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 detecting laser-induced fluorescence of nascent fragment HCO in its transition B̃ 2A′–X̃ 2A′. Rotational states up to N=30 and K=3 of HCOX̃ 2A′ are populated and vibrational states (000), (010), (001) are detected. The K_a=1 doublet states and the two spin states for all vibrational levels detected are nearly equally populated. Much less rotational excitation is observed than the distributions calculated on a statistical model—phase space theory. This implies that dissociation occurs from the triplet channel with a small exit barrier. Small rotational excitation arises from the repulsive part of the exit barrier and the geometry of the transition state on the triplet surface. Experimental data 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 11.5% of available energy. These results indicate that the  other fragment C₂H₃ has 3.2 kcal/mol of translation and 42.5 kcal/mol of internal energy; hence, 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 pathway of the radical channel from the triplet surface.