Experimental determination of the rate constants of the reactions of HO2 + DO2 and DO2 + DO2

The rate constants of the reactions of DO₂ + HO₂ (R1) and DO₂ + DO₂ (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO₂ and DO₂ by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO₂ was generated by photolyzing Cl₂ in the presence of CH₃OH and O₂. Low concentrations of DO₂ were generated simultaneously by adding low concentrations of D₂O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH₃OD and thus formation of DO₂. Excess DO₂ was generated by photolyzing Cl₂ in the presence of CD₃OD and O₂, small concentrations of HO₂ were always generated simultaneously by isotopic exchange between CD₃OD and residual H₂O. The rate constant k₁ at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D₂O concentration k₁ = (1.67 ± 0.03) × 10^?12 × (1 + (8.2 ± 1.6) × 10^?18 cm³ × D₂O] cm^?3) cm³ s^?1. The rate constant for the DO₂ self-reaction k₂ has been measured under excess DO₂ concentration, and the DO₂ concentration has been determined by fitting the HO₂ decays, now governed by their reaction with DO₂, to the rate constant k₁. A rate constant with insignificant pressure dependence was found: k₂ = (4.1 ± 0.6) × 10^?13 (1 + (2 ± 2) × 10^?20 cm³ × He] cm^?3) cm³ s^?1 as well as an increase of k₂ with increasing D₂O concentration was observed: k₂ = (4.14 ± 0.02) × 10^?13 × (1 + (6.5 ± 1.3) × 10^?18 cm³ × D₂O] cm^?3) cm³ s^?1. The result for k₂ is in excellent agreement with literature values, whereas this is the first determination of k₁.