Absolute cross-section measurements for electron-impact ionization of Na-like ions—${\mathrm{Mg}}^{+}$, ${\mathrm{Al}}^{2+}$, and ${\mathrm{Si}}^{3+}$

Measured cross sections for single ionization of ${\mathrm{Mg}}^{+}$, ${\mathrm{Al}}^{2+}$, and ${\mathrm{Si}}^{3+}$ by electron impact are reported. Crossed beams of electrons and ions have been employed to study the absolute cross sections as a function of collision energy in detail. Near threshold the cross sections for ${\mathrm{Mg}}^{+}$ and ${\mathrm{Al}}^{2+}$ are roughly 70% of the predicted direct-ionization cross sections, while ${\mathrm{Si}}^{3+}$ is in reasonable agreement with the predictions. Contributions to the total cross section by indirect processes, principally inner-shell-excitation autoionization, are specifically identified in each case and compared with theoretical results. These comparisons demonstrate specific failures of the predictions which rely on addition of excitation cross sections to the direct-ionization cross section.