Precision measurements of absolute line strengths of the HO2 radical in the ν1 and ν2 vibrational bands

Abstract

The hydroperoxy radical (HO₂), a key species in atmospheric chemistry, necessitates accurate quantitative measurements and precise spectroscopic characterization, both of which are essential for laboratory investigations and field observations. Herein, we present precision measurements of the absolute line strengths of the HO₂ radical in the ν₁ and ν₂ vibrational bands by utilizing synchronized two-color time-resolved dual-comb spectroscopy in the mid-infrared region. By simultaneously measuring the byproduct HCl near 3.3 μm and the HO₂ radical near 7.1 μm following flash photolysis of Cl₂/CH₃OH/O₂ gas mixtures, we were able to determine the absolute line intensities for several HO₂ ν₂ transitions, using the well-known line strength of HCl as a reference. Furthermore, the line intensities for the HO₂ ν₁ transitions were obtained relative to the accurately characterized line strengths of the HO₂ ν₃ or ν₂ transitions, by simultaneous probing of the HO₂ radical in the ν₁ band (2.9 μm) and either the ν₃ band (8.9 μm) or the ν₂ band (7.1 μm). Through high-resolution spectral analysis based on the vibration–rotation parameters of HO₂, the absolute intensities of the ν₁ and ν₂ vibrational bands of HO₂ were determined to be 18.5 ± 1.5 and 26.6 ± 1.3 km mol⁻¹, respectively. Additionally, the HO₂ fundamental band strengths were calculated using different levels of theory, and compared with the experimental results. This work presents precise experimental data together with theoretical comparisons of the absolute line strengths of HO₂, contributing to the revision of the spectral database for the HO₂ radical and supporting the validation of computational methods for predicting infrared intensities of radical vibrational bands.