Nishida Jun, Fayer Michael D
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Chem Phys. 2014 Apr 14;140(14):144702. doi: 10.1063/1.4870436.
Functionalized organic monolayers deposited on planar two-dimensional surfaces are important systems for studying ultrafast orientational motions and structures of interfacial molecules. Several studies have successfully observed the orientational relaxation of functionalized monolayers by fluorescence depolarization experiments and recently by polarization-resolved heterodyne detected vibrational transient grating (HDTG) experiments. In this article we provide a model-independent theory to extract orientational correlation functions unique to interfacial molecules and other uniaxial systems based on polarization-resolved resonant third-order spectroscopies, such as pump-probe spectroscopy, HDTG spectroscopy, and fluorescence depolarization experiment. It will be shown (in the small beam-crossing angle limit) that five measurements are necessary to completely characterize the monolayer's motions: I(∥)(t) and I(⊥)(t) with the incident beams normal to the surface, I(∥)(t) and I(⊥)(t) with a non-zero incident angle, and a time averaged linear dichroism measurement. Once these measurements are performed, two orientational correlation functions corresponding to in-plane and out-of-plane motions are obtained. The procedure is applicable not only for monolayers on flat surfaces, but any samples with uniaxial symmetry such as uniaxial liquid crystals and aligned planar bilayers. The theory is valid regardless of the nature of the actual molecular motions on interface. We then apply the general results to wobbling-in-a-cone model, in which molecular motions are restricted to a limited range of angles. Within the context of the model, the cone angle, the tilt of the cone relative to the surface normal, and the orientational diffusion constant can be determined. The results are extended to describe analysis of experiments where the beams are not crossing in the small angle limit.
沉积在平面二维表面上的功能化有机单分子层是研究界面分子超快取向运动和结构的重要体系。多项研究已通过荧光去极化实验，以及最近通过偏振分辨外差检测振动瞬态光栅（HDTG）实验，成功观测到功能化单分子层的取向弛豫。在本文中，我们基于偏振分辨共振三阶光谱技术，如泵浦 - 探测光谱、HDTG光谱和荧光去极化实验，提供了一种与模型无关的理论，以提取界面分子和其他单轴体系特有的取向相关函数。结果表明（在小光束交叉角极限下），需要进行五次测量才能完全表征单分子层的运动：入射光束垂直于表面时的I(∥)(t)和I(⊥)(t)、非零入射角时的I(∥)(t)和I(⊥)(t)，以及一次时间平均线性二色性测量。一旦进行了这些测量，就可以得到对应面内和面外运动的两个取向相关函数。该方法不仅适用于平面表面上的单分子层，还适用于任何具有单轴对称性的样品，如单轴液晶和取向平面双层膜。无论界面上实际分子运动的性质如何，该理论都是有效的。然后，我们将这些一般结果应用于锥体内摆动模型，其中分子运动被限制在有限的角度范围内。在该模型的框架内，可以确定锥角、锥体相对于表面法线的倾斜度以及取向扩散常数。结果被扩展以描述光束不在小角度极限下交叉的实验分析。
