ABSTRACT
Core hole spectroscopy is a powerful tool for interpreting the electronic, magnetic, and geometric structure of molecules, solutions, proteins, or solids. The complex mechanism behind the origin of these spectra means that their analysis invariably requires detailed theoretical simulations, which have traditionally been performed using multiple scattering1-3 or multiplet theories.4-8 However, rapid experimental developments, as well as continuous improvements in the instrumentation are enhancing the sensitivity of core hole experiments and consequently ner details of the spectra are being uncovered.4,9,10 In addition, time-resolved experiments with femtosecond temporal resolution11-14 and picosecond second-order spectroscopies15 can now be achieved. Finally, the advances brought about by the X-ray free electron lasers 16 offer exciting possibilities
4.1 Introduction .................................................................................................. 117 4.2 Basics of X-ray Absorption and Scattering .................................................. 118 4.3 Data Reduction ............................................................................................. 120
4.3.1 XANES and EXAFS ........................................................................ 121 4.3.2 Fluorescence Yield Measurements, XES and RIXS ........................ 122
4.4 A Qualitative Interpretation .......................................................................... 122 4.4.1 Pre-Edge and Electronic Transitions ................................................ 123 4.4.2 Edge Shifts ........................................................................................ 123 4.4.3 Extended Fine Structure .................................................................. 123
4.5 Calculating the Spectra .................................................................................124 4.5.1 XANES ............................................................................................. 125
4.5.1.1 Multiple Scattering Theory ................................................ 125 4.5.1.2 Full Potential Approaches .................................................. 127
4.5.2 EXAFS ............................................................................................. 129 4.5.2.1 EXAFS Equation ............................................................... 129 4.5.2.2 Fourier Transform .............................................................. 131 4.5.2.3 Wavelet Transform ............................................................. 133 4.5.2.4 Fitting EXAFS Spectra ...................................................... 133
4.5.3 Electronic Transitions: Pre-Edge and L2/3-Edges ............................. 135 4.5.3.1 Time-Dependent Density Functional Theory .................... 135 4.5.3.2 Multiplet Theory ................................................................ 137 4.5.3.3 Post Hartree-Fock Methods .............................................. 138
References .............................................................................................................. 143
for nonlinear core level spectroscopies17 and for achieving a temporal resolution of ≈10 fs, making it possible to unravel detailed insights into fundamental processes as they evolve in real time. Importantly, these developments call for high-level theoretical approaches so that one may accurately interpret the experimental results.
