H-2 production in the 440-nm photodissociation of glyoxal

H-2 has been detected following the photolysis of glyoxal at 440 nm using t he techniques of vacuum ultraviolet laser-induced fluorescence and (2 + 1) resonance enhanced multiphoton ionization. It is thus confirmed that a frac tion of the glyoxal excited at this wavelength dissociates into three photo fragments: HCOCOH --> H-2 + 2 CO. The most populated vibrational level of t hose observed was H-2 (v = 1), and in this level rotational states from J = 0-9 were detected. Doppler profiles of these lines provide estimates of th e translational energy and show a v parallel to J correlation. Of the avail able energy to the H-2 + 2CO products, 3.1% appears as rotational energy in H-2 (v = 1), 17.8% appears as the H-2 (v = 1) vibration, and 46.8% appears as H-2 (v = 1) translation. Excitation of the 7(0)(2) band produces somewh at more of the H-2 + 2CO channel than does excitation of the 0(0)(0), 5(0)( 1), or 8(0)(1) bands. These observations are consistent with a model in whi ch trans-->cis isomerization precedes dissociation. Rotational excitation w ith v parallel to J is caused by the v(7) torsional motion. The small degre e of rotational excitation, the production of H-2 in v = 1 and v = 2, and t he translational energy distribution are all consistent with ab initio calc ulations of the transition state structure.