Excitation-Autoionization Contributions to Electron Impact Ionization

Experimental measurements of electron impact ionization cross sections for the transition-series ions ${\mathrm{Ti}}^{+3\mathrm{}}$, ${\mathrm{Zr}}^{+3\mathrm{}}$, and ${\mathrm{Hf}}^{+3\mathrm{}}$ demonstrate that excitation-autoionization increases the cross sections by more than an order of magnitude over that anticipated for direct ionization. Theoretical predictions of both the energies and approximate magnitudes of these excitation cross sections provide confidence that this important indirect contribution to the total ionization can be estimated for many cases.