Contributions of excitation autoionization to electron-impact ionization of Mg-likeAl+,S4+,Cl5+, andAr6+ions

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.