标题： 在水冰上星际 NH$_3$形成过程中能量去哪了？ 计算研究 显示英文标题

标题： Where does the energy go during the interstellar NH$_3$ formation on water ice? A computational study

摘要： 在星际介质中最冷（10-20 K）的区域，星际尘埃的冰质表面作为化学反应的固态支持物。 在它们可能的作用中，第三体作用被提出，其中表面反应的反应能量在整个尘埃中耗散，稳定产物。 尽管这种能量耗散过程对天体化学有重要影响，但其在原子尺度上的理解仍很有限。 在这里，我们通过量子力学模拟研究了在水冰表面上通过连续的氢加成形成NH3的过程，并特别关注第三体作用。 我们首先表征了氢化反应和可能的竞争过程（即，氢抽提），其中氢加成比氢抽提更有利。 随后，我们通过从头算分子动力学模拟研究了氢化反应能量的命运。 结果表明，约58-90%的释放能量迅速被冰表面吸收，导致冰温度暂时升高。 区分了不同的能量耗散机制。 一种更普遍的机制是基于新形成物种的高度激发振动模式与冰中水分子的摆动模式之间的耦合。 第二种机制仅在NH$_3$形成期间出现，基于瞬时H$_3$O$^+$/NH$_2^-$离子对的形成，这显著加速了能量向表面的传递。 最后，讨论了我们的发现相对于星际介质中NH$_3$合成及其向气体中的化学脱附的天体物理意义。 显示英文摘要

摘要： In the coldest (10--20 K) regions of the interstellar medium, the icy surfaces of interstellar grains serve as solid-state supports for chemical reactions. Among their plausible roles, that of third body is advocated, in which the reaction energies of surface reactions dissipate throughout the grain, stabilizing the product. This energy dissipation process is poorly understood at the atomic scale, although it can have a high impact on Astrochemistry. Here, we study, by means of quantum mechanical simulations, the formation of NH3 via successive H-additions to atomic N on water ice surfaces, paying special attention to the third body role. We first characterize the hydrogenation reactions and the possible competitive processes (i.e., H-abstractions), in which the H-additions are more favourable than the H-abstractions. Subsequently, we study the fate of the hydrogenation reaction energies by means of ab initio molecular dynamics simulations. Results show that around 58--90\% of the released energy is quickly absorbed by the ice surface, inducing a temporary increase of the ice temperature. Different energy dissipation mechanisms are distinguished. One mechanism, more general, is based on the coupling of the highly excited vibrational modes of the newly formed species and the libration modes of the icy water molecules. A second mechanism, exclusive during the NH$_3$ formation, is based on the formation of a transient H$_3$O$^+$/NH$_2^-$ ion pair, which significantly accelerates the energy transfer to the surface. Finally, the astrophysical implications of our findings relative to the interstellar synthesis of NH$_3$ and its chemical desorption into the gas are discussed.