Polarisation potentials for positron-molecule collisions: Positron-H ₂ scattering

Model polarisation potentials for use in calculating positron-molecule cross sections are often based on approximations in which terms that depend on the sign of the charge of the projectile are neglected. Thus, these potentials do not fully distinguish between electron and positron scattering. The validity of using such potentials for positron-H₂ collisions is investigated by performing scattering calculations using electron- and positron-H₂ model polarisation potentials. Results show that, because of differences in the way electrons and positrons distort the target charge cloud, sign-dependent terms in the adiabatic potential can significantly influence the total cross section. Low-energy cross sections are also shown to be rather sensitive to how non-adiabatic polarisation effects are taken into account. Using a cut-off fully adiabatic positron-H₂ polarisation potential, theoretical total cross sections in reasonably good agreement with recent measurements are obtained.