Multi-step phase-cycling in a free-electron laser-powered pulsed electron paramagnetic resonance spectrometer
Multi-step phase-cycling in a free-electron laser-powered pulsed electron paramagnetic resonance spectrometer
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自由电子激光驱动脉冲电子顺磁共振波谱仪中的多步相位循环
DOI：
10.1039/c8cp01876f
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发表时间：
2018
影响因子：
3.3
通讯作者：
Sherwin, Mark S.
中科院分区：
文献类型：
--
作者：
Wilson, C. Blake;Aronson, Samuel;Clayton, Jessica A.;Glaser, Steffen J.;Han, Songi;Sherwin, Mark S.
电子顺磁共振(EPR)是化学、生物、物理和材料科学研究的有力工具，移动到100 GHz以上的频率可以显著受益。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)供电的EPR光谱仪，必须在振荡器的频率和功率水平上控制序列中脉冲的持续时间、功率和相对相位。本文报道了在自由电子激光EPR光谱仪中直接在240 GHz脉冲的kW功率水平上实现全准光四步相位循环的方法。利用精密加工的介质板改变240 GHz脉冲的光程长度来引入相移，这一过程称为光机移相器(POPS)的相位循环，而后处理则是数字接收相位循环。POPS方案被成功地用于减少实验的死区时间，使得在190K以上的温度下能够脉冲地进行快弛豫自旋系统的EPR，例如Gd配合物。POPS的相干转移路径选择被用来进行自旋回波弛豫实验，以测量在背景中存在强烈的有害FID信号时钻石中P1中心的相记忆时间。FEL-EPR的大激发带宽与相位循环相结合，实现了对瞬时电子光谱扩散的定量测量，由此估计出P1中心浓度在10%以内。最后，相循环使t1在室温下的三丁基水溶液中的饱和恢复测量成为可能--这是电子t1的第一次自由电子激光电子顺磁共振测量。
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.