Production of HCO from propenal photolyzed near 300 nm: Reaction mechanismand 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 (B) ov er tilde(2)A' - (X) over tilde(2)A' 0(0)(0). The yield is small and the wav elength range 300-287 nm of production is narrow. From the onset of yield o f HCO versus the photolysis wavelength, the threshold of the formation of C 2H3 + 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 K-a =2 of HCO are popu lated. The fluorescence intensity is corrected for both the quantum yield o f fluorescence and the effect of axis switching to yield the population of rotational states of HCO. The K-a = 1 doublet states and the two spin state s are about equally populated. The calculated distributions of HCO accordin g to phase-space theory disagree with the experimental data indicating a no nstatistical distribution. Hence, radical products are expected to emanate from the triplet surface with a small exit barrier; this process competes w ith intersystem crossing back to the ground electronic state to generate mo lecular products C2H3 + HCO so causing a small yield of HCO and rapid deple tion of excited propenal. This explains why the rate coefficient (>2 x 10(8 ) s(-1)) from the appearance curve of fragment HCO is greater than the calc ulated dissociation rate of triplet propenal. (C) 1999 American Institute o f Physics. [S0021-9606(99)00442-0].