Fast mode-to-mode vibrational energy transfer in D₂CO vapour: analysis of a complex pressure dependence

The kinetics of fast, rotationally specific vibrational energy transfer between the ν₆ and ν₄ vibrational modes of D₂CO displays a complex pressure dependence, which is suggestive of a unimolecular contribution to the ν₆→ν₄ transfer mechanism. This interpretation is discredited on the basis of an insufficient density of states of the low-energy portion of the (ν₄, ν₆) rovibrational manifold. The apparent anomaly is satisfactorily explained by detailed kinetic modelling which takes account of the finite duration of the pulsed infrared excitation and of the multi-channel nature of the collision-induced ν₆→ν₄ transfer mechanism.