University of Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Villeurbanne, France.
Phys Chem Chem Phys. 2021 Nov 10;23(43):24932-24941. doi: 10.1039/d1cp02258j.
Surface Second-Harmonic Generation (S-SHG) experiments provide a unique approach to probe interfaces. One important issue for S-SHG is how to interpret the S-SHG intensities at the molecular level. Established frameworks commonly assume that each molecule emits light according to an average molecular hyperpolarizability tensor (-2,,). However, for water molecules, this first hyperpolarizability is known to be extremely sensitive to their environment. We have investigated the molecular first hyperpolarizability of water molecules within the liquid-vapor interface, using a quantum description with explicit, inhomogeneous electrostatic embedding. The resulting average molecular first hyperpolarizability tensor depends on the distance relative to the interface, and it practically respects the Kleinman symmetry everywhere in the liquid. Within this numerical approach, based on the dipolar approximation, the water layer contributing to the Surface Second Harmonic Generation (S-SHG) intensity is less than a nanometer. The results reported here question standard interpretations based on a single, averaged hyperpolarizability for all molecules at the interface. Not only the molecular first hyperpolarizability tensor significantly depends on the distance relative to the interface, but it is also correlated to the molecular orientation. Such hyperpolarizability fluctuations may impact the S-SHG intensity emitted by an aqueous interface.
表面二次谐波产生 (S-SHG) 实验为探测界面提供了一种独特的方法。对于 S-SHG 来说，一个重要的问题是如何在分子水平上解释 S-SHG 强度。已建立的框架通常假设每个分子根据平均分子超极化率张量（-2，，）发光。然而，对于水分子，已知这种一阶超极化率对其环境极其敏感。我们使用具有显式不均匀静电嵌入的量子描述，研究了液体-蒸气界面内水分子的分子一阶超极化率。得到的平均分子一阶超极化率张量取决于相对于界面的距离，并且在液体中的任何地方实际上都遵守 Kleinman 对称性。在基于偶极子近似的这种数值方法中，对表面二次谐波产生 (S-SHG) 强度有贡献的水层小于一纳米。这里报告的结果质疑了基于界面上所有分子的单个平均超极化率的标准解释。不仅分子一阶超极化率张量明显取决于相对于界面的距离，而且还与分子取向相关。这种超极化率波动可能会影响水相界面发出的 S-SHG 强度。
