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

Abstract

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 10R32 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 nonvanishing 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.