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

Abstract

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 Ã(1)A2–X̃(1)A1 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 5(0)(1) + 1(0)(1)4(0)(1) 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.

POPTARTS: A New Method to Determine Quantum Yields in a Molecular Beam

“…The zero-pressure quantum yields of HCO formation from HCHO photoylsis for our measurements are recorded in Figure . Shown on the same figure are previous measurements for a variety of bath gases and pressure, from the work of Troe (1 atm air), Ernest et al (100 Torr He), and Gorrotxategi Carbejo et al (200 Torr N 2 ) and the recommendations of an IUPAC working party (solid line) . It is broadly accepted that the quantum yields of HCHO photolysis are not pressure dependent at the wavelengths used in this work .…”

“…Literature HCO photolysis quantum yields for HCHO as a function of photolysis energy, from Troe, Ernest et al, Gorrotxategi Carbejo et al, and IUPAC recommendations . Zero-pressure measurements from this work are shown, with error bars, in red.…”