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

The collision energy dependence of cross-section for reactions where the reactants 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 there is a critical distance d where capture occurs. Special attention is given to the case where the critical separation is independent of energy and/or impact parameter. A modern example where this is the case is laser-induced association of atoms. The capture cross-section has the functional form σ=πd2(1−Vg(d)/ET) where for the collisions discussed the potential is attractive, Vg(d)<0. Such a cross-section is a decreasing function of the collision energy ET. The same functional form is also useful if the potential is repulsive, Vg(d)>0. For this well-known case, the cross-section is an increasing function of the collision energy.