Measurements of the UV absorption cross-sections for HO2 and CH3O2 in the gas phase

Relative values of the gas phase absorption cross-sections σ for hydroperoxy (HO2) and methylperoxy (CH3 O2) radicals were determined at 298 K over the wavelength λ ranges 210 – 250 nm and 210 – 280 nm respectively. These determinations were based on measurements of the initial optical density (OD)0 of each radical vs. wavelength immediately following its flash photolytic generation. Chlorine atoms produced by the photolysis (λ ⩾ 300 nm) of Cl2 were stoichiometrically converted into the desired peroxy radical via the reaction sequences or Cross-section ratios for the two radicals, determined in separate experiments at several wavelengths, agreed well with those calculated using data from the complete (OD)0 vs. λ sets and were used to establish the relative intensities of the absorption spectral curves for the two radicals. These spectral data were converted to absolute cross-sections using measurements of the fractional photodissociation of Cl2 per flash in conjunction with measurements of the initial methylperoxy radical OD as a function of Cl2 concentration. The results are compared with existing data in the literature for both relative and absolute absorption cross-sections and the effect of these new values on the existing kinetics data base for both radicals in discussed.