Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.
J Phys Chem A. 2020 Apr 30;124(17):3471-3483. doi: 10.1021/acs.jpca.9b11791. Epub 2020 Apr 16.
Surfaces and interfaces are ubiquitous in nature. From cell membranes, to photovoltaic thin films, surfaces have important function in both biological and materials systems. Spectroscopic techniques have been developed to probe systems like these, such as sum frequency generation (SFG) spectroscopies. The advantage of SFG spectroscopy, a second-order spectroscopy, is that it can distinguish between signals produced from molecules in the bulk versus on the surface. We propose a polarization scheme for third-order spectroscopy experiments, such as pump-probe and 2D spectroscopy, to select for surface signals and not bulk signals. This proposed polarization condition uses one pulse perpendicular compared to the other three to isolate cross-peaks arising from molecules with polar and uniaxial (i.e., biaxial) order at a surface, while removing the signal from bulk isotropic molecules. In this work, we focus on two of these cases: XXXY and YYYX, which differ by the sign of the cross-peak they create. We compare this technique to SFG spectroscopy and vibrational circular dichroism to provide insight to the behavior of the cross-peak signal. We propose that these singularly cross-polarized schemes provide odd-ordered spectroscopies the typically associated with even-ordered techniques.
表面和界面在自然界中无处不在。从细胞膜到光伏薄膜，表面在生物和材料系统中都具有重要的功能。已经开发出光谱技术来探测这些系统，例如和频产生（SFG）光谱学。SFG 光谱学是一种二阶光谱学，其优势在于它可以区分来自体相和表面的分子产生的信号。我们提出了一种用于三阶光谱学实验（如泵浦探测和 2D 光谱学）的偏振方案，以选择表面信号而不是体相信号。这种建议的偏振条件使用一个相对于其他三个垂直的脉冲来隔离来自表面具有极性和单轴（即双轴）有序的分子的交峰，同时去除来自各向同性体相分子的信号。在这项工作中，我们重点关注其中两种情况：XXXY 和 YYYX，它们通过它们产生的交峰的符号而有所不同。我们将这种技术与 SFG 光谱学和振动圆二色性进行比较，以提供对交峰信号行为的深入了解。我们提出，这些奇异的交叉偏振方案为奇数阶光谱学提供了通常与偶数阶技术相关的特性。
