Coriolis-assisted vibrational energy transfer in D₂CO/D ₂CO and HDCO/ HDCO collisions: Experiment and theory

The technique of time-resolved infrared-ultraviolet double resonance is used to characterize the rates and propensity rules for mode-to-mode vibrational (V-V) energy transfer in D₂CO/ D₂CO and HDCO/HDCO collisions. Such processes are found to be exceptionally efficient when collision-induced transfer is between the ν₆ and ν₄ modes of D₂CO or between the ν₅ and ν₆ modes of HDCO: in the case of D₂CO prepared in a specific ν₆ rovibrational state by the 10R 32 line of a CO₂ laser, the rate of V-V transfer to specific states of the ν₄ rovibrational manifold is approximately three times greater than the hard-sphere gas-kinetic collisional rate. This efficiency is much higher than for typical V-V transfer processes and approaches that of pure rotational relaxation, with the result that rotationally specific V-V transfer channels can be identified. The essential mechanism depends on the strong Coriolis coupling between the modes of D₂CO or HDCO involved, as demonstrated by a semiclassical theoretical treatment which considers only the electric dipole/dipole portion of the intermolecular potential. The combined effect of Coriolis and asymmetric-rotor perturbations causes mixing of rovibrational basis states and induces non vanishing matrix elements of the permanent electric dipole moment between the vibrational modes of interest. These effects are most pronounced at moderate values of the rotational quantum number K_a (∼4), because quantum-mechanical interferences tend to annihilate the transition moment induced by Coriolis coupling alone at higher values of K_a. The theory also assumes that particularly efficient V-V transfer channels arise from very small energy differences between initial and final states of the state-selected molecule, owing to the abundance of collision-partner molecules then available to yield a zero overall energy defect for the pair of colliding molecules. The predictions of the simple long-range theory adopted yield order-of-magnitude agreement with the experimental results; possible deficiencies of the theory are discussed. Also discussed are the wider implications of the results, with regard to collision-induced V-V transfer between discrete rovibrational levels of small polyatomic molecules in general, to intramolecular vibrational redistribution in congested rovibrational and rovibronic manifolds, and to mechanisms of infrared multiple-photon excitation.