State-resolved dissociation dynamics of glyoxal near the threshold for formation of fragment HCO

Fluorescence excitation spectra for transition à Au – X̃ Ag of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm, most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b . Fluorescence decays with quantum beats, resulting from coherent excitation of S1 and T1 states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and onset of disappearance of fluorescence of glyoxal, the threshold for formation of fragment HCO from trans-glyoxal is determined to be 394.4 nm. With data for heat of formation this threshold yields an exit barrier 2.560.5 kcal/mol. Signal of HCO for wavelengths greater than the threshold position up to 400.5 nm is observed and is assigned to arise from the S0 surface. Rotationally state-resolved appearance rates of HCO for a vibrational state near the dissociation threshold are measured. Relative to lifetimes obtained from eigenstates in the quantum-beat data, the appearance time of product HCO reflecting the overall depletion of glyoxal is on average longer. Some observed gateway states with enhanced yields of HCO are considered to result from strong singlet–triplet interaction. © 2003 American Institute of Physics. @DOI: 10.1063/1.1612478#