Two-electron capture from helium by fast α particles

Two-electron capture from helium by fast {α} particles is investigated. Working within the four-body framework of scattering theory, we perform detailed computations for differential as well as total cross sections, by means of the second-order Born distorted-wave (BDW) approximation, and find good agreement with the available experimental data. We also report the results obtained by means of the continuum distorted-wave (CDW) method in both the four-body formalism and the independent-particle model. The former fails to reproduce the measurement, whereas the latter emerges as a satisfactory method for {α}-He double-charge exchange. The reported study of the electronic continuum intermediate states clearly indicates that double-charge exchange is remarkably sensitive to inclusion of these long-range Coulomb effects, even at incident energies at which the Thomas double scattering is completely unimportant. This is in sharp contrast to the familiar situation at the comparable energies encountered in treating single-electron transfer in ion-atom collisions.