POSITRON SCATTERING ON ATOMS AND MOLECULES NEAR THE POSITRONIUM THRESHOLD

Effects found in the total and elastic-scattering cross sections of po sitrons on atoms and molecules near the positronium (Ps) formation thr eshold are examined on the basis of R-matrix theory. Ln the case of at omic targets, the energy dependence of the Ps-production cross section yields the dominant angular momentum of the outgoing Ps near threshol d. The predicted effects agree semiquantitatively with experimental el astic and total cross sections. The predictions suggest that Wigner cu sps in the elastic-scattering cross section should be very small for H e targets and largest for Xe targets. For molecules, the variability o f threshold effects in the total cross section suggests that this is c aused by variable threshold effects in the nonelastic cross sections. Using three-channel R-matrix theory, the effect is illustrated for a H -2 target.