Photochemical formation of HCO and CH3 on the ground S0 (A1′) 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 (T1) surface. A barrierless pathway to CH3+HCO radical products formed on the ground state (S0) 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 T1 barrier energy are quite different; HCO produced on S0 yields a more congested LIF spectrum with sharp rotational transitions, while HCO formed on the T1 surface displays fewer, more intense, Doppler-broadened lines. These differences have been further explored in the populations of the HCO Ka=1 doublets. Despite the upper and lower levels being almost isoenergetic, HCO formed on T1 preferentially populates the upper Kc state due to the geometry of the T1 transition state struc...