Experimental and Theoretical Investigation of the Reaction of NH2 with NO at Very Low Temperatures

The first experimental study of the low-temperature kineticsofthe gas-phase reaction between NH2 and NO has been performed.A pulsed laser photolysis-laser-induced fluorescence technique wasused to create and monitor the temporal decay of NH2 inthe presence of NO. Measurements were carried out over the temperaturerange of 24-106 K, with the low temperatures achieved usinga pulsed Laval nozzle expansion. The negative temperature dependenceof the reaction rate coefficient observed at higher temperatures inthe literature continues at these lower temperatures, with the ratecoefficient reaching 3.5 x 10(-10) cm(3) molecule(-1) s(-1) at T = 26 K. Ab initio calculations of the potential energy surface werecombined with rate theory calculations using the MESMER software packagein order to calculate and predict rate coefficients and branchingratios over a wide range of temperatures, which are largely consistentwith experimentally determined literature values. These theoreticalcalculations indicate that at the low temperatures investigated forthis reaction, only one product channel producing N-2 +H2O is important. The rate coefficients determined in thisstudy were used in a gas-phase astrochemical model. Models were runover a range of physical conditions appropriate for cold to warm molecularclouds (10 to 30 K; 10(4) to 10(6) cm(-3)), resulting in only minor changes (<1%) to the abundances ofNH(2) and NO at steady state. Hence, despite the observedincrease in the rate at low temperatures, this mechanism is not adominant loss mechanism for either NH2 or NO under darkcloud conditions.