Electron-impact ionization of Pb^q+ ions for q=1–10

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of Pb^q+ ions for q=1–10. We compare with two main theoretical methods. First, we check against configuration-average distorted-wave calculations, which include both direct-ionization and indirect excitation-autoionization contributions. Second, for ion stages Pb⁺ through to Pb⁵⁺, we calculate the dominant excitation-autoionization channels using level-resolved distorted-wave theory to evaluate the excitation cross sections. We find that for ion stages Pb⁺, Pb²⁺, and Pb³⁺, distorted-wave theory significantly overestimates the total-ionization cross section, due to an overestimation of the direct-ionization cross section from the 5d subshell. For ion stages Pb⁴⁺ through to Pb¹⁰⁺ there is good agreement between theory and experiment. We find evidence for significant metastable fraction in the ion beam of the experiment for ion stages Pb²⁺, Pb³⁺, Pb⁴⁺, Pb⁵⁺, and Pb⁶⁺. For ion stage Pb³⁺ we find that the level-resolved distorted-wave calculation of the excitation autoionization results in a slight reduction of the configuration-average theoretical results, due to splitting of levels within the autoionizing configurations. We also investigate two semiempirical methods of calculating the direct-ionization cross sections: namely, the Lotz method and the binary encounter Bethe method. We find that both methods provide results which are significantly lower than the distorted-wave method for the 5d-subshell direct ionization of Pb⁺, Pb²⁺, and Pb³⁺. For the higher ion stages, both methods are lower than the distorted-wave direct-ionization cross-section results, trending towards the distorted-wave results as the ion stage increases.