Rotationally specific mode–to–mode vibrational energy transfer in D2CO/D2CO collisions. II. Kinetics and modeling

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Abstract
Time‐resolved infrared‐ultraviolet double resonance (IRUVDR) spectroscopy is used to study the kinetics of collision‐induced rovibrational energy transfer between the ν6 and ν4 modes of D2CO in the vapor phase. As in paper I [J. Chem. Phys. 9 3, 8634 (1990)] of the series, attention rests on the existence of V–V transfer channels which are rotationally specific with respect to both J and Ka. Infrared excitation by the 10R(32) CO2 ‐laser line prepares D2CO in two discrete rovibrational states, (J,Ka,Kc)=(11,4,7) and (7,2,6), of the v6=1 vibrational manifold. D2CO/D2CO collisions then disperse this selected population to various states of the (ν4,ν6) rovibrational manifold, through a combination of rotational energy transfer (RET) and ν6→ν4 transfer. This yields an extensive range of (J,Ka) ‐resolved IRUVDR kinetic curves, demonstrating the collision‐induced evolution of rovibrational population and enabling that evolution to be modeled by means of a master‐equation approach. The features of the model of best fit are as follows: the dominant Ka ‐resolved channel of ν6→ν4 transfer is that with Ka=4→6; accompanying J‐resolved ν6→ν4 transfer channels favor ΔJ=0, with state–to–state rate constants scaling as J3.4; additional (J,Ka) ‐resolved ν6→ν4 channels allow a spread of J‐ and Ka ‐changing V–V transfer. These features are consistent with the accepted mechanism of ν6→ν4 transfer in D2CO, involving enhancement by a combination of Coriolis coupling and rotor asymmetry perturbations. In addition to ν6→ν4 transfer, RET provides the predominant channels of collision‐induced relaxation: J‐changing RET is described by a conventional fitting law based on the energy gap ‖ΔE‖ for the state‐selected molecule; Ka ‐changing RET favors even values of ΔKa and, contrary to previous expectations, is J selective with a propensity for ΔJ=0. The physical implications of these results are discussed.

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