Fellows Alexander P, Balos Vasileios, John Ben, Díaz Duque Álvaro, Wolf Martin, Thämer Martin
Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany.
Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), 28049 Madrid, Spain.
J Chem Phys. 2023 Oct 28;159(16). doi: 10.1063/5.0169534.
Second-order nonlinear spectroscopy is becoming an increasingly important technique in the study of interfacial systems owing to its marked ability to study molecular structures and interactions. The properties of such a system under investigation are contained within their intrinsic second-order susceptibilities which are mapped onto the measured nonlinear signals (e.g. sum-frequency generation) through the applied experimental settings. Despite this yielding a plethora of information, many crucial aspects of molecular systems typically remain elusive, for example the depth distributions, molecular orientation and local dielectric properties of its constituent chromophores. Here, it is shown that this information is contained within the phase of the measured signal and, critically, can be extracted through measurement of multiple nonlinear pathways (both the sum-frequency and difference-frequency output signals). Furthermore, it is shown that this novel information can directly be correlated to the characteristic vibrational spectra, enabling a new type of advanced sample characterization and a profound analysis of interfacial molecular structures. The theory underlying the different contributions to the measured phase of distinct nonlinear pathways is derived, after which the presented phase disentanglement methodology is experimentally demonstrated for model systems of self-assembled monolayers on several metallic substrates. The obtained phases of the local fields are compared to the corresponding phases of the nonlinear Fresnel factors calculated through the commonly used theoretical model, the three-layer model. It is found that, despite its rather crude assumptions, the model yields remarkable similarity to the experimentally obtained values, thus providing validation of the model for many sample classes.
由于二阶非线性光谱在研究分子结构和相互作用方面具有显著能力，它在界面系统研究中正成为一种越来越重要的技术。所研究的此类系统的特性包含在其固有二阶极化率中，这些二阶极化率通过所应用的实验设置映射到测量的非线性信号（例如和频产生）上。尽管这能产生大量信息，但分子系统的许多关键方面通常仍然难以捉摸，例如其组成发色团的深度分布、分子取向和局部介电性质。在此表明，这些信息包含在测量信号的相位中，并且至关重要的是，可以通过测量多个非线性路径（和频与差频输出信号）来提取。此外，表明这种新信息可以直接与特征振动光谱相关联，从而实现新型的高级样品表征和对界面分子结构的深入分析。推导了不同非线性路径对测量相位的不同贡献的基础理论，之后针对几种金属基底上的自组装单分子层模型系统通过实验证明了所提出的相位解缠方法。将获得的局部场相位与通过常用理论模型三层模型计算的非线性菲涅耳因子的相应相位进行比较。发现尽管该模型有相当粗略的假设，但它与实验获得的值具有显著的相似性，从而为许多样品类别提供了模型验证。
