Distorted-wave calculations for electron-impact ionization of Si⁴⁺and Si⁵⁺

Cross sections for electron-impact ionization of and ions have been calculated in the distorted-wave approximation. In the present calculations both direct-ionization and excitation-autoionization processes are included. For ions, the cross sections are calculated for ionization from both the ground configuration and the excited configuration. Excitation-autoionization substantially contributes to the ionization of the excited configuration and is approximately a factor of four above direct ionization at energies below the ground-state threshold. For ionization from the ground configurations of both and ions, the direct process dominates the total ionization cross section, and contributions of excitation-autoionization are very small. The present results are in reasonable agreement with the experimental measurements of Thompson and Gregory (Thompson J S and Gregory D C 1994 A 50 1377). Remaining discrepancies between the experiment and theory are discussed.