(e,2e) processes – an overview

An (e,2e) process is one where an electron, of well-defined energy and momentum, is fired at a target, ionizes it and the two exiting electrons are detected in coincidence. The energies and positions in space of these electrons are determined by the experiment, so in effect all but the spin quantum numbers are then known. We can, therefore, describe it as a kinematically complete experiment; if we could also measure all the spins we would have all the information from a scattering experiment that quantum mechanics will allow. The technique offers both the possibility of a direct determination of the target wave function and a profound insights into the nature of few-body interactions. What information you extract from such an experiment really depends on the kinematics you chose and the target you use. Integrated cross sections can be crude things and you need the full power of a highly differential measurement to tease out the delicacies of the interactions. Indeed, often the most intriguing effects turn up in peculiar geometries where the cross sections are small and where a number of relatively subtle few-body interactions are present.

In recent years, attempts to give a complete numerical treatment of electron impact ionization have made considerable progress. In particular, one should mention the pioneering close coupling work of Curran and Walters [1–3], the convergent close coupling approach, [4], the complex exterior scaling calculations, [5], and the propagating exterior complex scaling method, [6].