Theory and interpretation of L-shell X-ray absorption spectra

X-ray absorption near edge structure (XANES) directly reflects the electronic structure in a material. However, despite significant progress in XANES theory, the quantitative analysis of XANES is not fully developed and remains a challenge.; In this work, a detailed analysis of the L2,3 edge XANES in transition metals was performed using relativistic, self-consistent real space Green's function code FEFFS. Several prescriptions for taking into account core hole in calculations of x-ray absorption spectra (XAS) were discussed. It was found that in most cases of L2,3 edge XANES in transition metals, the initial state (ground state) calculations were in the best agreement with experimental data.; A procedure was developed for quantitative applications of the sum rules for XAS, e.g., for x-ray magnetic circular dichroism and for obtaining hole counts. The approach is based on theoretical atomic calculations of transformations relating various experimental spectra to corresponding operator-spectral densities. This approach overcomes the difficulties of background subtraction and hole-count normalization of other sum rule analysis methods and yields quantitative values for spin- and orbital-moments from experimental absorption spectra. The developed approach was theoretically tested and applied to experimental XAS data in Cu, Ni, Co, Fe, and other materials.; Hole counts obtained from XAS are often interpreted in terms of free-atom occupation numbers or Mulliken counts. We demonstrated that renormalized-atom (RA) counts are a better choice to characterize the configuration of occupied electron states in molecules and condensed matter. A projection-operator approach was introduced to subtract delocalized states and to determine such hole counts from XAS quantitatively. Theoretical tests for the s- and d-electrons in transition metals showed that the approach works well.; A formalism was developed based on time dependent local density approximation (TDLDA) theory that takes into account polarization-type many body effects. This effect, which is essentially a screening of the local x-ray field due to the Coulomb interaction, mostly affects dipole matrix elements. The effect is most important for soft x-rays with energies less than 1 keV. Results of the TDLDA calculations for the L3/L2 white-line ratio in 3d metals are in good agreement with experiment.