DOUBLE-CHARGE-EXCHANGE AT HIGH-IMPACT ENERGIES

In fast ion-atom collisions, double ionization always dominates the tw o-electron transfer. For this reason, an adequate description of doubl e charge exchange requires proper inclusion of intermediate ionization channels. This is even more important in two- than in one-electron tr ansitions. First-order Born-type perturbation theories ignore througho ut these electronic continuum intermediate states and hence provide ut terly unreliable high energy cross sections for two-electron capture p rocesses. Therefore, it is essential to use second- and higher-order t heories, which include the intermediate ionization continua of the two electrons in an approximate manner. In the present paper, a new secon d-order theory called the Born distorted wave (BDW) approximation is i ntroduced and implemented in the case of symmetric resonant double ele ctron capture from the ground state of helium by fast alpha particles. A genuine four-body formalism is adopted, in contrast to the conventi onal independent particle model of atomic scattering theory. The obtai ned results for the total cross sections are compared with the availab le experimental data, and satisfactory agreement is recorded. As the i ncident energy increases, a dramatic improvement is obtained in going from the CB1 to the BDW approximation, since the latter closely follow s the measurement, whereas the former overestimates the observed total cross sections by two orders of magnitude. This strongly indicates th at the role of continuum intermediate states is decisive, even at thos e incident energies for which the Thomas double scattering effects are not important. This is in sharp contrast to the case of one-electron transfer atomic reactions.