Atomic photoionization cross-sections beyond the electric dipole approximation

TL;DR

This paper introduces a new response theory-based method to compute atomic photoionization cross-sections beyond the electric dipole approximation, accounting for higher-order effects at high photon energies.

Contribution

It develops a novel computational approach using Gaussian orbitals and plane waves to evaluate photoionization beyond the electric dipole approximation for atoms and ions.

Findings

Corrections increase with photon energy and initial state size.

Plane wave approximation becomes valid above certain energy thresholds.

Corrections are generally below 10% up to 12 keV photon energy.

Abstract

A methodology is developed to compute photoionization cross-sections beyond the electric dipole (BED) approximation from response theory, using Gaussian type orbitals and plane waves for the initial and final states, respectively. The methodology is applied to compute photoionization cross-sections of atoms and ions from the first four rows of the Periodic Table. Analysing the error due to the plane wave description of the photoelectron, we find kinetic energy and concomitant photon energy thresholds above which the plane wave approximation becomes applicable. The correction introduced by going beyond the electric dipole approximation increases with photon energy and depends on the spatial extension of the initial state. In general, the corrections are below 10% for most elements, at a photon energy reaching up to 12 keV.