H₂ Production in the 440-nm Photodissociation of Glyoxal

H 2 has been detected following the photolysis of glyoxal at 440 nm using the techniques of vacuum ultraviolet laser-induced fluorescence and (2 + 1) resonance enhanced multiphoton ionization. It is thus confirmed that a fraction of the glyoxal excited at this wavelength dissociates into three photofragments: HCOCOH → H 2 + 2 CO. The most populated vibrational level of those 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 the translational energy and show a v || J correlation. Of the available 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 somewhat 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 which trans→cis isomerization precedes dissociation. Rotational excitation with v || J is caused by the v 7 torsional motion. The small degree of rotational excitation, the production of H 2 in v = 1 and v = 2, and the translational energy distribution are all consistent with ab initio calculations of the transition state structure.