Inelastic Losses and Multi-Electron Excitations in X-Ray Spectra

An approach is discussed for ab initio calculations of inelastic losses and multi-electron excitations in core level x-ray absorption spectra (XAS) The approach is based on first principles calculations of dielectric response using a real-space Green's function formalism. Extrinsic losses are calculated in terms of the photoeleetron self-energy using a many-pole representation of the dielectric response function and the GW approximation. Multi-electron excitations correspond to satellites in the spectral function, which is obtained from similar ingredients using a quasi-boson model. The theory accounts for extrinsic and intrinsic losses as well as interference terms, and yields quantitative estimates of inelastic mean free paths and the many-body amplitude factor S02{expression}. These results improve both the near edge and the extended fine structure.