Department of Physics, AlbaNova University Center, Stockholm University, S-106 91, Stockholm, Sweden.
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California, CA, 94025, USA.
Nat Commun. 2018 May 15;9(1):1917. doi: 10.1038/s41467-018-04330-5.
The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.
液态水的动力学特征具有多种时间尺度，范围从极快的弹道热运动，到较慢的分子扩散和氢键重排。在这里，我们利用相干 X 射线脉冲研究了从环境条件到过冷温度的水的亚 100fs 平衡动力学。这种新方法利用 X 射线散斑可见度光谱固有的能力，通过测量动量空间中的氧运动，以长度选择性测量平衡的分子间动力学。在第一个衍射峰处观察到的散斑对比度的衰减，反映了四面体配位，归因于在最初的 120fs 内分子尺度上的运动。通过与分子动力学模拟的比较，我们得出结论，从 328K 冷却到 253K 时的减速不是由于简单的热弹道运动，而是由于氢键的作用，即使在超过 25fs 的时间尺度上，笼效应也起着重要作用。
