Kinetics of the reaction HO 2 + NO 2 (+M) = HO 2 NO 2 using molecular modulation spectrometry

Rate constants for the reaction HO 2 + NO 2 (+ M) = HO 2 NO 2 (+ M) have been obtained from direct observations of the HO 2 radical using the technique of molecular modulation ultraviolet spectrometry. HO 2 was generated by periodic photolysis of Cl 2 in the presence of excess H 2 and O 2 , and k 1 was determined from the measured concentrations and lifetime of HO 2 with NO 2 present. k 1 increased with pressure in the range of 40–600 Torr, and a simple energy transfer model gave the following limiting second‐ and third‐order rate constants at 283 K: k 1 ∞ = 1.5 ± 0.5 × 10 −12 cm 3 /molec·sec and k 1 III = 2.5 ± 0.5 × 10 −31 cm 6 /molec·sec. The ultraviolet absorption spectrum of peroxynitric acid was also recorded in the range of 195–265 nm; it showed a broad feature with a maximum at 200 nm, σ max = 4.4 × 10 −18 cm 2 .