Single-electron–removal processes in collisions of positrons and protons with helium at intermediate velocities

Publisher Website
Google Scholar
Abstract
Total cross sections for single ionization and charge transfer have been calculated using the classical-trajectory Monte Carlo (CTMC) technique for collisions of both positrons and protons with helium. Analysis of the classical trajectories has helped to explain the differences in the collision mechanisms responsible for the observed relative magnitudes of the positron and proton electron-removal cross sections. In the intermediate collision velocity range (1.5 a.u. a.u.) it is found that because the positron is much smaller in mass than the proton, two dynamical effects occur leading to differences in their efficiency in electron removal. First, positrons are less likely to singly ionize helium than are protons since they possess less kinetic energy above the ionization threshold and accordingly there is a lower probability for ionization. Second, positrons are more likely to remove an electron from helium by charge transfer than are protons since they may be deflected by the target to large positive or negative scattering angles and be accelerated or decelerated to more readily momentum-vector match with an orbital electron. In the large-velocity regime (v>4.5 a.u.) positrons and protons are found to be equally likely to singly ionize helium, but positrons remain at least half an order of magnitude more likely to remove an electron by charge transfer.