11 - Target dependence of the triply differential cross section for low energy (e, 2e) processes

Summary

Introduction

In recent years, experimental studies on low and intermediate energy (e, 2e) processes have accumulated large amounts of triply differential cross section data. These (e, 2e) results, in which the energies and angles of both of the outgoing electrons produced in the electron-impact ionization process are specified, display strong electron-correlation effects. Owing to the difficulty involved in describing precisely various electron correlations, in particular, the Coulomb interaction between the two final-state continuum electrons, only approximate theoretical treatments have been carried out. At present, theoretical understanding of these data and the underlying effects are far from complete.

The near-threshold energy dependence of two electrons escaping from a positive ion has been studied theoretically by many authors using a number of methods. These studies cover the threshold behavior of the total and the differential cross sections for electron-impact ionization of atoms and ions.

In the early 1950's, Wannier applied to this problem the idea that the near-threshold energy-dependence of a reaction could be derived by investigating only the long-range interactions among its final products, without having a detailed knowledge about a small “reaction zone,” the size of which is of the order of magnitude of the Bohr radius. He revealed the importance of the configuration r₁ = −r₂ for the double escape of slow electrons from a positive ion by using methods of classical mechanics.