Positron scattering on atoms and molecules near the positronium threshold

Effects found in the total and elastic-scattering cross sections of positrons on atoms and molecules near the positronium (Ps) formation threshold are examined on the basis of R-matrix theory. In the case of atomic targets, the energy, dependence of the Ps-production cross section yields the dominant angular momentum of the outgoing Ps near threshold. The predicted effects agree semiquantitatively with experimental elastic and total cross sections. The predictions suggest that Wigner cusps in the elastic-scattering cross section should be very small for He targets and largest for Xe targets. For molecules, the variability of threshold effects in the total cross section suggests that this is caused by variable threshold effects in the nonelastic cross sections. Using three-channel R-matrix theory, the effect is illustrated for a H2 target.