Le Breton G, Loison C, Vynck K, Benichou E, Bonhomme O
Institut Lumière Matière, UMR5306-UCBL-CNRS, 10 rue Ada Byron, 69622 Villeurbanne CEDEX, France.
J Chem Phys. 2024 Oct 21;161(15). doi: 10.1063/5.0231240.
Second harmonic generation (SHG) is a nonlinear optical phenomenon where two photons at the frequency ω combine to form a single photon at the second-harmonic frequency 2ω. Since that second-order process is very weak in bulk isotropic media, optical SHG responses of interfaces provide a powerful and versatile technique to probe the molecular structure and dynamics of liquid interfaces. Both local dipole contributions and non-local quadrupole contributions can be interesting to investigate different properties of the interface, such as the molecular orientation or the charge density. However, a major difficulty is to comprehend the link between the S-SHG intensity and molecular details. This article reports a numerical approach to model the polarization-resolved SHG intensities of a model vapor/liquid interface of pure water. The influence of the interfacial local environment on the hyperpolarizability is taken into account using quantum mechanical/molecular mechanics calculations. The numerical predictions are in very good agreement with experiments. We detail the hypotheses made during the modeling steps and discuss the impact of various factors on the modeled SHG intensities, including the description of the exciting field in the interfacial layer, the effect of neighboring molecules on the second-harmonic polarization, and the presence of an additional static electric field at the interface.
二次谐波产生（SHG）是一种非线性光学现象，其中频率为ω的两个光子结合形成频率为二次谐波2ω的单个光子。由于这种二阶过程在体各向同性介质中非常微弱，界面的光学SHG响应提供了一种强大且通用的技术来探测液体界面的分子结构和动力学。局部偶极子贡献和非局部四极子贡献对于研究界面的不同性质（如分子取向或电荷密度）都可能很有趣。然而，一个主要困难是理解S-SHG强度与分子细节之间的联系。本文报道了一种数值方法，用于模拟纯水模型气/液界面的偏振分辨SHG强度。使用量子力学/分子力学计算考虑了界面局部环境对超极化率的影响。数值预测与实验结果非常吻合。我们详细说明了建模步骤中所做的假设，并讨论了各种因素对建模的SHG强度的影响，包括界面层中激发场的描述、相邻分子对二次谐波偏振的影响以及界面处额外静电场的存在。
