Electron‐Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. VIII. Ar xiii with New Features

Abstract

Ar XIII is found to be unique with new features in electron-ion recombination not seen in any other ion. The ion has been studied with the unified method, which provides a theoretically self-consistent set of atomic parameters for the inverse processes of photoionization and total electron-ion recombination. Unified method subsumes both the radiative recombination (RR) and dielectronic recombination (DR) within the framework of close-coupling formulations using the R-matrix method. A set of four DR "bumps," two in the low-temperature and two in the high-temperature regions, is found to exist in the recombination rates of Ar XIV + e → Ar XIII. This is in contrast to two typical DR "bumps," one at high temperature common for most ions and one at low temperature depending on the presence of near threshold autoionizing resonances in the bound-free process. Large-scale ab initio calculations have been carried out for photoionization and electron-ion recombination cross sections of Ar XIII. The ion is represented by a large close-coupling eigenfunction expansion of 37 core Ar XIV states from n = 2 and 3 complexes. This enables core excitations of type Δn = 0 and 1. The Δn = 1 transitions have much higher radiative decay rates than those of Δn = 0 and cause the fourth DR bump around 2 × 10 6 K. For a large number of bound states, Ar XIII exhibits more extensive resonant structures and wider PEC (photoexitation-of-core) resonances for n = 3 core states than those of n = 1 states. Hence, the high-energy regions of photoionization and recombination are dominated by these structures. A total of 684 bound states with valence electron n ≤ 10 and l ≤ 9 are found for Ar XIII. Total and partial photoionization cross sections of all bound states, state-specific recombination rates of 561 bound states, and total recombination rate coefficients at a large temperature range are presented for Ar XIII.