Kinetic Study of the Reactions of Ground State Atomic Carbon and Oxygen with Nitrogen Dioxide over the 50-296 K Temperature Range
Abstract
The kinetics of the reactions of nitrogen dioxide, NO2, with atomic oxygen and atomic carbon in their ground triplet states (3P) have been studied at room temperature and below using a supersonic flow (Laval nozzle) reactor. O(3P) and C(3P) atoms (hereafter O and C respectively) were created in-situ by the pulsed laser photolysis of the precursor molecules NO2 at 355 nm and CBr4 at 266 nm respectively. While the progress of the O + NO2 reaction was followed by detecting O atoms by a chemiluminescent tracer method, progress of the C + NO2 reaction was followed by detecting C atoms directly by vacuum ultra violet laser induced fluorescence at 116 nm. The measured rate constants for the O + NO2 reaction are found to be in excellent agreement with earlier work at higher temperatures and extend the available kinetic data for this process down to 50 K. The present work represents the first kinetics study of the C + NO2 reaction. Although both reactions display rate constants that increase as the temperature falls, a more substantial rate increase is observed for the O + NO2 reaction. The effects of these reactions on the simulated abundances of interstellar NO2 and related compounds were tested using a gas-grain model of the dense interstellar medium, employing expressions for the rate constants of the form, k(T) = α(T/300)β, with α = 1 × 10-11 cm3 s-1 and β = -0.65 for the O + NO2 reaction and α = 2 × 10-10 cm3 s-1 and β = -0.11 for the C + NO2 reaction. Although these simulations predict that gas-phase NO2 abundances are low in dense interstellar clouds, NO2 abundances on interstellar dust grains are predicted to reach reasonably high levels, indicating the potential for detection of this species in warmer regions.
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