, X-Ray Imaging and many other advanced experimental techniques. A special section of the book is devoted to applications of XAS in chemistry, materials and environmental sciences. A balanced combination of theory and practice in X-Ray Absorption Spectroscopy. Explains how to interpret and analyze spectroscopic data. Gives an overview of XAS applications with many case studies. Cover Half Title Also of interest X-ray Absorption Spectroscopy Copyright Dedication Acknowledgements Contents About this book About the authors 1. Introduction 1.1 First light 1.2 The origin of photon sciences 1.3 Early synchrotron XAS facilities 1.4 A first visit to SSRL 1.5 Rapid development 1.6 The generation game 1.7 X-ray absorption spectroscopy References 2. Sources of synchrotron radiation 2.1 Introduction 2.2 The Lorentz force 2.3 Electromagnetic radiation from accelerated charged particles 2.4 Synchrotron radiation spectrum and time structure 2.5 Beam current 2.6 Electron beam emittance 2.7 Spectral brightness 2.8 Polarization and coherence 2.9 Components of synchrotron radiation facilities 2.9.1 The electron source, linear accelerator and booster synchrotron 2.9.2 The radio frequency (RF) system 2.9.3 Magnetic lattice 2.10 Beam orbits and beam losses 2.11 Bend magnet sources 2.11.1 Superbend magnets 2.12 Wigglers and undulators Further reading 3. X-ray beamlines 3.1 Introduction 3.2 Hard X-ray monochromators 3.2.1 Monochromator crystals 3.2.2 Double-crystal monochromators 3.2.3 Crystal monochromator energy resolution and range 3.2.4 The harmonic rejection problem and crystal detuning 3.2.5 Monochromator crystal glitches 3.2.6 Monochromator designs 3.2.7 Some corrections to the simple picture 3.2.8 Specialized crystal monochromators 3.3 Soft X-ray monochromators 3.4 X-ray mirrors 3.4.1 Polarization and mirrors 3.4.2 X-ray mirror coatings 3.4.3 X-ray mirrors in harmonic rejection 3.4.4 X-ray mirror optical figures 3.5 The other pieces: filters, windows, masks and slits 3.6 Radiation shielding and personnel protection 3.6.1 Radiation shielding 3.6.1 Radiation shielding 4. Interactions of X-rays with matter 4.1 Background concepts – wave particle duality 4.2 Background concepts – quantum numbers 4.3 X-ray scattering 4.3.1 Elastic X-ray scattering 4.3.2 Inelastic X-ray scattering 4.4 X-ray absorption 4.5 Selection rules and photoexcitation 4.6 X-ray fluorescence and Auger emission 4.7 X-ray photoabsorption timescales 4.8 Nomenclature 4.9 X-ray absorption by molecules and solids 5. X-ray detectors and detector systems 5.1 Introduction 5.2 Gas ionization chambers 5.2.1 Ion chamber function 5.2.2 Proportional counters and Geiger-Muller detectors 5.2.3 Gas ionization chamber readout 5.2.4 Calculation of photon flux using a gas ionization detector 5.2.5 Choice of ion chamber fill gas 5.2.6 Ion chamber Soller slits 5.2.7 Gridded ion chamber designs 5.2.8 Fluorescent ion chamber detectors 5.3 Photodiodes 5.4 Energy-dispersive solid-state X-ray detectors 5.4.1 Germanium detectors 5.4.2 Lithium-drifted silicon detectors 5.4.3 Silicon drift detectors 5.4.4 Detector readout – analog pulse processing 5.4.5 Detector readout – digital signal processing 5.4.6 Energy resolution and noise 5.4.7 Count rates and detector dead times 5.4.8 Array detectors 5.4.9 Filters and Soller slits 5.4.10 Neutral X-ray filters 5.4.11 Escape peaks 5.4.12 Solid-state detector readout – binning versus fitting 5.5 Scintillation detectors 5.6 Advanced photon detectors 5.6.1 Transition edge sensor detectors 5.6.2 Superconducting tunnel junction detectors 5.7 Crystal optics-based photon detectors 5.7.1 Types of crystal diffractive optics 5.7.2 Log-spiral bent Laue detector systems 5.7.3 Bent crystal Bragg optics 5.7.4 Johann geometry 5.7.5 The Von Hamos spectrometer 5.7.6 Johansson geometry 5.8 Electron detectors 5.8.1 Gas-amplified total electron yield detector 5.8.2 The channeltron electron multiplier 5.8.3 Drain current 5.8.4 Energy-dispersive electron energy analysers Further reading References 6. The X-ray absorption spectroscopy experiment – I 6.1 Introduction 6.2 Hard X-ray experiments 6.3 Tender X-ray experiments 6.4 Soft X-ray experiments 6.5 In situ experiments References 7. The X-ray absorption spectroscopy experiment – II 7.1 Introduction 7.2 Solid sample preparation for hard X-ray transmittance measurements 7.2.1 Powder diluents 7.2.2 Adhesive tape 7.2.2.1 Samples as pressed discs 7.2.2.2 The powder-on-tape method of solid sample preparation 7.2.2.3 Powder plates 7.3 Calculation of total X-ray absorbance 7.4 Solid samples for tender and soft X-ray fluorescence/electron yiel 7.5 Solution sample preparation 7.5.1 Filling the sample cuvette 7.5.2 Freezing the sample 7.6 Safety considerations 7.7 Signal contamination 7.8 Data acquisition strategies for XAS 7.9 Incident X-ray energy calibration 8. The EXAFS 8.1 Introduction 8.2 X-ray photoabsorption 8.3 A simple expression for the EXAFS 8.4 The EXAFS equation 8.4.1 Central atom effects 8.4.2 The photoelectron mean free path 8.4.3 The photoelectron backscattering phase and amplitude 8.4.4 The Debye-Waller factor 8.5 Polarized EXAFS 8.6 Atomic X-ray absorption fine structure 8.7 Multiple scattering EXAFS References 9. The near-edge structure 9.1 Introduction 9.2 Electric dipole and electric quadrupole transitions 9.3 Chemical sensitivity of near-edge spectra 9.3.1 Oxidation state sensitivity 9.3.2 Sensitivity to chemical bonding 9.3.3 Sensitivity to coordination geometry 9.3.4 Site symmetry and the near-edge spectrum 9.4 Polarization dependence of near-edge spectra 9.5 Shake-down and shake-up transitions 9.6 Electron spin effects 9.6.1 Spin-orbit coupling 9.6.2 X-ray magnetic circular dichroism 9.7 Spectral linewidths 9.8 Multiplet splittings 9.9 Calculation of near-edge spectra 9.9.1 Multiplet calculations 9.9.2 Multiple scattering calculations 9.9.3 Density functional theory 9.9.4 Time-dependent density functional theory 9.10 Peak deconvolution of near-edge spectra References 10. Analysis I – EXAFS data reduction and analysis 10.1 Signal averaging and screening 10.2 Background removal and normalization – removing the pre-edge 10.3 Background removal and normalization – the spline 10.4 The EXAFS 10.5 Sensitivities of the EXAFS 10.5.1 Sensitivity to structural parameters and threshold energy 10.5.2 Sensitivity to backscatterer type 10.6 The EXAFS Fourier transform 10.6.1 k-Windowing 10.6.2 R-windowing – Fourier filtering 10.6.3 Extraction of phase and amplitude functions using Fourier methods 10.6.4 Wavelet transforms 10.6.5 Maximum entropy transforms 10.7 EXAFS curve fitting – goodness of fit, accuracies and precisions 10.7.1 Goodness of fit: metrics of fit quality 10.7.2 Accuracy and precision 10.8 EXAFS curve-fitting analysis 10.8.1 EXAFS bond length resolution 10.8.2 Number of relevant independent points in EXAFS analysis 10.8.3 EXAFS cancellation 10.8.4 Physical and chemical checks – do the derived structural parameters make sense? 10.8.5 Getting fooled – confusion of backscatterers and lack of control of ΔE0 10.8.6 EXAFS range 10.8.7 Reporting the results of EXAFS curve-fitting analysis 10.9 Multiple-scattering EXAFS analysis 10.10 Putting it all together . . . References 11. Analysis II – speciation 11.1 Linear combination analysis 11.2 Choice of standard compounds 11.3 Determining the number of components in linear combination analysis 11.4 Principal component analysis 11.5 Target factor analysis 11.6 Linear combination fits from singular value decomposition 11.7 Principal component analysis of the EXAFS 11.8 Data analytics References 12. Experimental artefacts 12.1 Introduction 12.2 Transmittance leakage effects 12.3 Fluorescence self-absorption 12.4 Electron yield sample charging effects 12.5 Solid-state detector dead time effects and related phenomena 12.5.1 Ice diffraction and detector dead time 12.5.2 Variations in peaking time of digital signal processing hardware 12.6 Energy calibration standard fluorescence leakage 12.7 Radiation damage 12.8 Problems with dark currents (offsets) 12.9 Inadequate ion chamber sweeping voltage 12.10 Preferred orientation of powders References 13. XAS imaging 13.1 Introduction 13.2 Micro-focus optics 13.2.1 Refractive optics 13.2.2 Reflective optics – X-ray capillaries and mirrors 13.2.3 Diffractive optics – Fresnel zone plates 13.3 Raster scanning the sample 13.4 Spectroscopic imaging – beyond elemental mapping 13.4.1 X-ray fluorescence imaging 13.4.2 Experimental geometries 13.4.3 μ-XAS 13.4.4 Spectroscopic imaging using a small number of incident energies 13.4.5 Full-spectrum spectroscopic imaging 13.4.6 Full-field and wide-field spectroscopic imaging 13.4.7 Three-dimensional methods – tomography 13.4.8 Three-dimensional methods – confocal X-ray fluorescence imaging 13.5 Sample environment References 14. High-energy resolution methods 14.1 Introduction 14.2 The Johann geometry array spectrometer 14.3 Hard X-ray emission spectroscopy (XES) and minor line XES 14.4 Tender X-ray XES 14.5 Soft X-ray XES 14.6 High-energy resolution fluorescence detected XAS (HERFD-XAS) 14.6.1 HERFD-XAS nomenclature 14.6.2 Selection of HERFD-XAS analyser crystals 14.6.3 Anatomy of a HERFD-XAS experiment 14.6.4 Importance of accurate emission line determination 14.6.5 HERFD-XAS of dilute systems 14.6.6 HERFD-XAS of concentrated systems 14.6.7 HERFD-XAS speciation analysis of mixtures with fluorescence chemical shifts 14.6.8 High-energy resolution electron yield detected X-ray absorption spectroscopy 14.7 The RIXS plane 14.8 X-ray Raman scattering References 15. XAS using diffraction and reflection 15.1 Introduction 15.2 Diffraction anomalous fine structure 15.2.1 X-ray diffraction and anomalous scattering 15.2.2 Analysis of the DAFS 15.2.3 Extraction of polarized spectra from powder DAFS 15.2.4 Metalloprotein and single-crystal DAFS 15.3 The Borrmann effect 15.4 Grazing incidence X-ray absorption spectroscopy References 16. New and future sources for XAS 16.1 Introduction 16.2 Inverse Compton X-ray sources 16.3 Laser-wakefield X-ray sources 16.4 XAS with free electron laser sources References Closing remarks Appendix A. Complex numbers A.1 Introduction A.2 The complex plane A.3 Exponential formulation A.4 Physical functions Appendix B. Fourier transforms B.1 Introduction B.2 Fourier series for periodic functions B.3 The Fourier transform B.4 The discrete Fourier transform B.5 Some properties of the Fourier transform B.6 Calculations of numerical derivatives and integrals B.7 Fourier transforms of some simple functions B.8 Fast Fourier transforms Appendix C. Elements of quantum mechanical nomenclature C.1 Introduction C.2 Wavefunctions and operators C.3 Eigenfunctions and eigenvalues C.4 Vectors and bra-ket notation C.5 The inner product C.6 Basis vectors C.7 The outer product C.8 Bra-ket in quantum mechanics C.9 Expectation values C.10 Fermi’s golden rule and XAS Appendix D. The EXAFSPAK analysis code D.1 History of the exafspak analysis code Index