Radical quantum yields from formaldehyde photolysis in the 30 400-32 890 cm^-1 (304-329 nm) spectral region: Detection of radical photoproducts using pulsed laser photolysis-pulsed laser induced fluorescence

The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLIF) technique across the 30 400-32 890 cm^-1 (304-329 nm) spectral region of the ùA₂-X̃¹A₁ electronic transition. The photolysis laser had a bandwidth of ∼0.09 cm^-1, which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (∼0.07 cm^-1), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO₂. This technique produced an "action" spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm^-1. Yields were normalized to a value of 0.69 at 31 750 cm ^-1 based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 50¹ + 10 ¹40¹ combination band centered at 31 125 cm^-1. This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.