Study of Acetone Photodissociation over the Wavelength Range 248−330 nm:  Evidence of a Mechanism Involving Both the Singlet and Triplet Excited States

Abstract

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 < or = lambda/nm < or = 327.5, 0.3 < or = p/Torr < or = 400 and 218 < or = T/K < or = 295. The acetyl yield was determined relative to that at 248 nm by conversion to OH by reaction with O2. 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: S1 when the Stern-Volmer plots are linear and both S1 and T1 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 approximately 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.

Rate coefficients for the reactions CH3 + Br2 (224–358 K), CH3CO + Br2 (228 and 298 K), and Cl + Br2 (228 and 298 K)

“…This diverges from the results of Blitz et al [27], who suggest that the quantum yield of CH 3 CO radicals is 0.61 at this pressure and wavelength, with an overall quantum yield of 0.98, inferring a CH 3 to CH 3 CO ratio of 2.21. Our data analysis was therefore repeated with Eq.…”

Ketone photolysis in the presence of oxygen: A useful source of OH for flash photolysis kinetics experiments

“…Longer wavelength photolysis will deposit less energy into the CH 3 CO fragment and less dissociation of the acetyl will occur. Additionally, at long wavelengths, the pressure and temperature dependence of the primary quantum yield needs to be considered [26]. Issues surrounding ketone photolysis and the OH yield from the acetyl + O 2 reaction will be more fully discussed in forthcoming papers.…”

OH yields from the CH3CO+O2 reaction using an internal standard

“…In the atmosphere, it can be formed by the reactions of radicals/atoms (OH, NO 3 , Cl, Br) with acetaldehyde [3] or by the photolysis of some ketones [4,5].…”

“…The suggested mechanism is shown in Scheme 1 [7]: Although of insignificant atmospheric importance in its own right, R1b has been used as a convenient experimental marker for the acetyl radical (in the presence of O 2 ) in a study of the atmospherically important photolysis of acetone and of other methylketones [4,5,12].…”