Sequential Two (Blue) Photon Absorption by NO₂ in the Presence of H₂ as a Source of OH in Pulsed Photolysis Kinetic Studies: Rate Constants for Reaction of OH with CH₃NH₂, (CH₃)₂NH, (CH₃)₃N, and C₂H₅NH₂ at 295 K

Abstract

The use of the sequential two-photon dissociation of NO2 (420−450 nm) in the presence of H2 as a novel source of OH in kinetic studies has been assessed. Two-photon absorption by NO2 at these wavelengths leads to both O(1D) and O(3P) production ((40% and 60%, respectively); the O(1D) is rapidly converted to OH in the presence of ≈1 × 1016 cm-3 H2 [O(1D) + H2 → OH + H]. The H atom reacts with NO2 to generate a further OH radical (H + NO2 → OH + NO). Laser fluences of ≈20−40 mJ cm-2 were used to generate between 2 × 1011 and 1 × 1012 cm-3 OH from an initial NO2 concentration of (5−13) × 1014 cm-3. OH [and O(3P)] were detected by resonance fluorescence. Rate constants for the reactions of some aliphatic amines with the OH radical have been obtained at 295 K. The results obtained (in cm3 s-1) are k(OH + CH3NH2) = (1.73 ± 0.11) × 10-11, k(OH + (CH3)2NH) = (6.49 ± 0.64) × 10-11, k[OH + (CH3)3N] = (3.58 ± 0.22) × 10-11, and k(OH + C2H5NH2) = (2.3 ) × 10-11. Data for (CH3)2NH were also obtained using the 193 nm photolysis of N2O/CH4 as the O(1D) source; the rate constant above is the average value obtained using both methods. The error limits include both statistical (2σ) and systematic errors.