Laser assisted charge transfer reactions in slow ion-atom collisions: Coupled dressed quasimolecular-states approach

Semiclassical coupled dressed-quasimolecular-states (DQMS) approaches are presented for the nonperturbative treatment of charge transfer reactions at low collision velocities and high laser intensities. The DQMS are first obtained via the Floquet theory. The laser assisted collision process can then be treated as the electronic transitions among the DQMS driven by the nuclear motion only. The expansion of the total electronic wave function in a truncated DQMS basis results in a set of coupled adiabatic equations. The adiabatic DQMS and their associated quasienergies (depending parametrically upon the internuclear separation R) exhibit regions of avoided crossings, where the electronic transition probabilities are large due to strong radial couplings induced by the nuclear movement. By further transforming the adiabatic DQMS into an appropriate diabatic DQMS representation, defined via the vanishing of the aforementioned radial couplings, we obtain a new set of coupled diabatic equations which offer computational advantage. The method is illustrated by a case study of the laser assisted charge exchange process He⁺⁺+H(1s) +ℏω→He⁺(n=2)+H⁺, in a two-state approximation, for the velocity range from 1.5×10⁵ to 2×10⁷ cm/s and for the laser intensity in the range of 0.4 to 4.0 TW/cm². Results of exact coupled diabatic DQMS calculations are presented along with several approximation calculations, using first order perturbation theory, the Magnus approximation, and the average cross section.