RESONANCES ABOVE THE IONIZATION THRESHOLD IN POSITRON HYDROGEN SCATTERING

Resonances appearing above the ionization threshold in coupled-state c alculations of positron-atom scattering are discussed. Calculations in the six-state approximation Ps(1s,2s,2p) + H(1s,2s,2p), which show su ch resonance structure, are compared with a more extensive 18-state ap proximation Ps(1s, 2s, 3sBAR, 4sBAR, 2p, 3pBAR, 4pBAR, 3dBAR, 4dBAR) (H (1s, 2s, 3sBAR, 4sBAR, 2p, 3pBAR, 4pBAR, 3dBAR, 4dBAR) in which ch annels other than 1s, 2s and 2p are represented by pseudostates. The r esults strongly indicate that the above ionization threshold resonance s observed in the six-state approximation, and in other small basis se t calculations, are not real. It is suggested that they are a conseque nce of the neglect, or inadequate representation in other approximatio ns, of ionization channels. In the six-state approximation the positro nium component of the system wavefunction attempts to represent the mi ssing ionization channels but in so doing produces unreal resonances a bove the ionization threshold. More generally, it is suggested that, i n coupled-state calculations of positron-atom scattering, the atom par t of the system wavefunction will try to compensate for defects in the positronium component and vice versa. When the defects are serious, f or example, the omission of important ionization channels, unusual spu rious behaviour is to be expected.