Electron Capture in Ion-Atom Collisions

Abstract

The rearrangement process is one of the more outstanding problems in atomic collision theory. The molecular, near-adiabatic approach is well documented at low energies; but a fully consistent description of the momentum transfer to the captured electron has not yet been developed although some progress has been made with electron translation factors (see Briggs’ lecture in this volume). At high energies collisions may be considered to be impulsive and perturbative. However, unlike the excitation process, the convergence of standard perturbation series has not yet been assessed, neither has the appropriate first order approximation been generally isolated. Let us here mention two problems in this connection. First of all, there is more than one natural expansion parameter since the perturbations in initial and final channels are different. Secondly, initial and final states are non-orthogonal during the collision. Unless special care is excerted this may lead to spuriously large transition amplitudes in approximative calculations. The situation is illustrated in figure 1, where |i> and |f> represent the initial and final states while |s> represents the state of the system. We consider a case where the system is only weakly disturbed during the collision, i.e. |s>≃|i> for all t.