The absolute absorption cross-section of the ethyl peroxy radical C2H5O2 in the Ã←X˜ electronic transition with the peak wavelength at 7596 cm−1 has been determined by the method of dual wavelengths time resolved continuous wave cavity ring down spectroscopy. C2H5O2 radicals were generated from pulsed 351 nm photolysis of C2H6/Cl2 mixture in presence of 100 Torr O2 at T = 295 K. C2H5O2 radicals were detected on one of the CRDS paths. Two methods have been applied for the determination of the C2H5O2 absorption cross-section: (i) based on Cl-atoms being converted alternatively to either C2H5O2 by adding C2H6 or to hydro peroxy radicals, HO2, by adding CH3OH to the mixture, whereby HO2 was reliably quantified on the second CRDS path in the 2ν1 vibrational overtone at 6638.2 cm−1 (ii) based on the reaction of C2H5O2 with HO2, measured under either excess HO2 or under excess C2H5O2 concentration. Both methods lead to the same peak absorption cross-section for C2H5O2 at 7596 cm−1 of σ = (1.0 ± 0.2) × 10−20 cm2. The rate constant for the cross reaction between of C2H5O2 and HO2 has been measured to be (6.2 ± 1.5) × 10−12 cm3 molecule−1 s−1.

The self-reaction of the ethyl peroxy radical (C 2 H 5 O 2 ) (R1) has been studied using laser photolysis coupled to a selective time resolved detection of two different radicals by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared range: C 2 H 5 O 2 was detected in the Ã-electronic transition at 7596 cm -1 , and HO 2 was detected in the 2 1 vibrational overtone transition at 6638.21 cm -1 . Radicals were generated from pulsed 351 nm photolysis of C 2 H 6 / Cl 2 mixture in presence of O 2 . The reaction can proceed via a radical maintain channel leading to two C 2 H 5 O radicals (R1a) or to stable products (R1b/1c). Because C 2 H 5 O radicals react subsequently with O 2 leading to HO 2 , which in term reacts rapidly with C 2 H 5 O 2 , knowledge of the branching ratio is indispensable for retrieving the rate constant. A strong disagreement exists about the rate constant and the branching ratio between the IUPAC recommendation (Atkinson et al., ACP, 3525, 2006) and a recent, more direct measurement (Noell et al., JPC A, 6983, 2010). The rate constant for the self-reaction has been found at k 1 = (1.0±0.2) ×10 -13 cm 3 s -1 with the branching fraction of the C 2 H 5 O radical channel being  = (0.31±0.06), being in contradiction with the IUPAC recommendation, but confirms the most recent measurement and indicates that the current recommendation for this reaction should be revised. The reaction of C 2 H 5 O 2 with the ethoxy radical (C 2 H 5 O) can also been involved in the reaction mechanism, and its rate constant is also extracted from modelling. It is found to be k 9 = (7±1.5) ×10 -12 cm 3 s -1 , two times slower than the only available measurement from Noell et al. JPC A, 6983, 2010.

The rate constant for the self-reaction of the acetonyl peroxy radicals, CH3C­(O)­CH2O2, has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CH3C­(O)­CH2O2 radicals have been generated from the reaction of Cl atoms with CH3C­(O)­CH3, and the concentration time profiles of four radicals (HO2, CH3O2, CH3C­(O)­O2, and CH3C­(O)­CH2O2) have been determined by cw-CRDS in the near-infrared. The rate constant for the self-reaction was found to be k = (5.4 ± 1.4) × 10–12 cm3 s–1, in good agreement with a recently published value (Zuraski, K., et al. J. Phys. Chem. A 2020, 124, 8128); however, the branching ratio for the radical path was found to be ϕ1b = (0.6 ± 0.1), which is well above the recently published value (0.33 ± 0.13). The influence of a fast reaction of Cl atoms with the CH3C­(O)­CH2O2 radical became evident under some conditions; therefore, this reaction has been investigated in separate experiments. Through the simultaneous fitting of all four radical profiles to a complex mechanism, a very fast rate constant of k = (1.35 ± 0.8) × 10–10 cm3 s–1 was found, and experimental results could be reproduced only if Cl atoms would partially react through H-atom abstraction to form the Criegee intermediate with a branching fraction of ϕCriegee = (0.55 ± 0.1). Modeling the HO2 concentration–time profiles was possible only if a subsequent reaction of the Criegee intermediate with CH3C­(O)­CH3 was included in the mechanism leading to HO2 formation with a rate constant of k = (4.5 ± 2.0) × 10–14 cm3 s–1.

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The self-reaction of acetylperoxy radicals (CH3C(O)O2•) (R1) as well as their reaction with methyl peroxy radicals (CH3O2•) (R2) have been studied using laser photolysis coupled to a selective time resolved detection of three different radicals by cw-CRDS in the near-infrared range: CH3C(O)O2• was detected in the Ã-X˜ electronic transition at 6497.94 cm−1, HO2• was detected in the 2ν1 vibrational overtone at 6638.2 cm−1, and CH3O2• radicals were detected in the Ã-X˜ electronic transition at 7489.16 cm−1. Pulsed photolysis of different precursors at different wavelengths, always in the presence of O2, was used to generate CH3C(O)O2• and CH3O2• radicals: acetaldehyde (CH3CHO/Cl2 mixture or biacetyle (CH3C(O)C(O)CH3) at 351 nm, and acetone (CH3C(O)CH3) or CH3C(O)C(O)CH3 at 248 nm. From photolysis experiments using CH3C(O)C(O)CH3 or CH3C(O)CH3 as precursor, the rate constant for the self-reaction was found with k1 = (1.3 ± 0.3) × 10−11 cm3s−1, in good agreement with current recommendations, while the rate constant for the cross reaction with CH3O2• was found to be k2 = (2.0 ± 0.4) × 10−11 cm3s−1, which is nearly two times faster than current recommendations. The branching ratio of (R2) towards the radical products was found at 0.67, compared with 0.9 for the currently recommended value. Using the reaction of Cl•-atoms with CH3CHO as precursor resulted in radical profiles that were not reproducible by the model: secondary chemistry possibly involving Cl• or Cl2 might occur, but could not be identified.