Nonspherical Potential, Vibronic and Local Field Effects in XRay Absorption

The theory of x-ray absorption near edge structure (XANES) is complicated due to the difficulty of treating chemical, many-body, and core-hole effects. To address these problems we introduce a full potential, real space Green's function approach for calculations of both local electronic structure and x-ray spectra. Our approach includes inelastic losses as well as screened core-hole and local field effects. The latter are due to the screening of the x-ray electric field. The approach is illustrated in calculations for periodic solids and for small molecules. Calculations for the K-edge absorption of Si crystals show significant improvements in the XANES compared with that calculated with spherical, muffin-tin potentials. However, for systems of high symmetry like the S K-edge in the SF6 molecule, we find that the nonspherical corrections are small. Instead, local atomic displacements are responsible for additional XANES peaks. Local field effects are shown to be important at the diamond K-edge, which effectively counteract the core-hole interaction.