Alignment of hydrogen atoms in e + and e − ionising collisions

Abstract

Positron and electron impact-ionisation cross sections of atomic hydrogen have been calculated using a classical quasi free-fall atomic model, that is the atomic model with the zero angular momentum radial orbit approximated by a strongly elongated ellipse. A three-body numerical calculation has been carried out and the result compared with the experiment. It has been found that a spin-less atomic model neglecting the spin properties of the electron cannot describe correctly atomic collisions in the low energy range. Analysis has shown that at the beginning of the main, violent stage of a collision, when the Coulomb interactions play a dominant role and spin-orbit coupling can be neglected, hydrogen atoms are already, due to the long distance interaction with a projectile, reoriented in space: the quasi-radial ellipse is slightly inclined towards the incoming positron and it is declined outwards from the incoming electron.