Inelastic molecular collisions with an orientation-averaged interaction energy

The importance of molecular orientations for vibrational-translational energy transfers between diatomic molecules has been investigated. An angle-dependent potential function is assumed, and it is averaged over the orientations and vibrations of colliding molecules. For I₂? I₂ and Cl₂? Cl₂, it is found that the calculated average vibrational transition probability for a colinear collision is over-estimated by large factors (1/γ) compared to that obtained when all possible molecular orientations are considered. At 300⁰K, 1/γ = 34.4 for I₂? I₂ and 17.6 for Cl₂? Cl₂, while it is 6.8 and 5.9 for N₂? N₂ and O₂? ₂, respectively. It is also shown that 1/γ decreases rapidly as temperature increases. At 2000⁰K, 1/γ ≈? 3 for I₂? I₂, Cl₂? Cl₂, and N₂? N₂, while it is ≈? 2.5 for O₂? O₂. In general, when the molecules are large, and when strong attractive forces act between them, 1/γ is very large at low temperatures (<1000⁰K).