A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+

Abstract

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+.