ELECTRON IMPACT IONIZATION OF ATOMS

Abstract

The work reported in this thesis aims at investigating the ionization process, at intermediate and high energies, in which a beam of charged particles is bombarded on to a helium target. The electron impact ionization of atoms and ions is a topic both of fundamental and practical importance in modern atomic physics. Quite a significant progress, both experimental as well as theoretical, has been achieved in the understanding of the ionization of atoms by the impact of charged particles during the last decade. The ionizing collisions. are quite complicated because even 4 the simplest electron impact ioniza-tion process, namely that of atomic hydrogen, involves a three—particle continuum state. The most complete description of an ionization event is provided by determining the energy and momentum of all the particles involved in the collision. The total cross section is less meaningful because various interesting features of the differential cross section get smoothered during the summing and averaging over kinematical parameters of the ejected and scattered electrons. A more accurate test of a theoretical model is, therefore, provided by the cross sections that are differential in nature with respect to one or more parameters. The triple differential cross section (MS) gives the most detailed information about a (e, 2e) reaction since all the kinematic parameters are determined, namely the energy of the incident electron E0 , the energies Ea and Eb , and directions (ea' Oa) and (eb 9 (I)b) of the two outgoing electrons, the initial state of the target and the final state of the residual ion. We have concentrated on triple differential cross sections for electron impact ionization of helium, since helium is the most extensively studied case, in the literature on atomic ionization, and it is the simplest multielectron atom. On the experimental side, it is relatively easy to carry out experi-ments with helium as target. A good comparison between theory and experiment is therefore possible in this case.