Douglas Kevin M, Lucas Daniel, Walsh Catherine, Blitz Mark A, Heard Dwayne E
School of Chemistry, University of Leeds, Leeds LS2 9JT, U.K.
School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, U.K.
J Phys Chem A. 2023 Aug 31;127(34):7205-7215. doi: 10.1021/acs.jpca.3c03652. Epub 2023 Aug 17.
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.
已对NH与NO之间气相反应的低温动力学进行了首次实验研究。采用脉冲激光光解 - 激光诱导荧光技术在有NO存在的情况下生成并监测NH的时间衰减。测量在24 - 106 K的温度范围内进行，通过脉冲拉瓦尔喷嘴膨胀实现低温。文献中在较高温度下观察到的反应速率系数的负温度依赖性在这些较低温度下继续存在，在26 K时速率系数达到3.5×10 cm³分子⁻¹ s⁻¹。将势能面的从头算计算与使用MESMER软件包的速率理论计算相结合，以便在很宽的温度范围内计算和预测速率系数及分支比，这些结果在很大程度上与实验测定的文献值一致。这些理论计算表明，对于该反应所研究的低温下，只有一个生成N + HO的产物通道是重要的。本研究中确定的速率系数被用于一个气相天体化学模型。模型在一系列适用于冷到暖分子云的物理条件下运行（10到30 K；10⁴到10⁶ cm⁻³），结果在稳态下NH和NO的丰度仅有微小变化（<1%）。因此，尽管在低温下观察到反应速率增加，但在暗云条件下该反应机制对于NH或NO都不是主要的损失机制。
