Intramolecular vibrational energy relaxation induced by van der Waals molecule fragmentation: The systems C6H6−nDn⋅He1,2

We report the results of experiments which probe the vibrational energy redistribution which accompanies fragmentation of deuterobenzene–helium van der Waals molecules. Branching ratios for vibrational relaxation to various final states were obtained for initially prepared levels 61-He1,2, 61162-He1, 111171-He1, and 172-He1 in several deuterobenzene compounds. The probabilities of transitions to final states in the relaxation from 61-He1 show general agreement with an energy gap law, and relaxation from 61162-He1, 111171-He1, and 172-He1 favors small vibrational quantum number changes. The experiments were undertaken to illuminate how molecular symmetry influences the choice of pathways for vibrational energy redistribution associated with van der Waals molecule predissociation. The data for the various C6H6−nDn⋅He1,2 systems, however, show very different relaxation patterns. These patterns cannot readily be explained by the usual intramolecular vibrational coupling mechanisms.