Kinetic Study of the Reactions of Ground State Atomic Carbon and Oxygen with Nitrogen Dioxide over the 50-296 K Temperature Range

TL;DR

This study measures the reaction rates of atomic carbon and oxygen with nitrogen dioxide at very low temperatures, providing new kinetic data and insights into interstellar chemistry.

Contribution

First kinetic measurements of C + NO$_2$ reaction over 50-296 K, extending understanding of these reactions in interstellar environments.

Findings

Rate constants increase as temperature decreases.

O + NO$_2$ reaction has a more substantial rate increase at lower temperatures.

Predicted high NO$_2$ levels on interstellar dust grains in warmer regions.

Abstract

The kinetics of the reactions of nitrogen dioxide, NO$_2$, with atomic oxygen and atomic carbon in their ground triplet states ($^3$P) have been studied at room temperature and below using a supersonic flow (Laval nozzle) reactor. O($^3$P) and C($^3$P) atoms (hereafter O and C respectively) were created in-situ by the pulsed laser photolysis of the precursor molecules NO$_2$ at 355 nm and CBr$_4$ at 266 nm respectively. While the progress of the O + NO$_2$ reaction was followed by detecting O atoms by a chemiluminescent tracer method, progress of the C + NO$_2$ 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 + NO$_2$ 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 + NO$_2$ reaction. Although both reactions display rate constants that increase as the temperature falls, a more substantial rate increase is observed for the O + NO$_2$ reaction. The effects of these reactions on the simulated abundances of interstellar NO$_2$ and related compounds were tested using a gas-grain model of the dense interstellar medium, employing expressions for the rate constants of the form, $k(T) = \alpha(T/300)^\beta$, with $\alpha = 1 \times 10^{-11}$ cm$^3$ s$^{-1}$ and $\beta = -0.65$ for the O + NO$_2$ reaction and $\alpha = 2 \times 10^{-10}$ cm$^3$ s$^{-1}$ and $\beta = -0.11$ for the C + NO$_2$ reaction. Although these simulations predict that gas-phase NO$_2$ abundances are low in dense interstellar clouds, NO$_2$ abundances on interstellar dust grains are predicted to reach reasonably high levels, indicating the potential for detection of this species in warmer regions.