Collisional ionization and elastic scattering in alkali-halogen atom collisions

Close-coupling (two-state) computations for collisional ionization and elastic scattering in Na+I and Li+I collisions have been made, using realistic model potentials, at a large range of postthreshold energies. At the higher energies a generalized JWKB procedure was employed which yields a very accurate scattering matrix when compared to a fully quantal solution. The computed elastic and inelastic total and differential cross sections are discussed and compared to the predictions of more approximate methods. The rainbow angles in the angular distribution are compared to those predicted by the classical deflection function obtained from the trajectories along adiabatic or diabatic potential curves (based on the assumption of a single transition point). It is found that at all energies considered these rainbow positions agree best for trajectories computed via the adiabatic representation rather than the diabatic representation. The double rainbow pattern noted in the elastic ionic channel can be interpreted only within the adiabatic representation. Landau-Zener-Stueckelberg (LZS) computations for the energy dependence of the collisional ionization cross section have also been made. The LZS curve jumping mechanism is in better agreement with the quantal results when the transition is taken to occur at the adiabatic coupling point (R_p) rather than the diabatic crossing point (R_x). Moreover, depending on the nature of the potentials, there may also be an additional transition region at small internuclear distances, which is not present in the LZS scheme but may be the source of oscillations found in the post maximum energy dependence of the collisional ionization cross section. The systematics of the collisional ionization cross sections for all alkali metal-halogen atom (M+X) collisions are discussed and the energy dependence of the cross section is computed using realistic potentials. In terms of the reduced velocity (at the crossing point) v/K_p, it is noted that only Li+I and Na+I are in the strong coupling region. For all other systems the cross section is found to decrease with increasing collision energy.