Dynamic screening effects in x-ray absorption spectra

Calculations of x-ray absorption for soft x rays are often dependent on screening of the x-ray field and the photoelectron--core-hole interaction. Though screening is usually calculated with static screening models, we find that L-shell x-ray absorption in 3d transition metals is sensitive to dynamic screening effects. This screened interaction is calculated here using a generalization of the time-dependent local-density approximation, based in part on the Bethe-Salpeter equation. For computational efficiency, our approach uses a local screening approximation based on a projection onto a local atomic basis. The approach yields efficient calculations of the spectra in terms of screened transition matrix elements, and can be implemented straightforwardly within a real-space Green's-function approach. Calculations for rare-gas solids demonstrate the effectiveness of this local model, and also give reasonable agreement with the observed fine structure. Calculations based on a dynamic-screening model account for the observed deviations of the L₃/L₂ intensity branching ratio from the 2: 1 value of independent-electron theory.