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Abstract
We describe a new triply tuned (e(-), (1)H, and (13)C) 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 (2)H 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 (1)H signal
enhancement (epsilon approximately 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.