Particle-hole cumulant approach for inelastic losses in x-ray spectra

TL;DR

This paper introduces a practical cumulant-based method to accurately calculate inelastic losses in x-ray spectra, accounting for many-body excitations, and explains their effects on spectral features in XPS and XAS.

Contribution

It presents a novel approach combining cumulant representation, real-time DFT, and GW approximation to model inelastic losses in x-ray spectra.

Findings

The method captures spectral broadening and satellite peaks.

It explains the cancellation between extrinsic and intrinsic losses.

Results are demonstrated for weakly and correlated systems.

Abstract

Inelastic losses in core level x-ray spectra arise from many-body excitations, leading to broadening and damping as well as satellite peaks in x-ray photoemission (XPS) and x-ray absorption (XAS) spectra. Here we present a practical approach for calculating these losses based on a cumulant representation of the particle-hole Green's function, a quasi-boson approximation, and a partition of the cumulant into extrinsic, intrinsic and interference terms. The intrinsic losses are calculated using real-time, time-dependent density functional theory while the extrinsic losses are obtained from the GW approximation of the photo-electron self-energy and the interference terms are approximated. These effects are included in the spectra using a convolution with an energy dependent particle-hole spectral function. The approach elucidates the nature of the spectral functions in XPS and XAS and explains the significant cancellation between extrinsic and intrinsic losses. Edge-singularity effects in metals are also accounted for. Illustrative results are presented for the XPS and XAS for both weakly and more correlated systems.