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 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 O2 and 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 21 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 of C2H5O2,7596 cm-1 = (1.0±0.2) × 10-20 cm2. The rate constant for the cross reaction between of C2H5O2 and HO2 has been measured to be (6.5±1.6) × 10-12 cm3 molecule-1 s-1.