A laser photolysis/LIF study of the reactions of O(3P) atoms with CH3 and CH3O2 radicals

Abstract

Kinetics and mechanisms of the reactions CH₃+ O → CH₂O+ H (1), and CH₃O₂+ O → CH₃O+ O₂(2) at 298 K have been investigated using 193 nm laser co-photolysis of N₂O–azomethane mixtures for the simultaneous generation of O(³P) atoms and CH₃(N₂ as diluent)-CH₃O₂(O₂ as diluent) radicals combined with LIF for the time-resolved detection of reaction products. From measurements of CH₂O formation the rate coefficients k₁ was determined as k₁=(1.1 ± 0.4)× 10^–10 cm³ s^–1. Concurrent yield studies suggest that CH₂O accounts for the total loss of CH₃. The rate coefficient k₂ was determined from observation of CH₃O formation to be k₂=(4.3 ± 2.0)× 10^–11 cm³ s^–1. Although in this reaction OH radicals and CH₂O were also observed, a detailed product analysis combined with computer simulation suggests that these result from unavoidable parallel and consecutive reactions. Hence it is concluded that CH₃O + O₂ is the dominant product channel of reaction (2). Energy diagrams are presented which suggest that the dynamics of both reactions are determined by primary capture mechanism forming highly vibrationally excited adducts and followed by rapid subsequent decomposition, viz. CH₃+ O → CH₃O*→ CH₂O + H (1) CH₃O₂+ O → CH₃O^*₃→ CH₃O + O₂. (2) As a result of high excitation energies and low exit barriers k₁ and k₂ are not expected to be pressure-dependent.