SCHWINGER MULTICHANNEL METHOD FOR POSITRON-MOLECULE SCATTERING

We present a formulation for low-energy positron-molecule collisions b ased on the Schwinger multichannel method (SMC) which was previously a dapted for electron-molecule scattering. We have preserved very import ant features of the SMC such as its ability to deal with electronicall y inelastic scattering and its applicability to molecular targets of a rbitrary geometry. Our implementation also allows for the inclusion of a substantial number of closed electronic channels to represent polar ization effects, which are very important at low impact energies. As a n application, to illustrate the form with which polarization is repre sented in this formulation, we have calculated elastic integral and di fferential cross sections for e+-H-2 and e+-CH4 collisions. We include d all possible single-particle excitations from the target and found s ubstantial disagreement with respect to model potential calculations a nd attenuated-beam experimental data. We discuss possible reasons for these discrepancies.