太赫兹Laser场激发下液态水的强各向异性
Strong Anisotropy in Liquid Water upon Librational Excitation using Terahertz Laser Fields
摘要：
由于旋转激发能通过氢键网络的超快耗散，跟踪均匀液体中水分子的激发是具有挑战性的。在这里，我们展示了强大的瞬态各向异性的液态水通过librational激发使用单色泵浦探测实验在12.3太赫兹。我们推导出chi ^3的三阶响应，比以前报道的光学范围内的值高出三个数量级。使用一个理论，取代非线性响应与材料响应性能服从分子动力学模拟，我们表明，旋转阻尼运动的水分子在振动带共振驱动在这个频率，这可以解释增强的各向异性的液体由外部太赫兹场。通过添加盐（MgSO4），水合水反而由离子的局部电场主导，导致可以被THz脉冲动态扰动的水分子的减少。
英文摘要：
Tracking the excitation of water molecules in the homogeneous liquid is challenging due to the ultrafast dissipation of rotational excitation energy through the hydrogen-bonded network. Here we demonstrate strong transient anisotropy of liquid water through librational excitation using single-color pump-probe experiments at 12.3 THz. We deduce a third order response of chi^3 exceeding previously reported values in the optical range by three orders of magnitude. Using a theory that replaces the nonlinear response with a material response property amenable to molecular dynamics simulation, we show that the rotationally damped motion of water molecules in the librational band is resonantly driven at this frequency, which could explain the enhancement of the anisotropy in the liquid by the external Terahertz field. By addition of salt (MgSO4), the hydration water is instead dominated by the local electric field of the ions, resulting in reduction of water molecules that can be dynamically perturbed by THz pulses.
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