Validity of the Langevin capture model for charge exchange processes at thermal energies

Abstract

A detailed coupled state quantum mechanical calculation using an adiabatic basis is analysed to investigate the validity of the Langevin model for very low energy charge transfer processes in ion–atom collisions. Taking as an example the N3+/H system, it is shown that the success of the Langevin model in describing the energy variation of the cross section is due to the accuracy of the phase averaging procedure in the double passage through the avoided crossing. On the other hand, the model does not yield the correct isotopic dependence of the cross sections. These can only be accurately determined by a coupled state calculation. At low thermal and sub-thermal energies less than 1 meV, the phase averaging procedure breaks down and quantum tunnelling effects become important. In these conditions, the energy variation of the cross section exhibits an appreciable departure from the Langevin model.