Photochemical formation of HCO and CH3 on the ground S0 (1A') state of CH3CHO

The dynamics of the photodissociation of CH3CHO into CH3+HCO products have been investigated at energies between 30 953 and 31 771 cm(-1), spanning the threshold for radical production on the triplet (T-1) surface. A barrierless pathway to CH3+HCO radical products formed on the ground state (S-0) surface was discovered and established to be an important reaction channel in acetaldehyde photodissociation throughout this wavelength range. HCO laser induced fluorescence (LIF) spectra recorded from CH3CHO dissociated above and below the T-1 barrier energy are quite different; HCO produced on S-0 yields a more congested LIF spectrum with sharp rotational transitions, while HCO formed on the T-1 surface displays fewer, more intense, Doppler-broadened lines. These differences have been further explored in the populations of the HCO K-a=1 doublets. Despite the upper and lower levels being almost isoenergetic, HCO formed on T-1 preferentially populates the upper K-c state due to the geometry of the T-1 transition state structure. In contrast, HCO formed on S-0 produces equal population in each of the upper and lower K-a=1 components. Product state distributions (PSDs) showed that HCO formed on S-0 is born with an approximately statistical distribution of population in the available product states, modeled well by phase space theory. HCO formed on the T-1 surface, in contrast, has a PSD that can be characterized as arising from impulsive dynamics. Previous discrepancies in the height of the T-1 barrier are discussed following the observation that, once the T-1 channel is energetically accessible, there is competition between the S-0 and T-1 pathways, with the dominance of the triplet channel increasing with increasing photolysis energy.