POSITRONIUM-ATOM SCATTERING

Calculations of total cross sections for Ps(ls) scattering by atomic h ydrogen, helium, and argon are reported for the energy range 0-150 eV. The results for atomic hydrogen have been evaluated exactly within th e first Born approximation. For collisions with helium and argon in wh ich the target remains in its initial state (so called target elastic collisions) it is assumed that the positronium scatters off a frozen t arget atom and a coupled positronium pseudostate approximation is then used to calculate the cross sections. For collisions in which the tar get 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 Waiters has been employed. It is found that for the li ght targets, hydrogen and helium, target inelastic collisions become d ominant above 45 and 105 eV, respectively, while for the heavier argon atom, target elastic scattering is always more important. Except at t he lowest energies, and for both target elastic and target inelastic c ollisions, positronium ionization is the main outcome of the collision for all three atoms. There is an encouraging degree of agreement at t he higher energies with the total cross-section measurements of Zafar et al. and Laricchia et al. for helium and argon. The present approxim ations do not include electron exchange between the positronium and th e atom which may be the main source of disagreement between theory and experiment elsewhere.