Nonreactive molecular encounters

Low-energy molecular collisions are discussed from a point of view that emphasizes the role of the internal degrees of freedom of the colliding molecules. The theory centers attention on the mechanism of energy transfer from kinetic energy of relative motion into internal energy and vice versa. It is shown that irrespective of the over-all energy transfer (which may be zero), the collision event proceeds via steps of energy migration between internal and relative degrees of freedom. This feature is absent in the available first-order theories. An approximation scheme which includes this feature even in first order is suggested, and a self-consistent-field scheme where each collision partner moves in the average field of the other partner is discussed. The migration of kinetic energy into internal degrees of freedom results in the temporary formation of bound states. Such intermediates are of importance in the study of intermolecular forces, vibrational excitations, contact charge-transfer complexes, and three-body recombinations. The collision of two hydrogen atoms is discussed in the adiabatic approximation, where the temporary bound state is due to charge re-distribution. The mechanism of three-body recombination is discussed and a recombination rate constant is calculated.