Contributions of excitation autoionization to electron-impact ionization of Mg-like Al+, S4+, Cl5+, and Ar6+ ions.

Abstract

The R-matrix method is used to calculate the contributions of excitation autoionization to electron-impact ionization in Mg-like ${\mathrm{Al}}^{+}$, ${\mathrm{S}}^{4+}$, ${\mathrm{Cl}}^{5+}$, and ${\mathrm{Ar}}^{6+}$ ions. The ground 3${s}^{2}$ ${\mathrm{}}^{1}$S state together with the autoionizing states arising from the 2${p}^{5}$3${s}^{2}$3p, 2${p}^{5}$3${s}^{2}$3d, 2${p}^{5}$3${s}^{2}$4s, 2s2${p}^{6}$3${s}^{2}$3p, 2s2${p}^{6}$3${s}^{2}$3d, and 2s2${p}^{6}$3${s}^{2}$4s configurations are included in the R-matrix expansion. Large numbers of bound terms of appropriate symmetry are included in the expansion to account for the resonant-excitation--double-autoionization process. The total ionization cross sections are obtained by combining the cross sections for inner-shell excitation with direct-ionization cross sections. Our calculation shows an abrupt and significant rise in the cross sections, in agreement with recent crossed-beam measurements of Howald et al. The importance of indirect processes over the direct process increases with increasing ionic charge along the sequence.