Abstract
A methodology is developed to compute photoionization cross sections beyond the electric dipole 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. Analyzing 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.
Highlights
Photoionization is the physical process in which atoms, molecules, or solids emit electrons upon irradiation
IV A), we analyze the performance of the plane wave (PW) approximation for the final state by comparing our calculated photoionization cross sections to calculated results from the literature and experimentally measured data
We compared our calculated cross sections to the experimental data and to the calculated dipole photoionization cross sections from the literature. This comparison showed that the PW approximation for the final state is applicable far from the ionization threshold
Summary
Photoionization is the physical process in which atoms, molecules, or solids emit electrons upon irradiation. A step beyond such a simple comparison is to include in the calculation the probability of ionization of a particular state This can be performed by multiplying the projected density of states with the corresponding photoionization cross section calculated for atomic orbitals within the dipole approximation, as performed, for example, in Refs. By using Gaussian type orbitals (GTOs) and plane waves (PWs) for the initial and final states, respectively, we compute ND cross sections (denoted σND throughout this article) for the atoms in the first four rows of the periodic table, alongside their most common ions, and estimate the magnitude of the ND correction as a function of photon energy for each system. The complete set of data for all atoms and ions included in this study is provided at https://www.theochem.kth.se/BED_atomic_cross_sections
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