Two Dimensional TeraHertz Spectroscopy of Molecular Liquids

在这个由化学部化学结构动力学和机理 A (CSDM-A) 和化学测量与成像 (CMI) 项目资助的项目中，加州理工学院的杰弗里·布莱克 (Geoffrey Blake) 和汤姆·米勒 (Tom Miller) 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分子液体和混合物（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）。 这些太赫兹激光器还设计用于产生极短的光脉冲（几飞秒，其中飞秒是一秒的万亿分之一）。 然后，飞秒太赫兹激光脉冲用于探测液体分子与溶解在液体中的其他反应分子相互作用时的运动。太赫兹脉冲还可以控制液体中分子的方向，并检查两个反应分子之间或分子与液体之间的能量如何流动。布莱克和米勒教授将实验数据与液体和分子簇的计算机模拟相结合，建立更好的模型来理解液体中的化学反应。这项工作的更广泛影响包括通过加深对液体溶剂化分子和影响化学反应性的了解而带来的潜在社会效益，以及为研究生和本科生设计和建造先进实验仪器和复杂计算机模拟提供培训的机会。 布莱克和米勒研究小组还与当地的 K-12 社区进行接触，例如通过加州理工学院预科科学计划 (CAPSI) 计划（布莱克教授）和米勒教授的团队研究产生综合教育工具 (TRIDENT) 计划。该项目重点研究不同温度下纯液体和混合物的非线性太赫兹光谱。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) 红外光谱和核磁共振 (NMR) 光谱揭示了液体行为的许多细节。将此类工具扩展到太赫兹范围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太赫兹脉冲来实现这一目标，这两个脉冲到达样品时可以在受控延迟范围内进行扫描，随后由第三个 800 nm 脉冲探测液体的各向异性和拉曼光谱。 通过绘制三个脉冲之间相对延迟的液体响应，可以获得 2D-THz-THz-拉曼 (2D-TTR) 光谱。该方法本质上抑制了系统的线性响应，因此对分子间液体势能面、电偶极矩和拉曼极化率面的非谐性敏感。 为了确定观察到的响应的起源，通过分子簇和液体模型的理论计算来解释实验观察结果。集群工作涉及高精度电子结构计算，而液体模拟则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.