D. Hathiramani

Electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+

For the first time absolute cross sections for electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+ at electron energies from below threshold up to about 6 keV have been measured using the crossed-beams technique. All measured cross sections are in very good agreement with available distorted-wave exchange calculations and the semiempirical Lotz formula. The behaviour of the classically scaled cross sections along the hydrogen isoelectronic sequence is discussed. A recently developed scaling technique allows us to predict all non-relativistic cross sections along the isoelectronic sequence on an absolute scale.

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.

Electron-impact single ionization of low-charged titanium ions

The electron-impact single ionization of low-charged Ti ions was studied both experimentally and theoretically. Absolute cross sections for ions in charge states q = 1 to 6 except 3 were measured using a crossed-beams technique as well as calculated with the configuration-average distorted-wave method, including both direct ionization and excitation-autoionization contributions. For charge states q = 4, 5 and 6, the measurements are in good agreement with the calculations considering that large components of the incident ion beam were in metastable configurations. The cross sections for single ionization of and were determined in part with a high-resolution energy-scan method. Besides contributions of excitation-autoionization mechanisms, a remarkable number of structures arising from resonant-excitation double-autoionization (REDA) processes could be observed.

Electron impact single ionization of multiply charged iron ions

Employing the animated crossed-beams technique, absolute cross sections for electron impact single ionization of the iron isonuclear sequence are measured for charge states q = 1-6 for electron energies from threshold up to 1 keV, as well as for the intermediate charge states q = 9,10 up to an electron energy of 5 keV. The cross sections observed for Feq+ ions in charge states q = 1 - 4, 9 and 10 show a significant ionization signal below the respective ground state threshold resulting from ions in excited, long-lived metastable states in the parent ion beam. In the case of Fe5+ and Fe6+ no metastable components are found. The experimental results are in good agreement with the theoretical predictions if excitation-autoionization processes are taken into account. Our measured cross sections are also in good agreement with experimental results of other groups.

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

Cross sections for electron-impact double ionization of Wq+ ions in charge states q=1-6 as well as triple ionization in charge states q=1-4 have been measured using the crossed-beams technique. A new semiempirical formula for multiple ionization of three or more target electrons is in very good agreement with the measurements. Together with previously obtained experimental data for single ionization of tungsten ions, the measured cross sections are used to study the influence of multiple ionization on the Wq+ charge state evolution if tungsten atoms are exposed to an electron flux at 700 eV energy. It is shown that the charge state evolution is significantly influenced by multiple-ionization processes.

Electron impact double ionization of multiply charged iron ions

Using the crossed-beams technique, cross sections for electron impact double ionization of Feq+ ions in charge states q = 1 - 6 (except 2) are measured in the energy range from the respective ionization thresholds up to 1 keV, partly up to 5 keV. The data for Fe4+, Fe5+ and Fe6+ show significant contributions from inner-shell processes. Using a high-resolution energy-scan technique, we study narrow resonances in the cross sections for Fe4+ and Fe5+. Besides contributions from excitation-autoionization processes, a remarkable number of structures arising from resonant-excitation triple-autoionization processes are observed. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Tawara, respectively.

Recent results on multiple ionization and fragmentation of negatively charged fullerene ions by electron impact

Employing the dynamic crossed-beams technique, the absolute cross sections of the electron-impact multiple ionization and fragmentation of mass-selected negatively charged fullerene ions Cm− → Cm−n2+ (m=60, 70, 84; q=1, 2, 3; n=0, 2, 4) were measured. The electron energy varied from the respective threshold up to 1 keV. A scaling law was observed for the cross-section magnitude as a function of the fullerene size m and the charge state q of product ions. The data indicate that different mechanisms account for the detachment of an extra electron from the negatively charged fullerene and the formation of a positively charged ion, respectively. Moreover, the multiple ionization of a fullerene anion is found to be a sequential process. A novel ionization mechanism is proposed which might be expected to be valid for all negatively charged molecular or cluster ions able to shield the attached electron from the incident electron.

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 IONIZATION OF TITANIUM IONS

In an animated crossed-beams experiment the following absolute cross sections σq, q+n for single (n = 1), double (n = 2), triple (n = 3) and quadruple (n = 4) ionization have been measured for the reaction e + Tiq+ → Ti(q+n)+ + (n + 1)e σ5,6 (n = 1); σ2,4 (n = 2); σ2,5 (n = 3); σ2,6 (n = 4). The measurements have been performed in an energy range from threshold up to 1 keV, partly to 6 keV. The cross section for single ionization of ions in charge state q = 5 shows both contributions below the ground state threshold caused by the ionization of ions in excited, metastable states in the parent ion beam as well as from excitation-autoionization processes. The cross section is described well by distorted-wave calculations. The cross sections for multiple ionization show considerable contributions from inner shell processes. Semiempirical formulae are in reasonable agreement with the experimental data.

Electron impact ionization of the hydrogen-like ions: B4+, C5+, N6+ and O7+

Absolute electron impact ionization cross sections of hydrogen-like ions B4+, C5+, N6+ and O7+ have been measured at electron energies from below threshold up to about 6 keV using the crossed-beams technique. The cross sections are in good agreement with several theories and are compared with other available cross section data. The scaling behaviour of the cross sections along the hydrogen isoelectronic sequence is discussed.