Validity of the adiabatic approximation for vibrational energy transfer in collisions between diatomic molecules

Accurate quantum mechanical transition probabilities for vibrational to vibrational and vibrational to translational energy transfer in collinear collisions of two hydrogen molecules were calculated for a model potential by numerical integration of the Schroedinger equation. The accuracy of these calculations was checked by conservation of flux and convergence tests and by verifying that they satisfied a rigorous theorem proven by Clarke and Thiele. These accurate results were then used to determine the range of validity of the adiabatic approximation of Thiele and Katz as applied to this model system by Clarke and Thiele. This approximation yielded probabilities which agreed with the accurate ones to 10% or better for total energies E up to 6.1 (in units of diatom vibrational quanta). However, at E = 7.9 these approximate results could be off by a factor of 1.5 or higher. (Auth.)