Production of HCO from propenal photolyzed near 300 nm: Reaction mechanism and distribution of internal states of fragment HCO

The photodissociation dynamics of propenal in the near UV region is studied by detecting fluorescence of nascent fragment HCO in its transition B̃ 2A′−X̃ 2A′000. The yield is small and the wavelength range 300–287 nm of production is narrow. From the onset of yield of HCO versus the photolysis wavelength, the threshold of the formation of C2H3+HCO is determined to be 95.9±0.6 kcal/mol. At photolysis energy 34 032 cm−1 rotational states up to N=14 for K=0 and Ka=2 of HCO are populated. The fluorescence intensity is corrected for both the quantum yield of fluorescence and the effect of axis switching to yield the population of rotational states of HCO. The Ka=1 doublet states and the two spin states are about equally populated. The calculated distributions of HCO according to phase-space theory disagree with the experimental data indicating a nonstatistical distribution. Hence, radical products are expected to emanate from the triplet surface with a small exit barrier; this process competes with intersystem crossing back to the ground electronic state to generate molecular products C2H4+CO so causing a small yield of HCO and rapid depletion of excited propenal. This explains why the rate coefficient (>2×108 s−1) from the appearance curve of fragment HCO is greater than the calculated dissociation rate of triplet propenal.