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

Abstract

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.