Coherent Spectroscopy and Coherent Control of Molecules and Materials

在该项目中，由化学部的化学结构动力学和机理-A（CSDM-A）计划资助，马萨诸塞州理工学院的基思纳尔逊教授正在开发新技术，该技术使用太赫兹（THz）频谱范围内的短脉冲光。太赫兹的范围仅高于手机等常见设备中使用的低频微波（或GHz）范围。磁共振测量（类似于磁共振成像或MRI）可以揭示有关分子结构和键合的关键信息，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，使用一个简单的，桌面大小的实验装置产生的短THz脉冲，使THz磁共振测量在大约一分钟。正在进行分子和生物分子与铁或类似元素的复合物的测量，以了解分子结构以及它们在化学过程中如何变化。该项目具有更广泛的影响，使化学，生物学和材料科学的科学界可以使用简便，廉价的THz测量，其应用范围涵盖能量收集，化学催化和生物医学测量。 除了为研究生和博士后科学家提供尖端光谱技术培训外，纳尔逊研究小组还开展了一项独特的推广计划（λ项目），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。该项目建立在最近的太赫兹电子顺磁共振（EPR）在两个线性政权的演示，其中自由-感应衰减（FID）的测量和在非线性制度，其中2维（2D）电子顺磁共振（EPR）进行。测量了具有零场分裂的高自旋过渡金属配合物的太赫兹电子顺磁共振谱。分裂对中心过渡金属离子周围的配体及其构型高度敏感。EPR测量之前可以有一个光（“泵”）脉冲，启动光化学变化，与相关的变化，自旋状态监测皮秒的时间尺度。在磁性材料的集体自旋态上展示的2D THz EPR测量正在扩展到多核有机金属络合物，以测量过渡金属离子之间的自旋-自旋相互作用，揭示络合物的基态和激发态电子结构的详细信息。除了太赫兹磁场相干驱动电子自旋的这些测量之外，组合的太赫兹电场和磁场分量还用于在实验中驱动手性分子的电和磁偶极矩，以寻求对映体分离的新方法。 如果能够实现对映体分离的改善，则可能会在化学和生物学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.