Polarisation potentials for positron-molecule collisions: positron-H, 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-H, 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-H, polarisation potential, theoretical total cross sections in reasonably good agreement with recent measurements are obtained.
The interaction of positrons with large atoms and molecules: theory
Calculations of positron–atom and positron–molecule scattering are reviewed. The polarized orbital results of the York group are found to be of the same reliability as recent experimental determinations in the elastic region for the noble gases. The temperature dependence of the annihilation rate is suggested as a sensitive point of contact between theory and experiment. Using a new, direct experimental determination of the positronium formation cross section below the ionization threshold for argon and helium, we have made an improved Ore-type estimate of the positronium fraction of the noble gases. The new estimates agree perfectly with experiment for the three lighter gases. However, for krypton and xenon, the new estimates are much too high, which suggests the existence of an unknown process which suppresses or depletes positronium. Recent calculations on other atoms and on simple diatomic molecules are also reviewed.
Ab Initio and Semiempirical Calculations of the Static Potential for Electron Scattering off the Nitrogen Molecule
Six calculations, three ab initio and three semiempirical, are presented for the static potential (i.e., the interaction potential energy of an electron with the unperturbed charge distribution) of a nitrogen molecule. The ab initio calculations are Hartree‐Fock calculations using contracted Gaussian basis sets capable of yielding wavefunctions (1) near the Hartree‐Fock limit, (2) near the sp‐limit, and (3) at the minimum basis set level. The semiempirical calculations are all within the INDO formalism. We find that the inclusion of d functions in the ab initio calculations is necessary for an accurate description of the anisotropy of the potential. We also find that the INDO calculations give the correct form of the potential at small electron‐molecule separation distances but are poor at large distances.