A crossed electron–ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of (i) absolute cross sections, (ii) kinetic energy release distributions (KERD) and (iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation (DE) and for dissociative ionization (DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i.e. for an electron energy around 35 eV, are found to be (9.69 ± 2.08) × 10−17 cm2 and (6.24 ± 1.33) × 10−17 cm2, for C+ and O+, respectively, and the corresponding threshold energies are found to be (8.5 ± 0.5) eV and (14.8 ± 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be (12.56 ± 2.38) × 10−17 cm2 around 125 eV and the corresponding threshold energy is (27.7 ± 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3–6% for both C+ and O+.

Absolute cross sections for electron impact ionization, dissociative excitation (DE) and dissociative ionization of N + 2 ions are measured in the energy range from threshold to 2500 eV. The animated crossed electron-ion beam method has been employed. The individual contributions of ionization products (N 2+2 ) and dissociation fragments (N + ), which have both identical mass-to-charge ratio and average velocity, are deduced from the analysis of product velocity distributions. Particular attention was paid to determining the transmission efficiency for dissociation fragments, since their collection was incomplete during the measurements. Threshold energies and kinetic energy released to dissociation fragments are measured. The role of states contributing to different reactions is discussed. For DE, the present results are found to be much smaller than the results of Peterson et al (1998). For ionization (single and dissociative), a satisfactory agreement with their result is obtained as well as with the prediction of Kim et al (2000) obtained in the binary-encounter Bethe approximation.

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Applying the animated beam method the total cross sections (TCS) for double ionization (DI) of helium by electrons are measured in the collision energy region from threshold to 3 keV. The TCS for the same process are calculated at intermediate and high incident energies in the first Born approximation (FBA). The radial and angular correlations between the bound electrons and repulsion between the ejected electrons are accounted. In order to go beyond the FBA the scattered electron is considered as a particle being in the Coulomb field of the nucleus, the charge of which is screened by the ejected electrons. On the basis of the obtained results some conclusions concerning the behaviour of the TCS in the intermediate and high-energy regions are drawn.

First absolute cross sections for electron impact single ionization of H3O+, HD2O+ and D3O+ and for dissociative excitation (DE) producing H+ and D+ are reported. The animated crossed electron-ion beam method has been employed in the energy range from threshold to 2500 eV. The maxima of these cross sections are found to be unusually small (<1×10-17 cm2). The ionization threshold energies are determined to be 24.7±0.5, 24.4±0.5 and 24.0±0.5 eV for H3O+, HD2O+ and D3O+, respectively. The observed DE threshold energies lie in the range 10-12.5 eV and the maximum kinetic energies released are between 3 and 4 eV. Significant differences are observed between the results obtained for the three isotopomers.