An improved classical approach quantum encounter treatment of collision-induced vibrational energy transfer. Application to He + CO (ni = 1, 2) Original Research Article

The vibrational energy transfer in a three-dimensional molecular collision is assumed, in the recently published Classical Approach Quantum Encounter (CAQE) theory [J. Chem. Phys. 99 (1993) 2567], to occur in a brief and sharply defined encounter which can be formulated in terms of an effectively one-dimensional atom-diatomic collision, with the parameters for the encounter reflecting the local features of the potential in the neighborhood of the collision,geometry. Here we describe improvements to the original CAQE theory. A mathematical recasting renders the theory more conducive to use with accurate quantum encounter methods. The first-order distorted wave Born approximation that was used in the original CAQE theory is here replaced by a numerically exact one-dimensional encounter method. We illustrate the need for the new method using collinear calculations for Ar + HCl and He + CO, and then apply it using a log-derivative method in a three-dimensional calculation of single and multiple quantum vibrational cross sections for He + CO. The results are judged by comparison to the results of coupled states and semiclassical calculations, with good agreement being obtained for all conditions over a wide range of energies.