Vibrationally mediated photodissociation of ethyne isotopologues and homologues revisited

Abstract

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 C 2 H 2 and C 2 HD to that in H 3 CCCH, D 3 CCCH and H 3 CH 2 CCCH. 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.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…15 Energy conservation arguments dictate that the cofragments formed when exciting at such long wavelengths must be ground (X) state C2H radicals, and measurements of the parent excited state lifetimes, 16 their fragmentation probabilities, 17 the product energy disposal, 18 how these quantities vary with excitation wavelength, 19 and companion ab initio theory 20 all suggest that dissociation occurs via coupling to one or more of the nest of triplet states on a relatively long timescale (long when compared to a typical C-H vibrational period). C-H bond fission following excitation of vibrationally excited C2H2 molecules at longer wavelengths has also been reported (at  ~243.1 21,22 and at 248.3 nm 23 ).…”