MRI: Development of a Free-Electron Laser for Ultrafast Pulsed Electron Paramagnetic Resonance

技术摘要：与核磁共振（NMR）一样，EPR在高磁场和高频率下变得更加强大，并且在脉冲而不是连续波（cw）模式下变得更加强大。高场高频脉冲EPR的主要瓶颈是缺乏高频率（100 GHz）、高功率（1 kW）、高长期频率稳定性和脉冲可编程性的电磁源。在之前的核磁共振基金和W. M. Keck基金会的资助下，世界上第一台由fel供电的脉冲EPR光谱仪在加州大学圣巴巴拉分校进行了演示。最引人注目的成就是极快的自旋操纵——自旋1 / 2电子在240千兆赫下在6纳秒内旋转了90度，比世界上第二快的240千兆赫光谱仪快了两个数量级，后者是基于固态源的。要开发的主要研究仪器是自由电子激光器（FEL），该激光器在240至500 GHz的频率（对应于8.5至18 T的磁场）上进行了电子顺磁共振（EPR）优化。这一发展很大程度上利用了加州大学圣巴巴拉分校25年来的基础设施、投资、机构承诺和专业知识。将对现有的6 MV静电加速器进行升级，并建造新的自由电子激光器（波动器+腔）。总之，这些改进将使240 GHz的可用峰值功率从300 W增加到10 kW，重复频率从1 Hz增加到10 Hz，并大大提高系统的长期稳定性和可靠性。新的FEL将使自旋1 / 2电子的90度旋转时间低于1ns，从而实现极快速自旋弛豫过程的分辨率。脉冲EPR的数据采集时间将至少减少1000倍。新的FEL和相关的EPR光谱仪将提供给国内和国际用户社区，并使材料科学，物理，化学和分子生物学的变革性研究成为可能。非技术摘要：世界上最亮的可调谐太赫兹辐射源将被开发出来，以比以往任何时候都更快的速度操纵电子自旋。这种超快的自旋操纵将使开创性的研究成为可能，其应用范围从开发廉价的太阳能电池到了解蛋白质分子如何结合在一起并调节生物体中能量、信息和物质的流动。电子和原子核都有一种叫做自旋的特性，这使得它们的行为像（非常微小的）磁铁。核磁共振（NMR）是磁共振成像（MRI）的基础，在核磁共振（NMR）中，一个强大的外部磁场使核自旋对齐，而强大的射频电磁辐射脉冲操纵原子核，以发现有关邻近原子的其他不可见信息。电子顺磁共振（EPR），以一种类似核磁共振的方式，使用外部磁场来排列电子自旋（而不是核自旋）。通常，微波频率的电磁辐射脉冲操纵这些电子，以了解更大范围内的局部环境。当使用极高频太赫兹辐射时，EPR变得更加强大。加州大学圣巴巴拉分校（UCSB）的自由电子激光器（FELs）以世界上最亮的可调谐太赫兹辐射源而闻名。最近，UCSB的研究人员证明，UCSB的一个FELs可以使电子自旋速度比以前快50倍。25兆赫。这个项目将资助建造一个更强大的自由电子激光器。新的FEL将被来自全国和世界各地的科学家使用，它的功率将比现有的强100倍，脉冲速度将提高10倍，从而使实验数据的获取速度至少提高1000倍。由这种新型自由电子激光器驱动的EPR光谱仪将创造一种前所未有的能力，可以在纳米尺度上观察分子、材料和器件的结构和超快动力学。

Development of a Free-Electron Laser for Ultrafast Electron Magnetic ResonanceTechnical abstract: Like nuclear magnetic resonance (NMR), EPR becomes much more powerful at high magnetic fields and frequencies, and in a pulsed rather than continuous wave (cw) modality. The major bottleneck for high-field, high-frequency pulsed EPR has been the absence of electromagnetic sources capable of high frequency (100 GHz), high power (1 kW), high long-term frequency-stability, and pulse-programmability. Supported by a previous MRI grant and an award from the W. M. Keck Foundation, the world's first FEL-powered pulsed EPR spectrometer has been demonstrated at UC Santa Barbara. The most dramatic achievement is extremely rapid spin manipulation-spin ½ electrons have been rotated by 90 degrees in 6 ns at 240 GHz, two orders of magnitude faster than the next fastest 240 GHz spectrometer in the world, which is based on a solid-state source. The major research instrumentation to be developed is a free-electron laser (FEL) that is optimized for electron paramagnetic resonance (EPR) at frequencies between 240 and 500 GHz (corresponding to magnetic fields between 8.5 and 18 T). This development heavily leverages 25 years of infrastructure, investment, institutional commitment, and expertise at UC Santa Barbara. The existing 6 MV electrostatic accelerator will be upgraded and a new free-electron laser (undulator + cavity) will be built. Together, these improvements will increase the peak power available at 240 GHz from 300 W to 10 kW, the repetition rate from 1 Hz to 10 Hz, and also greatly improve the long-term stability and reliability of the system. The new FEL will bring times for 90-degree rotations of spin ½ electrons below 1 ns, enabling resolution of extremely rapid spin relaxation processes. Data acquisition times for pulsed EPR will be reduced by at least a factor of 1000. The new FEL and associated EPR spectrometer will be made available to a national and international user community, and enable transformative studies in materials science, physics, chemistry and molecular biology.Non-technical abstract: The world's brightest source of tunable terahertz radiation will be developed to manipulate electron spins faster than has ever been possible. This ultrafast spin manipulation will enable pathbreaking studies with applications ranging from development of inexpensive solar cells to understanding how protein molecules fit together and move to regulate the flow of energy, information and matter in living organisms.Electrons and atomic nuclei both have a property called spin, which makes them behave like (very tiny) magnets. In nuclear magnetic resonance (NMR), which is the basis for magnetic resonance imaging (MRI), a strong external magnetic field aligns nuclear spins, while powerful pulses of radio-frequency electromagnetic radiation manipulate nuclei to discover otherwise invisible information about neighboring atoms. Electron paramagnetic resonance (EPR), in a fashion similar to NMR, uses an external magnetic field to align electron spins (rather than nuclear spins). Typically, pulses of microwave-frequency electromagnetic radiation manipulate these electrons to learn about local environments over larger neighborhoods. EPR becomes even more powerful when extremely high-frequency terahertz radiation is used.The free-electron lasers (FELs) at the University of California at Santa Barbara (UCSB) are famous as the world's brightest sources of tunable terahertz radiation. Recently, researchers at UCSB demonstrated that one of the UCSB FELs could be used to rotate electron spins 50 times faster than ever before at .25 terahertz. This project will fund the construction of an even more powerful FEL. The new FEL, which will be used by scientists from all over the nation and world, will be 100 times more powerful than the existing one, and will pulse ten times faster, enabling at least 1000 times more rapid acquisition of experimental data. The EPR spectrometer powered by this new FEL will create an unprecedented capability to observe the structure and ultrafast dynamics of molecules, materials and devices at nanometer length scales.