Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706, USA.
Proc Natl Acad Sci U S A. 2011 Dec 27;108(52):20902-7. doi: 10.1073/pnas.1115055108. Epub 2011 Dec 5.
In the last ten years, two-dimensional infrared spectroscopy has become an important technique for studying molecular structures and dynamics. We report the implementation of heterodyne detected two-dimensional sum-frequency generation (HD 2D SFG) spectroscopy, which is the analog of 2D infrared (2D IR) spectroscopy, but is selective to noncentrosymmetric systems such as interfaces. We implement the technique using mid-IR pulse shaping, which enables rapid scanning, phase cycling, and automatic phasing. Absorptive spectra are obtained, that have the highest frequency resolution possible, from which we extract the rephasing and nonrephasing signals that are sometimes preferred. Using this technique, we measure the vibrational mode of CO adsorbed on a polycrystalline Pt surface. The 2D spectrum reveals a significant inhomogenous contribution to the spectral line shape, which is quantified by simulations. This observation indicates that the surface conformation and environment of CO molecules is more complicated than the simple "atop" configuration assumed in previous work. Our method can be straightforwardly incorporated into many existing SFG spectrometers. The technique enables one to quantify inhomogeneity, vibrational couplings, spectral diffusion, chemical exchange, and many other properties analogous to 2D IR spectroscopy, but specifically for interfaces.
在过去的十年中，二维红外光谱已成为研究分子结构和动力学的重要技术。我们报告了异频检测二维和频（HD 2D SFG）光谱的实现，它是二维红外（2D IR）光谱的类似物，但对非中心对称系统（如界面）具有选择性。我们使用中红外脉冲整形来实现该技术，该技术可实现快速扫描、相循环和自动定相。从吸收光谱中提取出可能具有最高频率分辨率的重相位和非重相位信号，这些信号有时是首选的。使用该技术，我们测量了吸附在多晶 Pt 表面上的 CO 的振动模式。2D 光谱揭示了谱线形状的显著非均匀贡献，这可以通过模拟来量化。这一观察表明，CO 分子的表面构象和环境比以前工作中假设的简单“顶位”构型更为复杂。我们的方法可以直接应用于许多现有的 SFG 光谱仪。该技术能够定量分析非均匀性、振动耦合、光谱扩散、化学交换和许多其他类似于 2D IR 光谱的特性，但专门用于界面。
