Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

The extensive studies on photodissociation of rovibrationally pre-excited ethyne isotopologues and homologues are reviewed. These studies are utilized here for comparing the dynamics of energy flow in C₂H₂ and C₂HD to that in H₃CC≅CH, D₃CC≅CH and H₃CH₂CC≅CH. Evidence on the intramolecular dynamics comes from the extent of mode- and bond-selectivity in ethyne and its mono-deuterated variant and from the spectral linewidth in propyne and butyne. In the smaller species, rotational and vibrational mode- and bond-selective cleavage were observed, while in the larger ones mode-dependent lifetimes were found. The different behaviour of the two groups with regard to preferential bond-fission indicates that in the former the intramolecular vibrational distribution lifetime is on the ns timescale, whereas in the latter it is at least one order of magnitude shorter. These observations are interpreted following the recent progress in the understanding of the intramolecular dynamics of these species.