Cross sections for ionization of Mo and Mo + by electron impact

Abstract

Theoretical cross sections for electron-impact ionization of the neutral Mo atom and Mo + ion are reported. Both Mo and Mo + have many metastable levels near the ground level. We calculated ionization cross sections from the ground and two lowest metastable levels for both Mo and Mo +. The total ionization cross sections for each initial state consist of direct and indirect ionization cross sections. The direct ionization cross sections were calculated by using the binary-encounter Bethe (BEB) model. The indirect ionization cross sections resulting from numerous excitation–autoionization processes were calculated by using scaled Born cross sections for the excitation of 5s electrons to 5p, 4d electrons to 5p, and 4p electrons to 4d or 5s, whose excitation energies exceeded the lowest ionization energy of the initial states of Mo and Mo +. For the ground-state Mo, indirect processes contribute only about 5%, while for metastable Mo, indirect processes contribute almost 30% to the total ionization cross section. Unlike our experience with light atoms, contributions to indirect ionization from spin-forbidden and Δ n = 1 excitations, where n is the principal quantum number, are noticeable. For Mo + indirect processes contribute less than 15%. There are no experimental data to compare for Mo. The distorted-wave Born cross sections for Mo available in the literature accounted for direct ionization only, and hence are lower than in the present work at higher incident electron energies where excitation–autoionization contributes significantly. Our total cross section for the single ionization of Mo + is in good agreement with two sets of experimental data available in the literature, while the distorted-wave Born cross section reported with one of the experiments is almost a factor of two higher than the experiments at the cross section peak.