Neural-Network Assisted Study of Nitrogen Atom Dynamics on Amorphous Solid Water -- II. Diffusion
Neural-Network Assisted Study of Nitrogen Atom Dynamics on Amorphous Solid Water -- II. Diffusion
复制标题
非晶固体水氮原子动力学的神经网络辅助研究--II．
DOI：
10.1093/mnras/stab3631
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发表时间：
2021
影响因子：
4.6
通讯作者：
J. Kastner
中科院分区：
文献类型：
--
作者：
Viktor Zaverkin;G. Molpeceres;J. Kastner
原子和自由基在星际尘埃颗粒上的扩散是准确预测天文环境中分子丰度的基本成分。扩散率和扩散障碍的定量值通常严重依赖于经验规则。本文结合机器学习原子间势、元动力学和动力学蒙特卡罗模拟计算了吸附氮原子的扩散系数。利用这种方法，我们获得了非晶固体水表面氮原子在10 K时的扩散系数仅为（3.5±1.1）× 10−34 cm2s−1。因此，我们发现氮，作为轻和弱结合吸附剂的典型案例，在10 K时不能在裸露的无定形固体水中扩散。表面覆盖对扩散系数有很强的影响，通过在10 K时调制其值超过9-12个数量级，并使特定条件下的扩散成为可能。此外，我们发现原子隧穿的影响可以忽略不计。势能面平均扩散势垒（2.56 kJ mol−1）与由光面扩散系数得到的有效扩散势垒（6.06 kJ mol−1）差别很大，因此不适合进行扩散建模。我们的研究结果表明，氮在水冰上的热扩散是一个高度依赖于冰的物理条件的过程。
The diffusion of atoms and radicals on interstellar dust grains is a fundamental ingredient for predicting accurate molecular abundances in astronomical environments. Quantitative values of diffusivity and diffusion barriers usually rely heavily on empirical rules. In this paper, we compute the diffusion coefficients of adsorbed nitrogen atoms by combining machine-learned interatomic potentials, metadynamics, and kinetic Monte Carlo simulations. With this approach, we obtain a diffusion coefficient of nitrogen atoms on the surface of amorphous solid water of merely (3.5± 1.1)× 10−34 cm2s−1 at 10 K for a bare ice surface. Thus, we find that nitrogen, as a paradigmatic case for light and weakly bound adsorbates, is unable to diffuse on bare amorphous solid water at 10 K. Surface coverage has a strong effect on the diffusion coefficient by modulating its value over 9–12 orders of magnitude at 10 K and enables diffusion for specific conditions. In addition, we have found that atom tunneling has a negligible effect. Average diffusion barriers of the potential energy surface (2.56 kJ mol−1) differ strongly from the effective diffusion barrier obtained from the diffusion coefficient for a bare surface (6.06 kJ mol−1) and are, thus, inappropriate for diffusion modeling. Our findings suggest that the thermal diffusion of N on water ice is a process that is highly dependent on the physical conditions of the ice.