Ionization of W and W+ by electron impact

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 Δn=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 S73 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 P3 term of W is large where a few 6s2→5d6p6s2→5d6p and many 5p→5d,6s5p→5d,6s excitations increase the total ionization cross-section by as much as 25%. Our total cross-section for the single ionization of W+ is ∼15∼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.