Contribution of autoionization to total ionization rates

Abstract

Electron impact collisions with ground-state ions in a low-density plasma may cause ionization either by direct ejection of an electron from the ion, or by excitation of an inner-subshell electron to an autoionizing level. The importance of the indirect process in sodium-like Fe XVI is investigated by means of detailed calculations of excitation cross sections and of branching ratios for autoionization versus radiative decay; the estimated accuracy is about 20%. It is found that radiative decay reduces the autoionization rate (at kT = 300-500 eV) to only about 52% of the excitation rate; even so, the indirect ionization rate is about 1.4-1.6 times the direct ionization rate in this temperature range. The relative importance of indirect to direct ionization is approximately as great for Na-like Mo XXXII as for Fe XVI, but should be much smaller for most non-alkali-like ions except Cu-like, Zn-like, and perhaps Ga-like ions.