On the direct formation of HO2 radicals after 248 nm irradiation of benzene C6H6 in the presence of O2

Abstract

We present in this work the direct observation of HO2 radicals after irradiation of benzene C6H6 at 248 nm in the presence of O2. HO2 radicals have been unambiguously identified using the very selective and sensitive detection of continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO2 radicals were detected in the first vibrational overtone of the OH stretch at 6638.20 cm-1, using a DFB diode laser. This reaction might be important because 248 nm photolysis of H2O2 has often been used in the past for studying the OH•-initiated degradation of C6H6, often using a large excess of C6H6 over H2O2. The possible importance of the title reaction with respect to these former laboratory studies has been quantified through comparison with HO2• signals obtained from 248 nm photolysis of H2O2: one obtains under our conditions (excess O2 and total pressure of 6.6 kPa helium) from the 248 nm irradiation of identical initial concentrations [C6H6]=[H2O2] the following relative initial radical concentrations: [HO2•]=(0.28±0.05)×[OH•]. Experiments with various O2 concentrations have revealed that the origin of the HO2 radicals is not the reaction of H-atoms with O2, but must originate from the reaction of O2 with excited C6H6*. The quantum yield of C6H6* formation has been deduced to ϕ=0.2±0.1.