溶剂涨落和核量子效应对水分子超极化率的调控
Solvent Fluctuations and Nuclear Quantum Effects Modulate the Molecular Hyperpolarizability of Water
摘要：
二次谐波散射（SHS）实验提供了一种独特的方法来探测水介质中的非中心对称环境，从本体溶液到界面，活细胞和组织。在分析SHS实验中的一个中心假设是每个分子根据恒定的分子超极化张量$\boldsymbolβ^{(2)}$散射光。在这里，我们调查的水分子的超极化率对它的环境和内部的几何畸变的依赖性，以检验恒定的$\boldsymbolβ^{(2)}$的假设。我们使用量子化学计算的超极化率的分子嵌入在点电荷环境中获得的模拟散装水。我们证明了溶剂配置的异质性和分子几何形状的量子力学波动引入水的非线性光学响应的大的变化。这一发现有可能改变SHS实验的解释方式：特别是，H $_2$ O和D $_2$ O之间的同位素差异可以解释最近的二次谐波散射观测。最后，我们证明了一个简单机械学习框架可以准确地预测分子超极化率的波动。该模型占溶剂的微观不均匀性，并代表SHS实验定量建模的第一步。
英文摘要：
Second-Harmonic Scatteringh (SHS) experiments provide a unique approach to probe non-centrosymmetric environments in aqueous media, from bulk solutions to interfaces, living cells and tissue. A central assumption made in analyzing SHS experiments is that the each molecule scatters light according to a constant molecular hyperpolarizability tensor $\boldsymbolβ^{(2)}$. Here, we investigate the dependence of the molecular hyperpolarizability of water on its environment and internal geometric distortions, in order to test the hypothesis of constant $\boldsymbolβ^{(2)}$. We use quantum chemistry calculations of the hyperpolarizability of a molecule embedded in point-charge environments obtained from simulations of bulk water. We demonstrate that both the heterogeneity of the solvent configurations and the quantum mechanical fluctuations of the molecular geometry introduce large variations in the non-linear optical response of water. This finding has the potential to change the way SHS experiments are interpreted: in particular, isotopic differences between H$_2$O and D$_2$O could explain recent second-harmonic scattering observations. Finally, we show that a simple machine-learning framework can predict accurately the fluctuations of the molecular hyperpolarizability. This model accounts for the microscopic inhomogeneity of the solvent and represents a first step towards quantitative modelling of SHS experiments.
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