High-Field DNP and ENDOR with a Novel Multiple-Frequency Resonance Structure

Abstract

We describe a new triply tuned (e−, 1H, and 13C) resonance structure operating at an electron Larmor frequency of 139.5 GHz for dynamic nuclear polarization (DNP) and electron nuclear double-resonance (ENDOR) experiments. In contrast to conventional double-resonance structures, the body of the microwave cavity simultaneously acts as a NMR coil, allowing for increased efficiency of radiofrequency irradiation while maintaining a high quality factor for microwave irradiation. The resonator design is ideal for low-γ-nuclei ENDOR, where sensitivity is limited by the fact that electron spin relaxation times are on the order of the RF pulse lengths. The performance is demonstrated with 2H ENDOR on a standard perdeuterated bis-diphenylene-phenyl-allyl stable radical. In DNP experiments, we show that the use of this resonator, combined with a low microwave power setup (17 mW), leads to significantly higher 1H signal enhancement (ϵ ∼ 400 ± 50) than previously achieved at 5-T fields. The results emphasize the importance of optimizing the microwave B1 field by improving either the quality factor of the microwave resonator or the microwave power level.