Triple differential cross sections for ionization of helium

Abstract

The first chapter, of this thesis is devoted to the development of the theory for the ionization of an atom or ion by electron impact. We have pursued the particular approach adopted by Schiff and derived an expression for the triple differential cross section corresponding to the ionization process. Chapter 2 described alternative treatments of the problem and the various approximate models that have been used for calculations. Included amongst these approximate models is the first Born approximation and it is upon this that the calculations of chapters 3 and 4 are based. These calculations of the triple differential cross section for the ionization of helium by electron impact are restricted to impact energies below 260eV to enable a comparison with the experimental measurements of Ehrhardt et al. The calculations of Chapter 3 use a simple uncorrelated function to describe the target ground state and constitute a preliminary investigation of the TDC with particular regard to the relative importance of the direct, exchange and capture contributions. The results of these calculations indicate that the capture process makes negligible contribution to the TDC in the energy region considered and it is consequently ignored in the later calculations. In Chapter 4 the calculations of Chapter 3 are repeated using an accurate 6 configuration C. I. target wave function. The results obtained are compared both with experiment and results of calculations based on other approximate models. Finally in Chapter 5 we examine a number of points arising from the preceding chapters.