Coherent Spectroscopy and Coherent Control of Molecules and Materials

在这个由化学部化学结构动力学和机理A(CSDM-A)计划资助的项目中，麻省理工学院的Keith Nelson教授正在开发使用太赫兹(THz)频率范围内的短脉冲光的新技术。太赫兹的范围略高于手机等常见设备中使用的较低频率的微波(或GHz)范围。磁共振测量(类似于磁共振成像中的测量)可以揭示关于分子结构和成键的关键信息，但由于太赫兹光源的技术挑战，它们几乎从未在太赫兹频率下进行过。纳尔逊教授克服了这些挑战，在一个简单的桌面大小的实验装置中产生了短的太赫兹脉冲，在大约一分钟内进行了太赫兹磁共振测量。正在对分子和生物分子与铁或类似元素的络合物进行测量，以了解分子结构及其在化学过程中如何变化。该项目在为化学、生物和材料科学的科学界提供方便和廉价的太赫兹测量方面产生了更广泛的影响，其应用范围从能量收集到化学催化再到生物医学测量。除了为研究生和博士后科学家提供尖端光谱学技术培训外，Nelson研究小组还开展了一个独特的推广项目(Lambda项目)，在该项目中，高中生来到麻省理工学院进行动手实验，以了解现代光学和现代材料。该项目建立在最近THz电子顺磁共振(EPR)的演示基础上，其中包括测量自由感应衰变(FID)的线性区域和进行二维(2D)电子顺磁共振(EPR)的非线性区域。测量了由自旋-轨道耦合产生的具有零场分裂的高自旋过渡金属络合物的太赫兹EPR谱。裂解对中心过渡金属离子周围的配体及其构型高度敏感。在EPR测量之前，可以用光(“泵浦”)脉冲启动光化学变化，并在皮秒时间尺度上监测自旋状态的相关变化。为了测量过渡金属离子之间的自旋-自旋相互作用，对磁性材料集体自旋态的二维THzEPR测量正在扩展到多核金属有机配合物，揭示了配合物的基态和激发态电子结构的详细信息。除了这些太赫兹磁场相干地驱动电子自旋的测量之外，在寻找新的对映体分离方法的实验中，太赫兹电场和磁场联合分量被用来驱动手性分子的电偶极矩和磁偶极矩。如果能够实现改进的对映体分离，可能会在化学和生物方面产生进一步的影响。

In this project, funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Keith Nelson of the Massachusetts Institute of Technology is developing novel techniques that use short pulses of light in the terahertz (THz) frequency range of the spectrum. The terahertz range is just above the lower-frequency microwave (or GHz) range that is used in common devices like cellphones. Magnetic resonance measurements (similar to those in magnetic resonance imaging, or MRI) can reveal key information about molecular structure and bonding, but they have hardly ever been made at THz frequencies because of technological challenges with the sources of THz light. Professor Nelson overcomes these challenges using short THz pulses generated in a simple, tabletop-sized, experimental setup to make THz magnetic resonance measurements in about one minute. Measurements of molecular and biomolecular complexes with iron or similar elements are being conducted to understand molecular structures and how they change during chemical processes. The project has broader impacts in making facile and inexpensive THz measurement available to scientific communities in chemistry, biology, and materials science with applications spanning energy harvesting to chemical catalysis to biomedical measurement. In addition to providing training in cutting-edge spectroscopy techniques for graduate students and post-doctoral scientists, the Nelson research group also runs a unique outreach program (the Lambda Project) in which high school students come to MIT for hands-on experiments to learn about modern optics and modern materials.The project builds on recent demonstrations of THz electron paramagnetic resonance (EPR) in both the linear regime in which free-induction decays (FIDs) are measured and in the nonlinear regime in which 2-dimensional (2D) electron paramagnetic resonance (EPR) is carried out. THz EPR spectra of high-spin transition metal complexes with zero-field splittings that arise from spin-orbit coupling are measured. The splittings are highly sensitive to the ligands and their configuration around the central transition metal ion. The EPR measurement can be preceded by an optical ("pump") pulse that initiates photochemical changes, with associated changes in the spin states monitored on picosecond time scales. 2D THz EPR measurements, demonstrated on the collective spin states of magnetic materials, are being extended to multinuclear organometallic complexes in order to measure spin-spin interactions between the transition metal ions, revealing detailed information about the ground and excited-state electronic structures of the complexes. In addition to these measurements in which THz magnetic fields coherently drive electron spins, the combined THz electric and magnetic field components are being used to drive both electric and magnetic dipole moments of chiral molecules in experiments that seek new methods for enantiometric separation. Further impacts in chemistry and biology may be realized if improved enantiomeric separation can be achieved.