Photodissociation dynamics of propyne at 157 nm

Photodissociation of propyne at 157 nm has been investigated using photofra gment translational spectroscopy. Detailed investigation of various photofr agments from the deuterated compounds CD3CCH and CH3CCD, as well as the unl abeled propyne provides a uniquely clear picture of an inherently complex p rocess. Hydrogen atom elimination processes from both the CH3 group and the C=C-H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H eliminatio n shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.4 3 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl) . Molecular hydrogen elimination processes have also been observed, but wit h a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H-2 HD and D-2 photoproducts from various deuterated pro pyne molecules shows that the molecular hydrogen elimination process is not sensitive to the origin of the two hydrogen atoms. This implies that scram bling (or isomerization) of H atoms is important prior to dissociation at 1 57 nm excitation of propyne. Two different C-C bond breaking processes have also been observed; one process breaks the C-C single bond to form methyl and C2H radicals, while the other process forms CH2 and C2H2. The existence of the CH2 channel also indicates that isomerization of propyne is signifi cant prior to dissociation. The relative branching ratio of these two chann els is estimated to be 2.2:1 for CH3 and CH2 formation, respectively. (C) 2 000 American Institute of Physics. [S0021-9606(00)00913-2].