Pulsed electron-nuclear double resonance (ENDOR) at 140 GHz

We describe a spectrometer for pulsed ENDOR at 140 GHz, which is based on m icrowave IMPATT diode amplifiers and a probe consisting of a TE011 cavity w ith a high-quality resonance circuit for variable radiofrequency irradiatio n. For pulsed EPR we obtain an absolute sensitivity of 3 x 10(9) spins/Gaus s at 20 K. The performance of the spectrometer is 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 reductas e (RNR). The EPR spectrum of the RNR tyrosyl radical displays substantial g -anisotropy at 5 T and is used to demonstrate orientation-selective Davies- ENDOR. (C) 1999 Academic Press.