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 ultraviolet 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.