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.