Parameter-free calculations of optical response from the visible to X-rays

J. J. Rehr
UW Physics

The real-space Green's function (RSGF) approach has been used extensively to calculate core-level x-ray spectroscopies ranging from x-ray absorption spectra (XAS) including both EXAFS and XANES to inelastic x-ray scattering (IXS) and electron energy loss spectra (EELS). The theory has been implemented in the FEFF codes which yield semi-quantitative agreement with experiment and permit an interpretation in terms of geometrical and electronic properties of a material. Owing to the difficulty of treating various many-body effects, the calculations heretofore used of a number of free parameters and approximate treatments of inelastic losses and vibrational damping.

However, recent advances in theory now offer the prospect of parameter free calculations within the RSGF approach. These advances include improved treatments of the core-hole interaction, inelastic losses, and Debye-Waller factors. For example, instead of the final state rule, the core-hole interaction can be treated using time-dependent density functional theory (TDDFT) or RPA screening of the core-hole, as in the Bethe-Salpeter equation. This development has led to broad spectrum calculations of optical response from which an efficient many-pole model of dielectric response has been derived. This model in turn yields system-dependent self-energies and mean-free paths, as well as estimates of intrinsic losses due to multi-electron excitations, i.e., the XAS many-body amplitude factor S₀². Finally a first principles approach for the calculation of Debye-Waller factors has been developed based on DFT calculations of the dynamical matrix. Together with advances in algorithms, these developments have given rise to FEFF9, a new generation of parameter free RSGF codes developed my our group which permit broad spectra calculations from the UV to x-ray energies.

*Supported in part by the DOE, NSF, NIST and NIH.

References:

[1] "Theoretical Approaches to X-ray Absorption Fine Structure," J. J. Rehr and R. C. Albers, Rev. Mod. Phys. 72, 621 (2000).

[2] "Combined BSE/TDDFT approach for x-ray absorption calculations," A. L. Ankudinov, J. J. Rehr and Y. Takimoto, Phys. Rev. B 71, 165110 (2005).

[3] "Ab initio calculations of mean free paths and stopping powers," A. P. Sorini, J. J. Kas, J. J. Rehr, M. P. Prange and Z. H. Levine, Phys. Rev. B 74, 165111 (2006).

[4] "Many-pole model of inelastic losses in x-ray absorption spectra," J.J. Kas, A. P. Sorini, M. P. Prange, L. W. Cambell, and J. A. Soininen, Phys. Rev. B 76, 195116 (2007).

[5] "Theoretical X-Ray Absorption Debye-Waller Factors," Fernando D. Vila, J. J. Rehr, H. H. Rossner, H. J. Krappe, Phys. Rev. B 76, 014301 (2007).