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

Cross sections for electron-impact ionization of Si4+ and Si5+ ions have be en calculated in the distorted-wave approximation. In the present calculati ons both direct-ionization and excitation-autoionization processes are incl uded. 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 conf iguration. Excitation-autoionization substantially contributes to the ioniz ation of the 2s(2)2p(5)3s excited configuration and is approximately a fact or of four above direct ionization at energies below the ground-state thres hold. For ionization from the ground configurations of both Si4+ and Si5+ i ons, the direct process dominates the total ionization cross section. and c ontributions of excitation-autoionization are very small. The present resul ts are in reasonable agreement with the experimental measurements of Thomps on and Gregory (Thompson J S and Gregory D C 1994 Phys. Rev. A 50 1377). Re maining discrepancies between the experiment and theory are discussed.