Wavelength-dependent photolysis of glyoxal in the 290-420 nm region

We have studied the gas-phase photolysis of glyoxal, (CHO)(2), at 10 nm intervals in the 290-420 nm region by using dye laser photolysis coupled with cavity ring-down spectroscopy. Absorption cross sections of glyoxal have been measured. The HCO radical is its photodissociation product. The dependence of the HCO quantum yield on photodissociation wavelength, glyoxal pressure, and nitrogen buffer gas pressure has been determined. The HCO yields decrease with increasing glyoxal pressure in the 1-8 Torr range, owing to the increasing HCO radical reactions at higher glyoxal pressures and quenching by ground-state glyoxal. After separation of the contribution of HCO radical reactions, the aldehyde pressure quenching effect was still observed in the 320-420 nm region, and this effect increased with increasing wavelength. The HCO radical yields (all lambda) and the ratios of quenching to unimolecular decay rate constants of excited glyoxal (lambda greater than or equal to 320 nm) are given. The peak HCO yield is 2.01 +/- 0.08 (error quoted only includes la measurement uncertainty) at 390 nm, consistent with the occurrence of the (HCO)(2) + hv --> 2HCO channel. The HCO radical yields are around 1.56 +/- 0.22 in the 320-370 nm region, indicating the simultaneous occurrence of (HCO)(2) + hnu --> 2HCO and (HCO)(2) + hv --> HCO + H + CO channels. The HCO radical yields are 0.50 +/- 0.01, 0.68 +/- 0.02, and 0.84 +/- 0.07 at 290, 300, and 310 nm, respectively, which may suggest the opening of an additional photolysis channel at higher photon energies. The dependence of the HCO quantum yield on nitrogen buffer gas pressure was examined between 10 and 400 Torr. The HCO radical yields are independent of nitrogen pressure in the 290-370 nm range, but they decrease with increasing nitrogen pressure in the 380-420 nm region. A comparison of the wavelength-dependent HCO, radical yields with results obtained from previous dynamics and quenching studies provides insight into the mechanism of glyoxal photodissociation as a function of wavelength.