HCO Quantum Yields in the Photolysis of HC(O)C(O)H (Glyoxal) between 290 and 420 nm

Quantum yields, Phi, for the production of the formyl radical, HCO, in the photolysis of glyoxal were determined at 85 wavelengths, lambda, in the range of 290-420 nm at pressures between 50 and 550 Torr (N-2) at 298 K using pulsed-laser photolysis combined with cavity ring-down spectroscopy detection of HCO. HCO quantum yields were parametrized using a Stern-Volmer analysis to obtain extrapolated zero-pressure NCO quantum yields, Phi(0)(lambda), and values for the ratio of the rate coefficients for quenching and dissociation, k(q)/k(d)(lambda), at each wavelength. Phi(0)(lambda) varied smoothly with wavelength with a maximum value of similar to 1.8 in the range 300-385 nm with values decreasing to near 0 at 420 nm and 0.4 at 290 nm. k(q)/k(d)(lambda) was measurable at nearly all photolysis wavelengths and is well-represented by the relationship k(q)/k(d)(lambda) = (2.3 x 10(-20)) +(1.5 x 10(-19)) exp(-0.4 Delta E) (cm(3) molecule(-1)) where Delta E = ((28 571/lambda) - 72.5) (kcal mol(-1)), lambda is the photolysis wavelength (nm), and 72.5 kcal mol(-1) is the threshold for glyoxal photodissociation. Differences in our HCO quantum yield wavelength- and pressure-dependence with previous studies are discussed. The present HCO quantum yield data are appropriate for use in atmospheric model calculations, and revised wavelength-dependent photolysis branching ratios for the production of 2HCO, H2CO + O-2 and H-2 + 2CO at atmospheric pressure are presented.