Department of Physics, University of California, Santa Barbara, Santa Barbara, California, USA.
Phys Chem Chem Phys. 2018 Jul 4;20(26):18097-18109. doi: 10.1039/c8cp01876f.
Electron paramagnetic resonance (EPR) is a powerful tool for research in chemistry, biology, physics and materials science, which can benefit significantly from moving to frequencies above 100 GHz. In pulsed EPR spectrometers driven by powerful sub-THz oscillators, such as the free electron laser (FEL)-powered EPR spectrometer at UCSB, control of the duration, power and relative phases of the pulses in a sequence must be performed at the frequency and power level of the oscillator. Here we report on the implementation of an all-quasioptical four-step phase cycling procedure carried out directly at the kW power level of the 240 GHz pulses used in the FEL-powered EPR spectrometer. Phase shifts are introduced by modifying the optical path length of a 240 GHz pulse with precision-machined dielectric plates in a procedure we call phase cycling with optomechanical phase shifters (POPS), while numerical receiver phase cycling is implemented in post-processing. The POPS scheme was successfully used to reduce experimental dead times, enabling pulsed EPR of fast-relaxing spin systems such as gadolinium complexes at temperatures above 190 K. Coherence transfer pathway selection with POPS was used to perform spin echo relaxation experiments to measure the phase memory time of P1 centers in diamond in the presence of a strong unwanted FID signal in the background. The large excitation bandwidth of FEL-EPR, together with phase cycling, enabled the quantitative measurement of instantaneous electron spectral diffusion, from which the P1 center concentration was estimated to within 10%. Finally, phase cycling enabled saturation-recovery measurements of T1 in a trityl-water solution at room temperature - the first FEL-EPR measurement of electron T1.
电子顺磁共振（EPR）是化学、生物学、物理学和材料科学研究的有力工具，将其频率提高到 100GHz 以上将带来显著的益处。在由强大的亚太赫兹振荡器驱动的脉冲 EPR 光谱仪中，例如 UCSB 的自由电子激光（FEL）驱动的 EPR 光谱仪，必须在振荡器的频率和功率水平上控制脉冲序列中脉冲的持续时间、功率和相对相位。在这里，我们报告了在 FEL 驱动的 EPR 光谱仪中使用的 240GHz 脉冲的千瓦功率水平上直接执行的全准光学四步相移循环程序的实现。通过在称为带有光机械相移器的相移循环（POPS）的程序中使用精密加工的介电板来改变 240GHz 脉冲的光程长度来引入相移，同时在后期处理中实现数值接收相移循环。POPS 方案成功地用于减少实验死时间，从而使镝配合物等快速弛豫自旋系统在 190K 以上的温度下进行脉冲 EPR。使用 POPS 进行相干转移途径选择，以执行自旋回波弛豫实验，以在背景中存在强的不需要的 FID 信号的情况下测量金刚石中的 P1 中心的相位记忆时间。FEL-EPR 的大激发带宽与相移循环相结合，实现了瞬时电子光谱扩散的定量测量，由此估计 P1 中心的浓度在 10%以内。最后，相移循环使室温下三苯基水合物溶液中的 T1 的饱和恢复测量成为可能-这是 FEL-EPR 对电子 T1 的首次测量。
