On the capture cross-section for charge neutralization, recombination, photoassociation and other barrierless reactions

The collision energy dependence of cross-section for reactions where the re actants attract is discussed. Examples include charge recombination A(+) B- --> products, ion-molecule reactions and other curve crossing processes. The common characteristic of such processes is that on physical grounds th ere is a critical distance d where capture occurs. Special attention is giv en to the case where the critical separation is independent of energy and/o r impact parameter. A modern example where this is the case is laser-induce d association of atoms. The capture cross-section has the functional form s igma = pid(2) (1 - Vg(d)/E-T) where for the collisions discussed the potent ial is attractive, V-g(d) < 0. Such a cross-section is a decreasing functio n of the collision energy ET. The same functional form is also useful if th e potential is repulsive, V-g(d) > 0. For this well-known case, the cross-s ection is an increasing function of the collision energy. (C) 2001 Elsevier Science B.V. All rights reserved.