F. Scheuermann

Electron-impact single ionization of multiply charged aluminium ions

Absolute total cross sections for electron-impact single ionization of Alq + (q = 3-7) ions are investigated experimentally as well as theoretically. Employing the crossed-beams technique, the cross sections have been measured from below the respective ground state threshold up to 1 keV and partly up to 5.5 keV. The theoretical calculations were performed using the configuration-averaged {distorted-wave} method, including both direct ionization and excitation-autoionization contributions from the 2p-shell. The measurements are in good agreement with the calculations considering a small content of metastable ions in the incident ion beam. Because of the strong dominance by direct ionization processes the semiempirical Lotz formula can describe the measured data as well.

Double ionization of heavy positive ions by electron impact: empirical formula and fitting parameters for ionization cross sections

Electron-impact double-ionization processes of multi-electron heavy ions from Ti up to Bi (nuclear charge from Z = 22 up to Z = 83) are investigated for incident electron energies E < 50 Ith where Ith is the threshold energy for double ionization. The following ions are considered: Ti q+ (q = 1–6) ,F e q+ (q = 1, 3–6) ,N i q+ (q = 1–6) ,G a q+ (q = 1–6) ,K r q+ (q = 1–4) ,M o q+ (q = 1–6) ,P r q+ (q = 1–4) ,S m q+ (q = 1–6) ,W q+ (q = 1–6) ,P b q+ (q = 1–9) and Bi q+ (q = 1–10, 12). On the basis of experimental data, mostly obtained at an electron–ion crossed-beams set-up in Giessen, and quantum-mechanical considerations, a simple semi-empirical formula with six fitting parameters is developed taking into account the contribution of direct double ionization of two outer-shell electrons of the ions and also of single inner-shell ionization processes followed by autoionization with additional ejection of an electron as was suggested in our previous paper (Shevelko et al 2005). The formula obtained there was found to describe well the available experimental doubleionization cross sections within an accuracy of 20–30%. However, for multielectron, very heavy ions significant deviations of that formula from experiment are found in the low-energy region. These deviations are most likely caused by higher order processes, including inner-shell excitation and subsequent double autoionization (EDA). The tabulated parameters can be used for easy analytical representation of the double-ionization cross sections of heavy positive ions in the modelling of laboratory and astrophysical plasmas.

Electron-impact multiple ionization of singly and multiply charged molybdenum ions

Cross sections for electron-impact double ionization of ions in charge states q = 1-6 as well as triple ionization for charge states q = 1-4 have been measured using the crossed-beams technique covering the entire range from the respective threshold up to energies well beyond maximum. The results are compared with the scaling law of Fisher et al and the semi-empirical formula published by Shevelko and Tawara, respectively. A comparison between experimental data and those formulae shows that a further improvement of the semi-empirical approaches is necessary. Together with previously obtained data for single ionization of molybdenum ions, the measured cross sections are used to study the influence of multiple ionization on the charge-state evolution if molybdenum ions are exposed to an electron flux at 700 eV energy.

Electron-impact double ionization of Pbq+ ions for q = 1–9

Electron-impact double-ionization cross sections of lead ions in charge states from q = 1 to q = 9 have been measured. The cross sections were obtained by using the dynamic crossed-beams method at electron energies from 20 eV to 1000 eV. The experimental results for Pb3+ are compared with available semi-empirical formulas.