Ionization of W and W + by electron impact

Abstract

Theoretical cross-sections for electron-impact ionization of the neutral W atom and W + ion are reported. The direct ionization cross-sections were calculated by using the binary-encounter Bethe (BEB) model and the indirect ionization cross-sections resulting from numerous excitation-autoionization (EA) were calculated by using scaled Born cross-sections. Contributions to indirect ionization from spin-forbidden and Δ n = 1 excitations, where n is the principal quantum number, are noticeable unlike in light atoms. The single ionization cross-section of W + is increased by about 10% due to the indirect EA of 5p electrons in the range of the incident electron energies between 40 and 60 eV. In the case of neutral W the EA cross-sections are very small for the S 7 3 level which is the first metastable term of W and because the excitations to the high spin states are mostly in the bound spectrum below the ionization limit. On the other hand, the EA cross-section of the second metastable P 3 term of W is large where a few 6 s 2 → 5 d 6 p and many 5 p → 5 d , 6 s excitations increase the total ionization cross-section by as much as 25%. Our total cross-section for the single ionization of W + is ∼ 15 % higher at the peak than the two sets of experimental data available in the literature. Our cross-sections are compared to the scaled Born cross-sections derived from the formulas provided by McGuire and those derived from the semi-empirical formulas by Lotz.