CH3O Yield in the CH3+ O3Reaction Using the LP/LIF Technique at Room Temperature

CH 3 O(X 2 E) radicals have been detected by laser-induced fluorescence (LIF) for the first time as a product of the reaction between CH 3 and O 3 . CH 3 radicals were generated by excimer-laser photolysis of CH 3 Br at 193 nm in the presence of O 3 at room temperature and 100 Torr of He. The rate constant for reaction 1a (CH 3 + O 3 → CH 3 O + O 2 ) has been determined by monitoring the CH 3 O build-up as a function of reaction time and k 1a was found to be (9.68 ± 1.10) × 10 -14 cm 3 molecule -1 s -1 . The rate coefficient of the global reaction 1 (CH 3 + O 3 → products) has been determined by the numerical analysis of the CH 3 O temporal profiles, yielding a value of k 1 = (2.2 ± 0.3) × 10 -12 cm 3 molecule -1 s -1 . Reaction of CH 3 with NO 2 has also been studied under the same experimental conditions as reaction 1 and has been used to calibrate the CH 3 O LIF signal. On the other hand, the rate constant k 2 for reaction 2 (CH 3 O + O 3 → products) has been directly determined ( k 2 ( T = 298 K) = (2.53 ± 0.75) × 10 -14 cm 3 molecule -1 s -1 ) using the CH 3 ONO(λ = 193 nm)/O 3 /He system and was included in the chemical model used to describe the kinetics of CH 3 in the presence of O 3 . The branching ratio φ CH 3 O = k 1a / k 1 obtained was found to be (0.044 ± 0.013). This low yield of CH 3 O in reaction 1a can be explained not only by the formation of highly vibrationally excited CH 3 O and the subsequent prompt dissociation to produce CH 2 O + H but also by the direct formation of CH 2 O + HO 2 in reaction 1.