Production of HCO from propenal photolyzed at 193 nm: Relaxation of excited states and distribution of internal states of fragment HCO

The dynamics of photodissociation of propenal at 193 nm are studied by dete cting laser-induced fluorescence of nascent fragment HCO in its transition (B) over tilde (2)A'-(X) over tilde (2)A'. Rotational states up to N = 30 a nd K = 3 of HCO (X) over tilde (2)A' are populated and vibrational states ( 000), (010), and (001) are detected. The K-a = 1 doublet states and the two spin states for all vibrational levels detected are nearly equally populat ed. Much less rotational excitation is observed than the distributions calc ulated on a statistical model-phase space theory. This implies that dissoci ation occurs from the triplet channel with a small exit barrier. Small rota tional excitation arises from the repulsive part of the exit barrier and th e geometry of the transition state on the triplet surface. Experimental dat a yield an energy partitioning with translation, rotation, and vibration of HCO at 3.0, 1.3, and 1.5 kcal/mol, respectively, in total accounting for 1 1.5% of available energy. These results indicate that the other fragment C2 H3 has 3.2 kcal/mol of translation and 42.5 kcal/mol of internal energy; he nce, most C2H3 is expected to undergo secondary dissociation to C2H2 and H. Because the appearance of HCO is faster than that calculated based on the Rice-Ramsperger-Kassel-Marcus theory, other decay pathways dominate the pat hway of the radical channel from the triplet surface. (C) 2001 American Ins titute of Physics.