Abstract
The steady state photolysis of iso-butyraldehyde ( i-C 3H 7CHO) was studied in the presence of O 2 at 263 and 294 K at several incident wavelengths. The quantum yields of CO and C 3H 8 were measured. From them the primary quantum yield Φ(C 3H 8) of the molecular process to produce CO + C 3H 8 and the primary quantum yield Φ(rad) of the free-radical process to produce i-C 3H 7 + HCO were deduced. Likewise the flash photolysis of i-C 3H 7CHO was studied in the presence of air at 298 K. The transient absorption of RO 2 radicals was monitored, and relative quantum yields were obtained with 284.0, 302.5, 311.7, 325.0 and 330.5 nm incident radiation. The quantum yields were not pressure quenched, except for very slightly at 330.5 nm. They followed the same trends with incident wavelength as seen in the steady state photolysis. The mechanism describing the primary process is ▪ where R represents i-C 3H 7, A represents the aldehyde, the superscripts 1 and 3 represent excited singlet and triplet states respectively, the subscripts n and 0 represent excited and ground vibrational levels respectively and M represents O 2 and N 2. k 2a/ k 2 = 0.43 and k 2a/ k 2 = 0.31 at 253.7 nm and 280.3 nm respectively and k 2a/ k 2 = 0 at 302.2 nm and longer wavelengths. The quenching of 3A n by O 2 or N 2 is quite inefficient, the half-pressure for quenching increasing with temperature and incident energy. Under all conditions studied it is 1200 Torr or greater. However, quenching of 3A n by i-C 3H 7CHO is quite efficient with k 5/ k 8a = 32.1 ± 10.0 Torr at 312.8 nm and 294 K. The quenching of A n is insensitive to variations in temperature or incident energy. An approximate value for k 3/ k 4 was estimated to be about 108 Torr. From the mechanism, radical quantum yields could be estimated at atmospheric pressure at several wavelengths. These lead to atmospheric photodissociation coefficients for radical formation at 298 K of 7.6 × 10 −5 s −1 and 5.9 × 10 −5 s −1 for solar zenith angles of 30° and 58° respectively.
Published Version
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