Angular distributions of secondary electrons in fast particle-atom scattering

TL;DR

This paper analyzes the angular distribution of secondary electrons in fast particle-atom scattering, highlighting the significant role of non-dipole transitions and differences from photoionization, with detailed calculations for noble gas atoms.

Contribution

It provides a theoretical framework for electron angular distributions in fast collisions, emphasizing the enhanced non-dipole effects compared to photoionization.

Findings

Non-dipole contributions can be significantly larger than in photoionization.

Angular distributions differ notably even at small momentum transfer.

The ionizing field in collisions is longitudinal, affecting distribution patterns.

Abstract

We present the angular distribution of electrons knocked out from an atom in a fast charge particle collision at small momentum transfer. It is determined not only by dipole but also by quadrupole transitions, the contribution of which can be considerably enhanced as compared to the case of photoionization. There the non-dipole parameters are suppressed as compared to the dipole ones by the parameter \omega R/c << 1, where is the photon energy, R is the ionized shell radius and c is the speed of light. This suppression in fast electron-atom collisions can be considerably reduced: the corresponding expansion parameter \omega R/ \nu << 1 is much bigger than in photoionization, since the speed of the incoming electron is much smaller than c. In formation of the angular distribution it is decisively important that the ionizing field in collision process is longitudinal, while in photoionization - it is transversal. We illustrate the general formulas by concrete results for outer s-, p-, and some nd-subshells of multi-electron noble gas atoms Ar, Kr and Xe, at several transferred momentum values: q=0.0, 0.1, 1.1, 2.1. Even for very small transferred momentum q, i.e. in the so-called optical limit, the deviations from the photoionization case are prominent.