Anisotropy Parameters and Adiabatic Approximations for Two-electron Atoms and lons

In dynamial processes atomic systems evolve from a condensation region at small distances where all particles are close together to an asymptotic region where some of the constituent particles are free and accessible to measurement. This dynamical evolution is characterised by the Jost matrix. Evaluation of the Jost matrix generally involves complex calculations, but considerable simplification is achieved when the evolution can be described in terms of adiabatic or diabatic potential curves. For low energy ion-atom and atom-atom collisions standard molecular potential curves have long been used. For low energy electron-atom scattering and photo-ionisation similar molecular-like potential curves have been proposed. There is no a priori justification for the adiabatic approach in these latter systems, thus confrontation with experiment is crucial for further development of this theory. Anisotropy parameters represent a particularly appropriate probe of the various adiabatic representations. This is illustrated by studies of photo-ionisation of helium at the n = 2 threshold. Potential curve crossings are important here and their relevance to the anisotropy parameters is illustrated