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.