The coherent Coulomb ionisation of the 1sσ molecular orbital in slow asymmetric ion-atom collisions

Abstract

Within the Briggs perturbed united-atom (UA) model a simple cross section formula has been derived for ionisation of the 1s sigma molecular orbital (MO) to s continuum states from straight-line trajectories, in a form which exhibits the coherent contribution of both collision partners to this process. The corrections for Coulomb and relativistic effects as used in the present work preserve the structure of this formula. Interesting behaviour of the ionisation due to the target motion as a function of the target atomic number is predicted and explained by the relativistic contraction of the 1s sigma MO. By means of an approximate scaling law, the calculations are compared with 520 published experimental cross sections (30<ZUA<110, asymmetry parameter Q<0.3) spanning more than five orders of magnitude. The excellent agreement obtained suggests that the simulation, within the Briggs model, of the 1s sigma MO by a relativistic 1s atomic wavefunction with extended nuclear charge distribution and perturbed binding energy provides a good approximation in describing the direct ionisation in slow ion-atom collisions.