NEAR-THRESHOLD EFFECTS IN POSITRON-ATOM (MOLECULE) SCATTERING

The energy dependence of the total cross-sections for ionization by po sitron impact has been measured for He, Ne, Ar, CO2 and O-2. The inter play between this and other scattering channels has been investigated. The data for He, Ne and Ar has been interpreted using a combination o f R-matrix and threshold theories in the neighbourhood of the Ps forma tion threshold. In CO2, an enhancement of the Ps formation probability is found when the remnant ion is left in an excited state. In O-2, st ructure has been observed which indicates channel-coupling with excita tion to the Schumann-Runge continuum.