Two-state model for top-of-barrier processes

A two-state model applicable to top-of-barrier processes in ion-atom collisions at low velocity v is investigated. An expression for the Massey parameter of the transition n->n + 1 is derived and is used to compute ionization, capture, and excitation from states (of high principal quantum number in H+ + H collisions. The Wannier law is obtained at the ionization threshold. The dependence of ionization on the initial n is computed. Total capture cross sections are in good agreement with other calculations, but an unreasonably large fraction goes to resonant capture. It is concluded that multicrossing models, which do nor incorporate interference between different paths to the resonant state, overestimate resonant capture.