Study of acetone photodissociation over the wavelength range 248-330 nm: Evidence of a mechanism involving both the singlet and triplet excited states

Measurements of the acetyl yield from acetone photolysis have been made using laser flash photolysis/ laser induced fluorescence. Phi(total)(lambda, p, T) was determined over the ranges: 266 <= lambda/nm <= 327.5, 0.3 <= p/Torr <= 400 and 218 <= T/K <= 295. The acetyl yield was determined relative to that at 248 nm by conversion to OH by reaction with O-2. Linear Stern- Volmer plots (1/[OH] vs [M]) describe the data for lambda < 300 nm, but for lambda > 300 nm, nonlinear Stern- Volmer plots were observed. This behavior is interpreted as evidence for dissociation from two excited states of acetone: S-1 when the Stern- Volmer plots are linear and both S1 and T-1 when Stern- Volmer plots are nonlinear. A model for acetone photolysis is proposed that can adequately describe both the present and literature data. Barriers to dissociation are invoked in order to explain the dependence of pressure quenching of the acetone photolysis yields as a function of wavelength and temperature. This pressure quenching was observed to become more efficient with increasing wavelength, but it was only above similar to 300 nm that a significant T dependence was observed, which became more pronounced at longer wavelengths. This is the first study to observe a T-dependent Phi(total)(lambda, p, T). A parametrized expression for Phi(total)(lambda, p, T) has been developed and is compared against the recommended literature data by running box model simulations of the atmosphere. These simulations show that acetone photolysis occurs more slowly at the top of the troposphere.