Influence of the impact parameter for ionization of argon by fast positrons and electrons on the postcollision kinematics and triply differential spectra

Classical trajectory Monte Carlo calculations for fast positron and electron impact ionization of argon are used to provide information about how the pre- and postcollision kinematics alter the triply differential spectra. It is shown that in addition to the scattering direction expected for a positive or negative projectile passing near a partially screened nuclear charge, scattering in the opposite direction also occurs. Independent of energy transfer, when the impact parameter is less than the mean radius of the $3p$ electron cloud, the expected direction of scattering always occurs. In contrast, for scattering in the opposite direction, the impact parameter is larger than the mean $3p$ radius for low energy transfer but decreases to values near the $3p$ radius for large energy transfer. The change in impact parameter and direction of scattering are shown to influence the recoil to binary triply differential cross-section ratios and explains the unexpectantly large ratios that have been observed at higher emission energies and may explain the larger ratio observed for electron impact.