Temperature-dependent relaxation of CO(v=1) by HD, D2, and He and of D2(v=1) by D2

Abstract

Rate constants for the vibrational deactivation of CO in collisions with HD, D₂, and He have been measured as a function of temperature using the laser excited vibrational fluorescence technique. Throughout the 109–630 °K range, CO–He and CO–HD samples exhibit a single exponential decay, dominated by V–T,R transfer, with rates increasing rapidly with temperature. Typical collision deactivation rate constants at 630 °K are 22.7 sec⁻¹ torr⁻¹ for He and 82.5 sec⁻¹ torr⁻¹ for HD, and, at 109 °K, 0.067 sec⁻¹ torr⁻¹ for He and 0.27 sec⁻¹ torr⁻¹ for HD. At low temperatures, diffusion and radiative decay become important contributions to the observed rates. In CO–D₂ mixtures, double exponential decay of CO fluorescence at large D₂ mole fractions is obtained, corresponding to rapid V–V transfer between the (v=1) vibrational levels of CO and D₂, followed by coupled V–T,R deactivation. The V–V transfer rate (ΔE=−850 cm⁻¹) increases from 0.26 sec⁻¹ torr⁻¹ at 202 °K to 69.4 sec⁻¹ torr⁻¹ at 633 °K. The V–T,R deactivation rate constant for excited CO by D₂ is significantly slower than that by HD and H₂, going from 0.089 sec⁻¹ torr⁻¹ at 156 °K to 3.7 sec⁻¹ torr⁻¹ at 450 °K, indicating the decreased importance of the rotational states of D₂ in the deactivation of CO than hypothesized for H₂ and HD. The deactivation of excited D₂ by collisions with D₂ also increases with temperature, ranging from 0.080 sec⁻¹ torr⁻¹ at 202 °K to 6.9 sec⁻¹ torr⁻¹ at 450 °K.