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

Cross sections for electron-impact ionization of Si4+ and Si5+ ions have been calculated in the distorted-wave approximation. In the present calculations both direct-ionization and excitation-autoionization processes are included. For Si4+ ions, the cross sections are calculated for ionization from both the 2s(2)2p(6) ground configuration and the 2s(2)2p(5) 3s excited configuration. Excitation-autoionization substantially contributes to the ionization of the 2s(2)2p(5)3s 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 Si4+ and Si5+ 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 Phys. Rev. A 50 1377). Remaining discrepancies between the experiment and theory are discussed.