Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
International Center for Quantum Materials, School of Physics, Peking University, 5 Yiheyuan Road, Haidian, Beijing 100871, China.
Phys Rev Lett. 2018 Dec 14;121(24):246101. doi: 10.1103/PhysRevLett.121.246101.
The orientational distribution of free O-H (O-D) groups at the H_{2}O- (D_{2}O-)air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ∼63°, substantially larger than previous estimates of 30°-40°. This discrepancy can be traced to erroneously assumed Gaussian or stepwise orientational distributions of free O-H groups. Instead, the MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface.
利用分子动力学（MD）模拟和和频发生（SFG）实验研究了 H_{2}O-（D_{2}O-）空气界面上游离 O-H（O-D）基团的取向分布。发现游离 O-H 基团相对于表面法线的平均角度约为 63°，远大于之前估计的 30°-40°。这种差异可以追溯到对游离 O-H 基团的高斯或逐步取向分布的错误假设。相反，MD 模拟和 SFG 测量揭示了一个广泛且呈指数衰减的取向分布。广泛的取向分布表明存在指向体相的游离 O-H 基团。我们将这种游离 O-H 基团的起源归因于水面上的毛细波的存在。
