New model for electron-impact ionization cross sections of atoms and molecules

Abstract

A new theoretical model for electron‐impact ionization cross sections for atoms and molecules is presented. The new model combines the binary‐encounter theory and the Bethe theory for electron‐impact ionization, and uses minimal theoretical data for the ground state of the target atom or molecule. Two versions of the model are presented. The first one, the Binary‐Encounter‐Dipole (BED) model, requires the knowledge of continuum oscillator strengths and produces the differential ionization cross section, i.e., energy distribution of ejected electrons. The differential cross section is then integrated over the ejected electron energy to obtain the total ionization cross section. The second version, the Binary‐Encounter‐Bethe (BEB) model, assumes a simple form of the continuum oscillator strength to obtain a compact and analytic form of the total ionization cross section. We found that both the BED and BEB models provide total ionization cross sections from threshold to several keV in incident energy within 5% to 15% of known experimental data for many neutral targets. The total ionization cross sections are expressed in compact analytic expressions suitable for use in modeling, e.g., of plasmas and radiation effects. We found that the BEB model is particularly effective in estimating total ionization cross sections of complex molecules.