A SPECTROMETER FOR DYNAMIC NUCLEAR-POLARIZATION AND ELECTRON-PARAMAGNETIC-RESONANCE AT HIGH-FREQUENCIES

A high-frequency dynamic nuclear polarization (DNP)/electron paramagnetic resonance spectrometer operating at 211 MHz for H-1 and 140 GHz for g = 2 paramagnetic centers (5 T static field) is described. The salient feature of the instrument is a cyclotron-resonance maser (gyrotron) which generates high-frequency, high-power microwave radiation, This gyrotron, which under conventional operation produces millisecond pulses at kilowatt powers, has been adapted to operate at similar to 100 W for 1 to 20 s pulses and in the continuous wave mode at the 10 W power level, Experiments combining DNP with magic-angle spinning (MAS) nuclear magnetic resonance were performed on samples consisting of 2% by weight of the free radical BDPA doped into polystyrene, Room-temperature DNP enhancement factors of 10 for H-1 and 40 for C-13 were obtained in the NMR-MAS spectra, Static DNP NMR has also been performed on samples containing nitroxides dissolved in water:glycerol solvent mixtures, Enhancements of approximately 200 have been obtained for low-temperature (14 K) H-1 NMR. A pulsed/CW EPR spectrometer operating at 140 GHz has been developed in conjunction with the DNP spectrometer, Microwave sources include Gunn-diode oscillators which provide low-power (20 mW) radiation, and the gyrotron, which has been used to deliver higher power levels in pulsed experiments, Results using this spectrometer are presented for continuous-wave and echo-detected EPR, electron spin-echo-envelope modulation (ESEEM), and Fourier-transform EPR. (C) 1995 Academic Press, Inc.