HIGH FREQUENCY ELECTRON ELECTRON DOUBLE RESONANCE SPECTROSCOPY

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ELDOR is a magnetic resonance technique which is used to determine motional dynamics, relaxation properties, and molecular structure in paramagnetie systems. The technique requires the irradiation of a sample at two distinct EPR (Electron Paramagnetic Resonance) spectral positions. In the experiment described here, this is accomplished by irradiation with two different microwave frequencies from two distinct sources. A saturating (pump) pulse at 139.6S GHz is applied to the sample for 1 1.tsec to 10 msec. The effect of this pulse at a different spectral position (150 MHz upfield) is then monitored using a spin echo detection (probe) sequence at 139.50 GHz. The detection pulses are typically 300-600 ns in duration. Three switches are used in the ELDOR experimental set-up: one to form the saturating (pump) pulse. one to switch the low power detection (probe) pulses, and one to protect the mixer from high power. The switches provided by Capital Technology Corporation (CTC) have been used successfully to gate the high frequency (139.65 GHz) saturating pulse produced by the gyrotron. For various reasons, relatively low power saturation pulses have been used to date. However, it is desirable to use as much power as can be provided by the microwave source. These CTC switches are the only ones currently manufactured which have the insertion loss, power-handling capability, and switching times required for a high power (10 W) ELDOR experiment. Further improvements in CTC switch rise/fall times will allow their use to form the detection pulses. This will allow higher powers to be used in the detection pulse sequence, improving the signal-to-noise ratio and increasing the available detection bandwidth. ELDOR experiments have been performed on frozen solution samples of 4-amino TEMPO in 60:40 glycerol/water at 10 K. For a 4-amino TEMPO concentration of 40 mM, clear evidence of spectral diffusion is observed; the probe electron spin echo is attenuated due to the pump pulse. This attenuation is maximal when the pump pulse irradiates at the first moment of the EPR line. No such attenuation of the probe electron spin echo is observed for 1 mM 4-amino TEMPO. These results indicate a significant dipolar coupling between unpaired electrons in the 40 mM sample. This dipolar coupling facilitates electron-electron cross-relaxation, which can be exploited via dynamic nuclear polarization (DNP) to increase of signal-to-noise ratios of solid state nuclear magnetic resonance (NMR).