Accuracy of Close-Coupling Approaches using Single-Centre Expansions for Positron-Lithium Scattering

For positron-atom scattering, the break-up channel contains both the effects of ionization and positronium formation. In a single-centre close-coupling expansion, these effects must be contained in the bound and continuum states. In previous R-matrix calculations for positron-lithium scattering, it was found that the cross sections for exciting the 3d state were anomalously large and it has been argued that the atomic d-states try to represent the positronium formation channels and in so doing overestimate the cross sections for excitation to this state. It has further been argued that the only way to avoid this problem is to use a two-centre basis set expansion. We have investigated this hypothesis for positron-lithium scattering using a single-centre convergent close-coupling method and have found that accurate N = 2 and 3 cross sections can be obtained using a single-centre expansion even for energies just above the ionization threshold.