Rate constant and branching ratio of the reaction of ethyl peroxy radicals with methyl peroxy radicals.

The cross-reaction of ethyl peroxy radicals (C 2 H 5 O 2 ) with methyl peroxy radicals (CH 3 O 2 ) (R1) has been studied using laser photolysis coupled to time resolved detection of the two different peroxy radicals by continuous wave cavity ring down spectroscopy (cw-CRDS) in their AÃ-X̃ electronic transition in the near-infrared region, C 2 H 5 O 2 at 7602.25 cm -1 , and CH 3 O 2 at 7488.13 cm -1 . This detection scheme is not completely selective for both radicals, but it is demonstrated that it has great advantages compared to the widely used, but unselective UV absorption spectroscopy. Peroxy radicals were generated from the reaction of Cl-atoms with the appropriate hydrocarbon (CH 4 and C 2 H 6 ) in the presence of O 2 , whereby Cl-atoms were generated by 351 nm photolysis of Cl 2 . For different reasons detailed in the manuscript, all experiments were carried out under excess of C 2 H 5 O 2 over CH 3 O 2 . The experimental results were best reproduced by an appropriate chemical model with a rate constant for the cross-reaction of k = (3.8 ± 1.0) × 10 -13 cm 3 s -1 and a yield for the radical channel, leading to CH 3 O and C 2 H 5 O, of ( ϕ 1a = 0.40 ± 0.20).