| Popis: |
We focus upon the coupled - state approach to positron - and positronium (Ps) - atom collisions. The concept of pseudostates as a way of including ionization channels is discussed. Examples are given from positron scattering by atomic hydrogen and the alkali metals. The present status of Ps - atom collisions is next considered with emphasis upon the Ps-H, He, Ne and Ar systems where large coupled - pseudostate calculations have been performed within the frozen target model. Discrepancies between theories and experiments are highlighted. It is argued that converged cross sections have been obtained within the frozen target model and that any remaining discrepancies must be associated with the neglect of target excitation, whether real or virtual. |
