Electron-impact ionization in the magnesium-isoelectronic sequence

Excitation-autoionization contributions to the electron-impact ionization of the Mg-like ions ${\mathrm{S}}^{4+}$, ${\mathrm{Cl}}^{5+}$, and ${\mathrm{Ar}}^{6+}$ are calculated in the distorted-wave approximation. The calculations indicate that the largest contributions are due to the 2p→3p monopole and 2p→3d dipole excitations from either the 2$p^6$3$s^2$ ground-state configuration or the 2$p^6$3s3p excited-state configuration. Excellent agreement with the experimental crossed-beam measurements of Howald et al. is obtained when one assumes that a substantial fraction of the ions coming from the electron cyclotron resonance ion source are in metastable states of the 2$p^6$3s3p excited-state configuration.