Charge-transfer process using the molecular-wave-function approach: The asymmetric charge transfer and excitation in Li + Na+ and Na + Li+

The charge-transfer processes occurring in collisions of Li + Na+ and Na + Li+ have been studied theoretically using the molecular-wave-function approach. The wave functions and Born-Oppenheimer breakdown terms were evaluated using rigorous methods. The six lowest molecular states (dissociating to the 2s and 2p atomic states on Li and to the 3s and 3p atomic states of Na) were included in the coupled equations. The transition probabilities were calculated using linear trajectories for a variety of impact parameters and ion velocities. We find that the over-all transition processes are well represented as a succession of simple two-state transition processes (Σ-Σ, Σ-Π, and Π-Π). The Σ-Σ two-state process can be described in terms of three steps involving (i) a coupling region as the atoms come together [(10-20)a0], (ii) an uncoupled phase changing region for shorter separatons (<10a0), and (iii) a decoupling region as the atoms depart [(10-20)a0]. On the other hand, in the molecular—wave-function formulation, the Σ-Π two-state transition process involves continuous coupling (for R<7a0). As a result the transition probabilities for Σ-Π coupling differs from that of Σ-Σ coupling, leading to rather different forms for the cross sections.