The Wavelength Dependence of the Photodissociation of Propionaldehyde in the 280−330 nm Region

Abstract

We have investigated the photodecomposition of propionaldehyde (C2H5CHO; propanal) at 5 nm intervals in the 280−330 nm region by using dye laser photolysis combined with cavity ring-down spectroscopy. Absorption cross sections were determined for propionaldehyde. The HCO radical was a fragment from photodissociation. The HCO radical yields, obtained by monitoring its transient absorption at 613.8 nm, decreased with increasing C2H5CHO pressure in the 1−10 Torr range due to the increasing HCO + HCO, HCO + C2H5, and HCO + C2H5CHO reactions at higher propionaldehyde pressures and quenching by ground state propionaldehyde. After separating the contribution of HCO radical reactions, the propionaldehyde pressure quenching effect was only observed at photolysis wavelengths longer than 315 nm. Values of zero-pressure HCO yields (all λ) and ratios of quenching to unimolecular decay rate constant of excited propionaldehyde (λ ≥ 315 nm) were given. The HCO yields (φHCO°) were 0.98 ± 0.06, 0.92 ± 0.06, 0.95 ± 0.08, 0.98 ± 0.11, 0.91 ± 0.05, and 1.08 ± 0.07 at 295, 300, 305, 310, 315, and 320 nm, indicating that C2H5CHO + hν → C2H5 + HCO is the dominant photolysis pathway. The HCO yields decreased at both the shorter-wavelength (280 nm) and the longer-wavelength (330 nm) ends. The wavelength dependence of the HCO yields from propionaldehyde photolysis was compared to that from t-pentanal ((CH3)3CCHO) photolysis. The HCO yields from t-pentanal photolysis decayed much more rapidly at the shorter-wavelength end, which might reveal the difference in the excited states singlet−triplet surface crossing of t-pentanal versus propionaldehyde. The dependence of the HCO yields on nitrogen buffer gas pressure was examined between 10 and 400 Torr. No dependence was observed. Cross section results were combined with HCO radical yields to estimate atmospheric photodissociation rate constants of propionaldehyde to form HCO as a function of zenith angle for cloudless conditions and at 760 Torr nitrogen pressure. Radical formation rate constants were 1.6 × 10-5 − 4.6 × 10-5 s-1 for zenith angles of 0−60°.