Many-electron effects in EXAFS
thesisposted on 15.12.2014, 10:40 by Mervyn. Roy
Extended X-ray absorption fine structure (EXAFS) spectroscopy is an experimental technique useful in the study of non-crystalline materials. EXAFS is analysed by comparing experimental to theoretical spectra. Many-electron effects in EXAFS are important, however, EXAFS theory has been developed using a single electron formalism with many-body effects included via empirical factors. Multiple electron excitations reduce the EXAFS amplitude and hence affect the determination of coordination numbers. At present, intrinsic amplitude reduction effects are modelled by a constant factor whilst extrinsic effects are modelled using an imaginary scattering potential or mean free path term.;This thesis is concerned with the many electron effects in EXAFS. Expressions are developed with which the EXAFS amplitude may be studied independently in the presence of a complex scattering potential. The Hedin-Lundqvist  potential, which is most commonly used in EXAFS analysis, is found to overestimate the extrinsic losses but fortuitously gives good agreement to the total losses to the EXAFS. It is concluded that an intrinsic reduction factor should not be used when data fitting with this potential. The Beni, Lee and Platzman  correlation potential is also investigated and found to be unsuitable for EXAFS calculations.;The intrinsic amplitude reduction factor is calculated in the high energy limit for all elements and found to give good agreement with experiment. Calculations with both tight binding and atomic initial state wavefunctions show that chemical effects are unimportant when determining the intrinsic amplitude reduction factor.;Time-dependent perturbation theory and a model form for the core hole - photoelectron system are used to calculate the multiple electron excitations following a photoabsorption. Screened and unscreened forms of the potential are investigated. The results agree well with experiment and may be used to approximate the amplitude losses to the EXAFS without the need for ad hoc parameters or complex scattering potentials.