Positronium–atom scattering

Calculations of total cross sections for Ps(1 s) scattering by atomic hydrogen, helium, and argon are reported for the energy range 0–150 eV. The results for atomic hydrogen have been evaluated exactly within the first Born approximation. For collisions with helium and argon in which the target remains in its initial state (so called target elastic collisions) it is assumed that the positronium scatters off a frozen target atom and a coupled positronium pseudostate approximation is then used to calculate the cross sections. For collisions in which the target atom is excited or ionized (target inelastic collisions) the first Born approximation is adopted. Here there is a significant problem in summing over all final states of the target and for this a scheme due to Hartley and Walters has been employed. It is found that for the light targets, hydrogen and helium, target inelastic collisions become dominant above 45 and 105 eV, respectively, while for the heavier argon atom, target elastic scattering is always more important. Except at the lowest energies, and for both target elastic and target inelastic collisions, positronium ionization is the main outcome of the collision for all three atoms. There is an encouraging degree of agreement at the higher energies with the total cross-section measurements of Zafar et al. and Laricchia et al. for helium and argon. The present approximations do not include electron exchange between the positronium and the atom which may be the main source of disagreement between theory and experiment elsewhere.