标题： 电荷界面处各向异性水结构的深度分辨振动SFG/DFG光谱学 显示英文标题

标题： Depth-Resolved Vibrational SFG/DFG Spectroscopy of the Anisotropic Water Structure at Charged Interfaces

摘要： 水在带电水界面的分子结构受到界面电场的影响，这些电场可以在纳米尺度距离上诱导分子取向的显著各向异性。 尽管具有重要意义，关于这种深度依赖的各向异性水结构的细节知之甚少，主要是由于缺乏适当的实验技术。 在此，我们介绍了一种时域光谱仪，专门用于在水界面获取深度分辨的非线性振动光谱。 该方法基于相位分辨的振动和频与差频光谱的结合，能够提取纳米尺度的深度信息。 同时，对获得的共振光谱线形的分析提供了对界面区域局部氢键结构和各向异性分子取向的见解。 在使用不溶性带电表面活性剂的测量中，我们证明所获得的数据可以作为深度的函数完整地重建非线性振动响应。 结果表明，在界面各向异性中存在两个明显的区域，其分子取向的优先程度有很大差异。 对深度依赖的振动响应的光谱分析进一步揭示了体相水中自然的局部氢键结构在整个界面区域保持完整，包括靠近表面电荷的水。 这些发现显著提高了我们对带电表面活性剂界面各向异性水结构的理解，并展示了我们深度分辨光谱技术的巨大潜力。 显示英文摘要

摘要： The molecular structure of water at charged aqueous interfaces is shaped by interfacial electric fields, which can induce significant anisotropy in the molecular orientations that extends over nanometer-scale distances. Despite great relevance, very little is known about the details of this depth-dependent anisotropic water structure, mainly due to the lack of appropriate experimental techniques. Here, we present a time-domain spectrometer specifically developed to acquire depth-resolved nonlinear vibrational spectra at aqueous interfaces. The approach is based on combining phase-resolved vibrational sum- and difference-frequency spectra and enables the extraction of depth information on the nanometer scale. The analysis of the acquired resonant spectral line shapes meanwhile yields insight into local hydrogen-bond structures and anisotropic molecular orientations across the interfacial region. In measurements with insoluble charged surfactants, we demonstrate that the obtained data allows for a full reconstruction of the nonlinear vibrational responses as function of depth. The results show the presence of two pronounced regions within the interfacial anisotropy with largely deviating degrees of preferential molecular orientations. A spectral analysis of the depth-dependent vibrational responses furthermore reveals that the natural local hydrogen-bond structure of bulk water remains intact throughout the interfacial region, including water in the direct proximity of the surface charges. These findings significantly refine our understanding of the anisotropic water structure at the interface to charged surfactants and showcase the large potential of our depth-resolved spectroscopic technique.