Ionization by electron impacts and ionization potential depression

TL;DR

This paper investigates how ionization potential depression (IPD) influences ionization cross-sections in dense plasmas, introduces a new analytical formula for ionization rates, and compares results with experimental data.

Contribution

It presents a new analytical formula for ionization cross-sections that combines existing models and fits atomic code data, improving rate calculations in dense plasma conditions.

Findings

IPD significantly affects ionization cross-sections at high densities.

The new formula provides accurate, fully analytical ionization rate calculations.

Results align well with experimental measurements on aluminum and CNO ions.

Abstract

We calculate the cross-section of ionization by free-electron impacts in high or moderate density plasmas. We show that the so-called ionization potential depression (IPD) strongly affects the magnitude of the cross-section in the high-density domain. We use the well-known IPD formulas of Stewart-Pyatt and Ecker-Kr\"oll. A more recent approach based on classical molecular dynamics simulation is also investigated. The latter provides an alternative way to calculate IPD values. At near-solid densities the effects of the free-electron degeneracy should be investigated. The rates are then calculated within the Fermi-Dirac statistics. We first use the semi-empirical formula of Lotz for ionization cross-section. The results may differ significantly from measured cross-sections or calculations with reliable atomic codes. Then, in a second step, we propose a new formula that combines the Lotz formula and a polynomial expansion in terms of the ratio of the energy of the incident electron and the ionization energy. The coefficients of the polynomial expansion are adjusted to fit the cross-section provided by robust atomic codes. A great advantage of the new formula is that it allows a fully analytical calculation of the ionization rate. Our results are compared to experiments measuring IPDs, cross-sections and rate coefficients on aluminum at high and moderate densities and on Be-like CNO ions.