Dynamics of ion-molecule complex formation at very low energies and temperatures

The dynamics of complex formation in collisions of rotationally excited diatomic molecules with ions at very low collision energies and translational temperatures is discussed. Under these conditions, the locking of the intrinsic angular momentum of the diatomic molecule to the collision axis occurs in the region of centrifugal barriers, and the motion of collision partners across these barriers bears quantum features (tunneling transmission and overbarrier reflection). The capture in this energy range is described by coupled radial wave equations that are solved numerically. In two limits, the respective rate coefficients are expressed analytically: for low collision energies (adiabatic channel approximation with classical relative motion of partners) and at zero collision energy (Bethe limit, s-wave capture). By comparison with accurate numerical quantum results, it is shown that these two limits are satisfactorily bridged by the so-called axially-nonadiabatic channel model that considers uncoupled relative motion of the colliding partners across effective potentials; the latter are generated by adiabatic channel potentials and Coriolis interaction in the perturbed rotor basis. The limits of applicability of the standard adiabatic channel model are discussed. The general approach is illustrated by the capture of H2 and HCl in collisions with ions.