High-frequency high-power DNP/EPR spectrometer operating at 7 T magnetic field

Abstract

One of the most essential prerequisites for the development of pulse Dynamic Nuclear Polarization (DNP) is the ability to generate high-power coherent mm-wave pulses at the electron precession frequencies corresponding to the magnetic fields of modern high-resolution NMR spectrometers. As a major step towards achieving this goal, an Extended Interaction Klystron (EIK) pulse amplifier custom-built by the Communications and Power Industries, Inc. and producing up to 140 W at 197.8 GHz, was integrated with in-house built NMR/DNP/EPR spectrometer operating at 7 T magnetic field. The spectrometer employs a Thomas Keating, Ltd. quasioptical bridge to direct mm-waves into a homebuilt DNP probe incorporating photonic bandgap (PBG) resonators to further boost electronic B_1e fields. Three-pulse electron spin echo nutation experiments were employed to characterize the B_1e fields at the sample by operating the homodyne 198 GHz bridge in an induction mode. Room-temperature experiments with a single-crystal high-pressure, high-temperature (HPHT) diamond and a polystyrene film doped with BDPA radical yielded < 9 ns π/2 pulses at ca. 50 W specified EIK output at the corresponding resonance frequencies and the PBG resonator quality factor of Q≈300. DNP experiments carried out in a “gated” mode by supplying 20 μs mm-wave pulses every 1 ms yielded ¹³C solid-effect DNP with gains up to 20 for the polystyrene-BDPA sample at natural ¹³C abundance. For a single-crystal HPHT diamond, the gated DNP mode yielded almost the same ¹³C enhancement as a low-power continuous wave (CW) mode at 0.4 W, whereas no DNP effect was observed for the BDPA/polystyrene sample in the latter case. To illustrate the versatility of our upgraded DNP spectrometer, room-temperature Overhauser DNP enhancements of 7–14 for ³¹P NMR signal were demonstrated using a liquid droplet of 1 M tri-phenyl phosphine co-dissolved with 100 mM of BDPA in toluene‑d₈.