CH $_4$在非晶态固体水中的扩散
Diffusion of CH$_4$ in amorphous solid water
摘要：
上下文挥发性物质在非晶态固体水冰上的扩散会影响星际介质中尘埃颗粒的化学性质，也会影响富含行星大气或彗星物质中存在的气体的捕获。目标。本工作的目的是提供CH $_4$在非晶态固体水（ASW）上的扩散系数，并了解它们如何受到ASW结构的影响。方法.在不同条件下生长H $_2$ O和CH $_4$的冰混合物，并通过红外光谱或通过低温石英晶体微量天平的质量损失监测CH $_4$的升华。扩散系数从实验数据中获得，假设系统服从菲克扩散定律。蒙特卡罗模拟模拟了不同的非晶态固体水冰结构的研究，并用于再现和解释实验结果。结果甲烷在非晶态固体水中的扩散系数在42 ~ 60 K范围内为10 $^{-12}$ ~ 10 $^{-13}$ cm $^2$ s $^{-1}$。我们表明，扩散可以不同的一个数量级取决于非晶态固体水的形态。水冰内部的多孔性以及由孔隙合并形成的网络增强了孔隙内物种的扩散，实验得出的扩散速率不能用于我们的蒙特卡罗模拟来重现测量结果。结论.我们的结论是，菲克定律可以用来描述在宏观尺度上的扩散，而蒙特卡罗模拟描述的微观尺度上的捕获的物种在冰（及其运动）被认为是。
英文摘要：
Context. The diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material. Aims. The aim of the work is to provide diffusion coefficients of CH$_4$ on amorphous solid water (ASW), and to understand how they are affected by the ASW structure. Methods. Ice mixtures of H$_2$O and CH$_4$ were grown in different conditions and the sublimation of CH$_4$ was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. Diffusion coefficients were obtained from the experimental data assuming the systems obey Fick's law of diffusion. Monte Carlo simulations modeled the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results. Results. Diffusion coefficients of methane on amorphous solid water have been measured to be between 10$^{-12}$ and 10$^{-13}$ cm$^2$ s$^{-1}$ for temperatures ranging between 42 K and 60 K. We showed that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. The porosity within water ice, and the network created by pore coalescence, enhance the diffusion of species within the pores.The diffusion rates derived experimentally cannot be used in our Monte Carlo simulations to reproduce the measurements. Conclusions. We conclude that Fick's law can be used to describe diffusion at the macroscopic scale, while Monte Carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered.
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