A 139.5 GHZ PULSED EPR AND ENDOR SPECTROMETER

我们已经购买并安装了一台新的EPR光谱仪， 缓解对139.5 GHz EPR需求的显著增长 光谱仪时间 主要的新组件是一个S T磁铁（Magrex） 带有q0.4 T超导扫描线圈，允许进入系统 电子g值在1.8-2.2的范围内;这表示 与之前的q0.075 T扫描范围相比， NMR/DNP磁体。 购买或建造的额外设备 包括现场锁定系统、可编程直流电源， 现场扫描，一台Power Macintosh电脑， National Instruments GPIB数据采集卡和Labview软件。 的这种组合 硬件和软件提供了一个灵活而强大的平台， 用来控制光谱仪 微波板已经 修改为包含新的电源和安装方案， 减少振动和信号损失的直接连接， 桥到磁铁。 这些变化导致了巨大的 改善信噪比性能和稳定性。 在室温 温度在银圆柱谐振腔，灵敏度 光谱仪的CW测量值为2- 3 x 10~ spins/高斯 模式，并且在脉冲模式下大约差一个数量级。 灵敏度的增加足够大， 检测1 mM核糖核苷酸还原酶的酪氨酰自由基 (RNR)，而以前我们只能获得CW光谱。 脉冲 然而，模式性能受到低微波功率的限制：900 上述实验条件下的脉冲长度约为 四百纳秒。 因此，如下文更详细描述的，新的 具有增加的输出功率的微波源（30 mW，与 目前2毫瓦）和相位开关能力已订购， 应该大大提高脉冲的灵敏度 光谱仪 我们已经（从乌克兰的V. Krymov）订购了一台新的 大功率大相位脉冲四相微波源 切换能力。 该网络由一个连续波雪崩管振荡器 注入锁定到我们目前的139.S GHz古恩二极管源。 到 克服与开关和相位相关的功率损耗 移位，一系列的雪崩二极管放大器将用于后，每个 相位和脉冲切换级。 我们预计输出功率为30 mW 对应于900个脉冲长度为~.60 ns， 我们的EPR探针 这种新的微波源将允许相位循环 并使一系列更复杂的脉冲EPR成为可能 与先前在高频下进行的实验相比， 例如自旋锁定、相位循环、COSY、SECSY和双量子 实验 我们还完成了140 GHz的建设， ENDOR探头和传输线。 可变RF频率为 是由PTS合成器生成的 脉冲和四个正交相位 在40 MHz的中频下产生，RF放大 在混合到所需的频率后， 频率. 第二个RF合成器可以实现三重 共振实验 基于高Q值的精密探针 建立了射频电路。 这种类型的谐振电路，由一个 LC并联电路和一个匹配电容串联，常见于 固态核磁共振 我们扩展了结构， 在实验中。 两个千分尺和一个数字编码器驱动器 电容器和电容器的位置由计算机控制。 一 校准曲线允许设置测微计位置， 在整个频率扫描范围内进行调谐和匹配。 典型 RF 1800脉冲长度目前为7 gs，用于1H自旋。

We have purchased and installed a new EPR spectrometer to alleviate a significant increase in demand for 139.5 GHz EPR spectrometer time. The major new component is a S T magnet (Magnex) with a q0.4 T superconducting sweep coil, allowing access to systems with electron g-values in the range 1.8-2.2; this represents a significant improvement over the q0.075 T sweep range of the previous NMR/DNP magnet. Additional equipment purchased or constructed includes: a field lock system, a programmable DC power supply for the field sweep, a Power Macintosh computer with National Instruments GPIB data acquisition cards and Labview software. This combination of hardware and software provides a flexible and robust platform from which to control the spectrometer. The microwave board has been modified to incorporate new power supplies and a mounting scheme that reduces vibrations and signal losses by the direct connection of the bridge to the magnet. These changes have resulted in large improvements in signal-to-noise performance and stability. At room temperature in a silver cylindrical resonant cavity, the sensitivity of the spectrometer was measured to be 2-3x 10~ spins/gauss in CW mode, and approximately one order of magnitude worse in pulsed mode. The increase in sensitivity is large enough to now permit echo detection of the tyrosyl radical of 1 mM ribonucleotide reductase (RNR), whereas previously we could only obtain CW spectra. Pulsed mode performance is, however, limited by low microwave power: the 900 pulse length for the above experimental conditions is approximately 400 nanoseconds. Therefore, as described below in more detail, a new microwave source with increased output power (30 mW compared to the present 2 mW) and phase switching capability has been ordered and should dramatically improve the sensitivity of the pulsed spectrometer. We have ordered (from V. Krymov in the Ukraine) a new pulsed four-phase microwave source with greater power and phase switching capability. The network consists of a CW IMPATT oscillator injection-locked to our current 139.S GHz Gunn diode source. To overcome the power losses associated with switching and phase shifting, a series of IMPATT diode amplifiers will be used after each phase and pulse switching stage. We anticipate 30 mW output power from this source corresponding to 900 pulse lengths of - .60 ns for our EPR probe. This new microwave source will allow for phase cycling of pulses and make possible a host of more sophisticated pulsed EPR experiments than have been performed previously at high frequencies, such as spin-locking, phase cycling, COSY, SECSY, and double quantum experiments. We have also completed the construction of our 140 GHz ENDOR probe and transmission line. The variable RF frequencies are generated with a PTS synthesizer. Pulses and four orthogonal phases are generated at an intermediate frequency of 40 MHz, RF amplification is performed with an AMT amplifier after mixing up to the desired frequency. A second RF synthesizer can be implemented for triple resonance experiments. A sophisticated probe based on a high Q RF-circuit was built. This type of resonance circuit, consisting of a LC parallel circuit and a matching capacitor in series, is common in solid state NMR. We extended the construction to allow for RF sweep during the experiment. Two micrometers and a digital encoder drive the positions of both capacitors and are controlled by computer. A calibration curve allows the setting of the micrometer position for tuning and matching through the entire frequency scan range. Typical RF 1800 pulse lengths are currently 7 gs for 1H spins.