A 139.5 GHZ PULSED EPR AND ENDOR SPECTROMETER

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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.