A 140 GHz pulsed EPR/212 MHz NMR spectrometer for DNP studies

Abstract

We described a versatile spectrometer designed for the study of dynamic nuclear polarization (DNP) at low temperatures and high fields. The instrument functions both as an NMR spectrometer operating at 212MHz (1H frequency) with DNP capabilities, and as a pulsed-EPR operating at 140GHz. A coiled TE011 resonator acts as both an NMR coil and microwave resonator, and a double balanced (1H, 13C) radio frequency circuit greatly stabilizes the NMR performance. A new 140GHz microwave bridge has also been developed, which utilizes a four-phase network and ELDOR channel at 8.75GHz, that is then multiplied and mixed to obtain 140GHz microwave pulses with an output power of 120mW. Nutation frequencies obtained are as follows: 6MHz on S=1/2 electron spins, 100kHz on 1H, and 50kHz on 13C. We demonstrate basic EPR, ELDOR, ENDOR, and DNP experiments here. Our solid effect DNP results demonstrate an enhancement of 144 and sensitivity gain of 310 using OX063 trityl at 80K and an enhancement of 157 and maximum sensitivity gain of 234 using Gd-DOTA at 20K, which is significantly better performance than previously reported at high fields (⩾3T).