Two Dimensional TeraHertz Spectroscopy of Molecular Liquids

在这个由化学系的化学结构动力学和机理A（CSDM-A）和化学测量和成像（CMI）计划资助的项目中，加州理工学院的Geoffrey Blake和Tom米勒教授正在使用新的激光技术来研究纯分子液体和混合物的性质，特别是那些含有氢键的液体和混合物（例如水和醇）。具体来说，新的激光工具涉及太赫兹（THz）频率的光（该频率范围位于光谱的红外和微波部分之间）。 这些太赫兹激光器也被设计用于产生极短的光脉冲（几飞秒，其中飞秒是一秒的千万亿分之一）。 然后，飞秒太赫兹激光脉冲被用来探测液体分子的运动，因为它们与溶解在液体中的其他反应分子相互作用。太赫兹脉冲还可以控制液体中分子的方向，并检查能量如何在两个反应分子之间或分子与液体之间流动。布莱克和米勒联合收割机教授将实验数据与液体和分子簇的计算机模拟相结合，以建立更好的模型来理解液体中的化学反应。这项工作的更广泛的影响包括潜在的社会效益，从增加了解如何液体溶剂化分子和影响化学反应性，以及培训研究生和本科生的机会，在设计和建造先进的实验仪器和复杂的计算机模拟。 Blake和米勒的研究小组也参与当地的K-12社区，例如通过加州理工学院大学预科科学计划（CAPSI）计划（Blake教授）和米勒教授的团队研究产生综合教育工具（TRIDENT）计划。该项目侧重于各种温度下纯液体和混合物的非线性THz光谱。液体结构和动力学的详细分子图像推动了我们对化学和生物学的理解。非线性二维（2D）红外和核磁共振（NMR）光谱已经揭示了液体行为的许多细节。将这种工具扩展到太赫兹区域将直接探测分子间力支持的液体动力学。该研究项目通过结合两个强烈的THz脉冲来实现这一点，这些脉冲到达样品时可以在受控的延迟内扫描，随后的液体各向异性和拉曼光谱由800 nm的第三个脉冲探测。 通过绘制液体响应在三个脉冲之间的相对延迟上的图，获得2D-THz-THz-拉曼（2D-TTR）光谱。这种方法固有地抑制了系统的线性响应，因此对分子间液体势能面和电偶极矩以及拉曼极化率面中的非谐性敏感。 为了确定所观察到的响应的起源，实验观察与分子团簇和液体模型的理论计算解释。簇的工作涉及高精度的电子结构计算，而液体模拟是基于分子动力学方法。

In this project funded by the Chemical Structure Dynamics and Mechanism A (CSDM-A) and Chemical Measurement and Imaging (CMI) Programs of the Chemistry Division, Professors Geoffrey Blake and Tom Miller of the California Institute of Technology are using new laser techniques to study the properties of pure molecular liquids and mixtures, especially those containing hydrogen bonds (for example water and alcohols). Specifically, the new laser tools involve light at terahertz (THz) frequencies (this frequency range lies between the infrared and microwave parts of the light spectrum). These THz lasers are also designed to produce extremely short pulses of light (a few femtoseconds, where a femtosecond is one quadrillionth of one second). The femtosecond THz laser pulses are then used to probe the motions of the liquid molecules as they interact with other reactive molecules dissolved in the liquid. The THz pulses also make it possible to control the orientation of the molecules in the liquid, and to examine how energy flows between two reacting molecules or between the molecules and the liquid. Professors Blake and Miller combine the experimental data with computer simulations of liquids and clusters of molecules to build better models for understanding chemical reactions in liquids. The broader impacts of this work include potential societal benefits from an increased understanding of how liquids solvate molecules and influence chemical reactivity, as well as opportunities for the training of graduate students and undergraduates in the design and construction of advanced experimental instrumentation and complex computer simulation. The Blake and Miller research groups also engage the local K-12 community, for example via the Caltech Pre-College Science Initiative (CAPSI) program (Prof. Blake) and Prof. Miller's Team Research yielding Integrated Educational Tools (TRIDENT) program.The project focuses on the nonlinear THz spectroscopy of neat liquids and mixtures at various temperatures. Detailed molecular pictures of the structure and dynamics of liquids drive our understanding of chemistry and biology. Nonlinear two dimensional (2D)-infrared and nuclear magnetic resonance (NMR) spectroscopies have revealed many specifics of liquid behavior. Extending such tools into the THz regime would directly probe liquid dynamics supported by intermolecular forces. This research project does so by combining two intense THz pulses whose arrival at the sample can be swept over controlled delays, with the subsequent anisotropy and Raman spectrum of the liquid probed by a third pulse at 800 nm. By mapping out the liquid response over the relative delays between the three pulses, a 2D-THz-THz-Raman (2D-TTR) spectrum is obtained. This method inherently suppresses the linear response of the system, and so is sensitive to anharmonicities in the intermolecular liquid potential energy surface and the electric dipole moment and Raman polarizability surfaces. To determine the origins of the observed response, the experimental observations are interpreted with theoretical calculations both of molecular clusters and liquid models. The cluster work involves high accuracy electronic structure calculations, while the liquid simulations are based on molecular dynamics approaches.