New experimental data on double-ionization cross sections of neutral atoms, positive and negative ions by electron impact are reported. These and other relevant data are compared with recent multiple-ionization semiempirical formulae extended for the case of two-electron ionization. Theory gives analytical dependence of the double-ionization cross sections on the main parameters of the collision system in a wide energy range. For the He atom, He-like ions and H − ion high-energy asymptotic behaviour of the double-ionization cross sections are considered on the basis of a quantum mechanical treatment. Suggested cross section scaling for fast collisions, employing a target-effective nuclear charge, is in good agreement with experimental data.

Differential cross sections for the elastic scattering of electrons by multiply charged ions have been measured at scattering angles between 32 • and 148 • . Mainly at backward angles the cross section deviates remarkably from the Rutherford cross section showing oscillations due to the structure of the ionic core. Experimental results obtained for Ar 8+ ions and different Xe q+ ions (3 q 6) are found to compare well with theoretical calculations.

Absolute cross sections for electron impact ionization of multiply charged argon ions (Arq+) have been measured in an animated crossed electron–ion beam experiment. Results are reported for single ionization (SI) (q = 4–6, 8, 10 and 11) ions and for double ionization (DI) (q = 5–11). The measurements cover the energy range from threshold to about 6000 eV. The direct process is seen to dominate SI for q = 8, 10 and 11. For charge states 4–6, excitation–autoionization processes are seen to largely contribute to the ionization signal. In addition, metastable states are unambiguously identified for Ar5+ and Ar6 +. Ionization–autoionization (IA) from the L shell accounts predominantly for DI of Arq+ (q = 5, 6). For charge states 7–11 direct DI is found to decrease strongly with respect to IA from the K shell.

Absolute cross section measurements for double ionization of and ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from ionization threshold to approximately 2.5 keV. Present results are found to be smaller than the experimental result of Steidl et al (1995 Proc. 19th Int. Conf. Physics of Electronic and Atomic Collisions (Whistler) ed J B A Mitchell et al p 564). The role of slow positive ions trapped in the electron beam could explain the observed discrepancy. The study of double ionization processes along the fluorine isoelectronic sequence (for , and ions) shows that the results for multiply charged ions are strongly lower than predicted by classical scaling. A systematic threshold energy shift is observed in the sequence and, for , the position of the cross section maximum is found to be at an unusually high energy. The autoionizing 1P state belonging to the configuration of atomic oxygen is clearly seen to contribute to the signal above 23.7 eV. Other autoionizing states are also seen to play a role around the K-shell ionization threshold. The Bethe-plot of present data shows that the high energy behaviour of cross sections is dominated by the , term for , while it is dominated by the logarithmic term for .

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