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

Time-resolved infrared-ultraviolet double resonance (IRUVDR) spectroscopy is used to study the kinetics of collision-induced rovibrational energy transfer between the v₆ and v₄ modes of D₂CO in the vapor phase. As in paper I [J. Chem. Phys. 93, 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 K_a. Infrared excitation by the 10R (32) CO₂-laser line prepares D₂CO in two discrete rovibrational states, (J,K_a,K_c) = (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 (v₄,v₆) rovibrational manifold, through a combination of rotational energy transfer (RET) and v6 → v₄ transfer. This yields an extensive range of (J,K_a)-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 K _a-resolved channel of v₆→ v₄ transfer is that with K_a= 4→6; accompanying J-resolved v₆- →v₄ transfer channels favor ΔJ = 0, with state-to-state rate constants scaling as J^3.4; additional (J,K_a)-resolved v₆→v₄ channels allow a spread of J- and K _a-changing V-V transfer. These features are consistent with the accepted mechanism of v₆-→v₄ transfer in D ₂CO, involving enhancement by a combination of Coriolis coupling and rotor asymmetry perturbations. In addition to v₆→v₄ 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; K_a-changing RET favors even values of ΔK_a and, contrary to previous expectations, is J selective with a propensity for ΔJ = 0. The physical implications of these results are discussed.