Close-Coupling Approach to Multiple—Atomic Ionization

Abstract

Photoionization is a collision process where a quantum of electromagnetic energy is absorbed by an atom and one or several atomic electrons are ejected into the continuum. In the independent electron approximation, at most practical photon energies, the electromagnetic field can only couple to a single electron. Changing the quantum state of the second or further electrons can only take place via many-electron correlation. Here, this correlation is understood broadly as the ability of atomic electrons to change, without any external field or interaction, a well-defined set of individual quantum numbers. Uncorrelated electrons in the ground state are labeled by the principle, angular, and magnetic quantum numbers n, l, m, respectively. The ground-state correlation make these quantum numbers only approximate. For instance, the 1s² pair of electrons in the ground—state helium atom can find itself, with a finite probability, in the 2s², 2p² and higher excited states. In an atomic continuum, an uncorrelated electron is labeled by its energy E and momentum k. However, due to elastic or inelastic scattering on other atomic electrons, one or both of these quantum numbers can change.