Threshold Photodouble Ionisation in Atoms and Molecules

Abstract

The process of single photon double ionisation is of particular interest because it is a means of studying the dynamics of two outgoing electrons in the field of a doubly charged ion: the fundamental, three body Coulomb problem. These two electrons are highly correlated and so photodouble ionisation measurements are a sensitive way of studying these correlations. Photodouble ionisation may occur through a direct process where the two photoelectrons are emitted simultaneously, or indirectly - for example via an excited singly charged state, which subsequently decays, by autoionisation, to an energetically lower lying doubly charged state. So far, less attention has been paid to these indirect processes as they constitute an extra degree of complexity to the overall photodouble ionisation process. Interestingly, however, these can have much larger cross-sections than those for the direct process, and so are beginning to receive increased attention^l,2. Most of the work on the study of correlations in photodouble ionisation has centred on atoms rather than molecules: the additional molecular motions are expected to complicate the basic three body Coulomb problem. In fact in the molecular case it is the spectroscopy of the doubly charged states created that has formed the centre of interest³.