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139.5 GHz EPR需求的显著增长 分光计时间。主要的新部件是S T磁体(MagneX) 使用Q0.4T超导扫描线圈，允许访问系统 具有在1.8-2.2范围内的电子g值；这表示 与以前的q0.075T扫描范围相比有了显著的改进 核磁共振/DNP磁体。购买或建造的其他设备 包括：现场锁定系统、可编程直流电源 FIELD SCEEP，一台搭载国家仪器的高性能麦金塔计算机 GPIB数据采集卡和LabVIEW软件。这一组合 硬件和软件提供灵活、强大的平台， 用来控制光谱仪。微波板已经被 经过修改以采用新的电源和安装方案， 通过直接连接到 桥接上磁铁。这些变化导致了巨大的 改善了信噪比性能和稳定性。在房间里 银柱形谐振腔中的温度，灵敏度 在连续波中测得光谱仪的自旋为2-3×10~3自旋/高斯 模式，在脉冲模式下大约差一个数量级。 灵敏度的提高足够大，现在允许回声 1 mM核糖核苷酸还原酶酪氨酸自由基的检测 (RnR)，而以前我们只能获得连续波谱。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 400纳秒。因此，正如下面更详细地描述的那样，新的 输出功率增加的微波源(与 目前为2 mW)，并且已经订购了相位切换能力 应该会显著提高脉冲的灵敏度 分光计。我们已经(从乌克兰的V.Krymov)订购了一种新的 功率和相位更大的脉冲四相微波源 交换能力。该网络由一个CW IMPATT振荡器组成 注入锁定到我们目前的139 GHz Gunn二极管光源。至 克服与开关和相位相关的功率损耗 移位后，将使用一系列IMPATT二极管放大器 相位和脉冲切换阶段。我们预计输出功率为30兆瓦 从该源对应于900个脉冲长度为-.60 ns 我们的EPR探测器。这种新的微波源将允许相位循环。 并使大量更复杂的脉冲EPR成为可能 比以前在高频下进行的实验更多， 例如自旋锁定、相位循环、COSY、SECSY和双量子 实验。我们还完成了我们的140 GHz的建设 恩多尔探头和传输线。可变的RF频率是 用PTS合成器生成。脉冲和四正交位相 以40兆赫的中频产生，射频放大 在混合到所需的信号后，使用AMT放大器执行 频率第二个射频合成器可以实现三倍频 共振实验。一种基于高Q值的复杂探测器 搭建了射频电路。这种类型的谐振电路由一个 LC并联电路和一个串联的匹配电容，是常见的 固体核磁共振。我们扩展了结构以允许射频扫描 在实验过程中。两个微米和一个数字编码器驱动器 两个电容器的位置均由计算机控制。一个 校准曲线允许设置千分尺位置 通过整个频率扫描范围进行调谐和匹配。典型 目前，1H自旋的RF 1800脉冲长度为7 GS。

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.