A kinetics study of reactions of HO2 and C2H5O2 using diode laser absorption spectroscopy

C2H5O2 radicals and HO2 radicals were produced by modulated photolysis of Cl2–C2H6–CH3OH-O2–N2 or azoethane–O2 mixtures. C2H5O2 was detected in absorption at wavelengths between 205 and 295 nm, but was generally monitored at 260 nm, where the absorption cross-section was determined to be (3.2 ± 0.38)× 10–18 cm2 molecule–1. HO2 was monitored either in the u.v. at 210 nm, or using a vibration–rotation line of the ν3 band in the i.r. near 1117 cm–1, using a tunable diode-laser source. In the absence of C2H5O2, HO2 displayed second-order kinetic behaviour owing to the reaction HO2+ HO2→ H2O2+ O2, for which k1=(1.6 ± 0.1)× 10–12 cm3 molecule–1 s–1 at 2.4 Torr and 295 K. C2H5O2 also exhibited second-order kinetics when in large excess over HO2. The observed rate constant, k3=(1.30 ± 0.16)× 10–13 cm3 molecule–1 s–1, was insensitive to variation of temperature in the range 266–347.5 K, and pressure in the range 27–760 Torr. Observation of the perturbation of the second-order kinetics of HO2 when C2H5O2 was present in large excess, which arises from the mutual reaction of HO2 and C2H5O2, provided a value of k4=(6.3 ± 0.9)× 10–12 cm3 molecule–1 s–1 at 2.4 Torr and 295 K.