Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, D-37077 Göttingen, Germany.
Science. 2012 Sep 21;337(6101):1529-32. doi: 10.1126/science.1225468.
The number of water molecules needed to form the smallest ice crystals has proven challenging to pinpoint experimentally. This information would help to better understand the hydrogen-bonding interactions that account for the macroscopic properties of water. Here, we report infrared (IR) spectra of precisely size-selected (H(2)O)(n) clusters, with n ranging from 85 to 475; sodium doping and associated IR excitation-modulated photoionization spectroscopy allowed the study of this previously intractable size domain. Spectral features indicating the onset of crystallization are first observed for n = 275 ± 25; for n = 475 ± 25, the well-known band of crystalline ice around 3200 cm(-1) dominates the OH-stretching region. The applied method has the potential to push size-resolved IR spectroscopy of neutral clusters more broadly to the 100- to 1000-molecule range, in which many solvents start to manifest condensed phase properties.
形成最小冰晶所需的水分子数量在实验上很难确定。这一信息将有助于更好地理解解释水的宏观性质的氢键相互作用。在这里，我们报告了精确尺寸选择的（H2O）（n）团簇的红外（IR）光谱，其中 n 从 85 到 475；钠离子掺杂和相关的 IR 激发调制光电离光谱允许研究以前难以处理的尺寸域。对于 n = 275 ± 25，首先观察到指示结晶开始的光谱特征；对于 n = 475 ± 25，在 3200 cm-1 左右，众所周知的结晶冰带主导 OH 伸缩区域。所采用的方法有可能将中性团簇的尺寸分辨 IR 光谱更广泛地推向 100 到 1000 个分子的范围，在这个范围内，许多溶剂开始表现出凝聚相的性质。
