High-field DNP and ENDOR with a novel multiple-frequency resonance structure

We describe a new triply tuned (e(-), H-1, and C-13) 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-gamma-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 H-2 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 H-1 signal enhancement (epsilon similar to 400 +/- 50) than previously achieved at 5-T fields. The results emphasize the importance of optimizing the microwave B-1 field by improving either the quality factor of the microwave resonator or the microwave power level. (C) 1999 Academic Press.