冰混合物中分离动力学的定量
Quantification of segregation dynamics in ice mixtures
摘要：
在原恒星包层中观察到的纯CO2冰的存在归因于热致冰分离，但缺乏定量的实验数据阻止了它作为温度探测器的使用。还需要定量分离研究来描述冰中扩散的特征，这是所有冰动力学和冰化学的基础。本研究的目的是量化的隔离机制和障碍，在不同的H2O：CO2和H2O：CO冰的混合物，涵盖了一系列的天体物理相关的冰厚度和混合比例。冰在（超）高真空条件下在16 - 50 K下沉积。然后通过红外光谱法在23 - 70 K下监测作为时间的函数的偏析。薄（8 - 37 ML）H2O：CO2/CO冰混合物分离顺序通过表面过程，其次是一个数量级较慢的散装扩散。较厚的冰（> 100 ML）通过快速的整体过程分离。因此，厚冰必须是更加多孔的或者通过不同的机制（例如相变）分离。在表面跳跃和电子对交换的蒙特卡罗模拟中定性地再现了薄冰的分凝动力学。实验确定的所有混合比例的表面偏析率遵循Ahrrenius定律，H2O：CO2的势垒为1080 [190] K，H2O：CO混合物的势垒为300 [100] K。在低质量恒星形成过程中，H2O：CO2的分离在30 K时就已经很重要了。当纯冰中混合物组分的平均粘结强度超过冰混合物中的平均粘结强度时，表面和体积偏析被认为是冰混合物的一般特征。
英文摘要：
(Abridged) The observed presence of pure CO2 ice in protostellar envelopes is attributed to thermally induced ice segregation, but a lack of quantitative experimental data has prevented its use as a temperature probe. Quantitative segregation studies are also needed to characterize diffusion in ices, which underpins all ice dynamics and ice chemistry. This study aims to quantify the segregation mechanism and barriers in different H2O:CO2 and H2O:CO ice mixtures covering a range of astrophysically relevant ice thicknesses and mixture ratios. The ices are deposited at 16-50 K under (ultra-)high vacuum conditions. Segregation is then monitored at 23-70 K as a function of time, through infrared spectroscopy. Thin (8-37 ML) H2O:CO2/CO ice mixtures segregate sequentially through surface processes, followed by an order of magnitude slower bulk diffusion. Thicker ices (>100 ML) segregate through a fast bulk process. The thick ices must therefore be either more porous or segregate through a different mechanism, e.g. a phase transition. The segregation dynamics of thin ices are reproduced qualitatively in Monte Carlo simulations of surface hopping and pair swapping. The experimentally determined surface-segregation rates for all mixture ratios follow the Ahrrenius law with a barrier of 1080[190] K for H2O:CO2 and 300[100] K for H2O:CO mixtures. During low-mass star formation H2O:CO2 segregation will be important already at 30[5] K. Both surface and bulk segregation is proposed to be a general feature of ice mixtures when the average bond strengths of the mixture constituents in pure ice exceeds the average bond strength in the ice mixture.
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