Experimental and Theoretical Investigation of the Reaction of NH with NO at Very Low Temperatures.

The first experimental study of the low-temperature kinetics of the gas-phase reaction between NH and NO has been performed. A pulsed laser photolysis-laser-induced fluorescence technique was used to create and monitor the temporal decay of NH in the presence of NO. Measurements were carried out over the temperature range of 24-106 K, with the low temperatures achieved using a pulsed Laval nozzle expansion. The negative temperature dependence of the reaction rate coefficient observed at higher temperatures in the literature continues at these lower temperatures, with the rate coefficient reaching 3.5 × 10 cm molecule s at = 26 K. Ab initio calculations of the potential energy surface were combined with rate theory calculations using the MESMER software package in order to calculate and predict rate coefficients and branching ratios over a wide range of temperatures, which are largely consistent with experimentally determined literature values. These theoretical calculations indicate that at the low temperatures investigated for this reaction, only one product channel producing N + HO is important. The rate coefficients determined in this study were used in a gas-phase astrochemical model. Models were run over a range of physical conditions appropriate for cold to warm molecular clouds (10 to 30 K; 10 to 10 cm), resulting in only minor changes (<1%) to the abundances of NH and NO at steady state. Hence, despite the observed increase in the rate at low temperatures, this mechanism is not a dominant loss mechanism for either NH or NO under dark cloud conditions.