DEVELOPMENT OF PULSED ELECTRON NUCLEAR DOUBLE RESONANCE (ENDOR) AT 140 GHZ

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We developed a spectrometer for pulsed ENDOR at 140 GHz, which is based on three new main units a magnet utilizing a 1 0.4 T superconducting sweep coil and a 2 H-NMR field lock system, a 139.5 GHz microwave bridge with four phase capabilities operating in reflection mode, and a probe for EPR/NMR double resonance. The probe consists of a TEOI I cavity adapted for RF irradiation and a high-quality resonance circuit with autotuning capability for variable radio frequency irradiation. For pulsed EPR we obtain 90* pulses of 60 - 70 ns and an absolute sensitivity of 3.109 spins/Gauss at 20 K. The performance of the spectrometer was demonstrated with pulsed ENDOR spectra of a standard bis-diphenylene-phenyl-allyl (BDPA) doped into polystyrene and of the tyrosyl radical from E. coli Ribonucleotide Reductase (RNR). The EPR spectrum of the RNR tyrosyl radical displays substantial g-anisotropy at 5 T and was used to demonstrate orientation selective Davies-ENDOR.