Collision-induced state-to-state energy transfer in perturbed rovibrational manifolds of small polyatomic molecules: mechanistic insights and observations

This paper considers collision-induced rovibrational energy transfer in small polyatomic molecules, measured on a state-to-state basis by rotationally selective, time-resolved optical double-resonance (DR) spectroscopy. Particular attention is given to the extent to which the rovibrational manifold of interest is energetically congested and/or affected by intramolecular perturbations. A semiclassical dynamical description is used to visualise how such perturbations, manifested in spectroscopic energies and transition probabilities, affect the efficiencies of collision-induced bovibrational energy transfer. Perturbation-induced enhancement (and, in some cases, inhibition) of V-V transfer efficiencies is examined, with regard to several DR measurements of perturbed manifolds of CO₂, D₂CO, C₂H₂, and C₂D₂ at various levels of vibrational excitation. Specific questions considered include: the feasibility of separating rotationally specific channels of collision-induced V-V transfer from the scrambling effects of rotational relaxation; how to characterise dynamical processes in which intermolecular perturbations exceed (and effectively override) those of intramolecular origin; the role of vibrational angular momentum in propensities and efficiencies of collision-induced V-V transfer; physically realistic molecular bases to minimise perturbation-induced contributions to rovibrational energy transfer; reconciliation of apparently contrasting dynamics of small and large polyatomic molecules, over a range of vibrational energies, spectral congestion, and molecular complexity.