Isotope Effects in Collisional VT Relaxation of Molecular Hydrogen

A simple exponential-potential model of molecular collisions leads to a twoparameter analytic expression for rates of collisionally induced vibrationaltranslation (VT) energy exchange that has been shown to be accurate over variations of orders of magnitude as a function of temperature in a variety of systems. This includes excellent agreement with reported experimental and theoretical results for the fundamental self-relaxation rate of molecular hydrogen H2(v = 1) + H2 → H2(v = 0) + H2. The analytic rate successfully follows the five-orders-of-magnitude change in experimental values for the temperature range 50-2000 K. This approach is now applied to isotope effects in the vibrational relaxation rates of excited HD and D2 in collision with H2: HD(v = 1)+H2 → HD(v = 0)+H2 and D2(v = 1)+H2 → D2(v = 0)+H2. The simplicity of the analytic expression for the thermal rate lends itself to convenient application in modeling the evolving vibrational populations of molecular hydrogen in shocked astrophysical environments