Vinh N Q, Sherwin Mark S, Allen S James, George D K, Rahmani A J, Plaxco Kevin W
Institute for Terahertz Science and Technology, University of California, Santa Barbara, Santa Barbara, California 93106, USA.
Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA.
J Chem Phys. 2015 Apr 28;142(16):164502. doi: 10.1063/1.4918708.
Because it is sensitive to fluctuations occurring over femtoseconds to picoseconds, gigahertz-to-terahertz dielectric relaxation spectroscopy can provide a valuable window into water's most rapid intermolecular motions. In response, we have built a vector network analyzer dielectric spectrometer capable of measuring absorbance and index of refraction in this frequency regime with unprecedented precision. Using this to determine the complex dielectric response of water and aqueous salt solutions from 5.9 GHz to 1.12 THz (which we provide in the supplementary material), we have obtained strong new constraints on theories of water's collective dynamics. For example, while the salt-dependencies we observe for water's two slower relaxations (8 and 1 ps) are easily reconciled with suggestions that they arise due to rotations of fully and partially hydrogen bonded molecules, respectively, the salt-dependence of the fastest relaxation (180 fs) appears difficult to reconcile with its prior assignment to liberations of single hydrogen bonds.
由于千兆赫兹至太赫兹介电弛豫光谱对飞秒到皮秒时间尺度上的波动敏感，它能够为研究水最快速的分子间运动提供一个有价值的窗口。为此，我们构建了一台矢量网络分析仪介电谱仪，能够以前所未有的精度测量该频率范围内的吸光度和折射率。利用它来确定水和盐水溶液在5.9吉赫兹至1.12太赫兹范围内的复介电响应（我们在补充材料中提供了相关数据），我们对水的集体动力学理论获得了强有力的新约束。例如，虽然我们观察到水的两个较慢弛豫过程（8皮秒和1皮秒）的盐依赖性很容易与以下观点相契合，即它们分别是由于完全和部分氢键合分子的旋转引起的，但最快弛豫过程（180飞秒）的盐依赖性似乎难以与其先前被归因于单个氢键的释放相契合。
