The wavelength and pressure dependence of the photolysis of propionaldehyde in air

Abstract

Propionaldehyde was photolyzed in air at 23 ± 1 °C at pressures up to 1 atm at several incident wavelengths: 334, 326, 313, 302, 280 and 254 nm. The two primary processes are ▪ ▪ The products measured were CO and C 2H 6, the former being a measure of the sum of both processes and the latter being a measure of molecular process (2). At all wavelengths Φ(CO) ≈ 1.0 in the low pressure limit. Primary process (2) is significant only with incident radiation at 254 and 280 nm. Its low pressure quantum yield is 0.39 at both wavelengths. At 254 nm there is no pressure quenching of C 2H 6, but at 280 nm most, but not all, of the C 2H 6 can be pressure quenched with a half-quenching pressure of 47 Torr of air. We suggest that primary process (2) comes from upper vibrational levels of the ground and excited singlet states. Primary process (1) is pressure quenched at all wavelengths. The pressure quenching follows Sterm—Volmer quenching at 326, 302, 280 and 254 nm. However, at 334 and 313 nm, the Stern—Volmer quenching curves show a marked deviation from linearity, and two states with different decomposition rates are indicated. We interpret primary process (1) as coming from upper vibrational levels of the triplet state but with the less reactive vibrational state being more readily quenched than the more reactive vibrational states to the ground vibrational level of the triplet. The half-quenching pressures increase from about 24 Torr at 334 nm to 280 Torr at 254 nm for the fast quench and from 325 Torr at 334 nm to 610 Torr at 280 nm for the slow quench. The more reactive states appear only at 334, 313 and 280 nm and always give a low pressure quantum yield of 0.60. At atmospheric pressure the average quantum yield for all six incident wavelengths for primary process (1) is 0.20 ± 0.08. This leads to atmospheric photodissociation coefficients of about 1.3 × 10 −5 s and about 9.7 × 10 −6 s −1 for solar zenith angles of 30° and 58.18° respectively.