Vibrational relaxation of CO (n=1) in collisions with H_2-. I. Potential energy surface and test of dynamical approximations

We present a fully quantal dynamical study of vibrational relaxation of CO in collisions with H₂ treating H₂ as a structureless projectile. The potential energy surface consists of a SCF part, including explicitly the variation with the CO bond distance, and a damped long range dispersion part. This potential model contains only two free parameters in the damping function which have been determined previously by fitting rotationally inelastic beam data for D₂-CO. The dynamical calculations are performed within the infinite order sudden approximation (IOSA) and the coupled states approximation (CSA). The IOSA relaxation cross sections are generally a factor of ∼1.5 smaller than the CSA cross sections independent on the initial rotational state of CO for j≲4-6. The relaxation rate constants are very sensitive to small changes of the interaction potential. After readjusting one of the damping parameters (within a range still acceptable with the beam data) the IOSA rate constants agree very well with the experimental ones for ortho H₂. It is demonstrated that on the same level of approximation the present rate constants are about three times larger than those obtained from the potential energy surface of Poulsen.