title: HCO quantum yields in the photolysis of HC(O)C(O)H (glyoxal) between 290 and 420 nm.
abstract: 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 HCO 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 approximately 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(-0.4DeltaE) (cm(3) molecule (-1)) where DeltaE = ((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, H(2)CO + O(2), and H(2) + 2CO at atmospheric pressure are presented.
authors.affiliation: Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305-3328, USA.
authors.forename: Karl J
authors.initials: KJ
authors.lastname: Feierabend
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Flad
authors.forename: S S
authors.initials: SS
authors.lastname: Brown
authors.forename: James B
authors.initials: JB
authors.lastname: Burkholder