We presents a fully quantal version of the Langevin model for the total rate
of exoergic ion-molecule reactions or inelastic processes. The model, which is
derived from a rigorous multichannel quantum-defect formulation of bimolecular
processes, agrees with the classical Langevin model at sufficiently high
temperatures. It also gives the first analytic description of ion-molecule
reactions and inelastic processes in the ultracold regime where the quantum
nature of the relative motion between the reactants becomes important.Comment: 5 pages, 3 figure