Lu Haiping, McCartney Stephanie A, Sadtchenko Vlad
Department of Chemistry, The George Washington University, Washington, DC 20052, USA.
J Chem Phys. 2007 Nov 14;127(18):184701. doi: 10.1063/1.2786101.
Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.
我们使用快速热脱附光谱法（这是我们实验室开发的一项新技术），研究了在温度为-2.0±1.5摄氏度时，厚度为3微米的含有厚度在10至300纳米范围内的D2O层的多晶H2O冰膜中HD同位素交换的动力学。根据我们在典型快速热脱附实验（3 - 4毫秒）期间的结果，同位素交换局限于位于多晶H2O和D2O冰边界处的一个宽度为50±10纳米的反应区。将这些数据与多晶介质中扩散的理论分析相结合，我们确定了水自扩散系数的可能值范围以及我们冰样品特有的晶界宽度。我们的分析表明，对于几纳米量级的晶界宽度，D2O沿晶界的扩散率在相同温度下必须比其在体相水中的扩散率至少低两个数量级。基于这些结果，我们认为，在低杂质浓度的极限情况下，多晶冰在温度高达-2摄氏度时不会发生晶界预熔化。
