A 200 GHz dynamic nuclear polarization spectrometer.

We present our experimental setup for both dynamic nuclear polarization (DNP) and electron paramagnetic resonance (EPR) detection at 7 T using a quasi-optical bridge for propagation of the 200 GHz beam and our initial results obtained at 4 K. Our quasi-optical bridge allows the polarization of the microwave beam to be changed from linear to circular. Only the handedness of circular polarization in the direction of the Larmor precession is absorbed by the electron spins, so a gain in effective microwave power of two is expected for circular vs. linear polarization. Our results show an increase in DNP signal enhancement of 28% when using circularly vs. linearly polarized radiation. We measured a maximum signal enhancement of 65 times that of thermal polarization for a (13)C labeled urea sample corresponding to 3% nuclear spin polarization. Since the time constant for nuclear spin polarization buildup during microwave irradiation is 10 times faster than the (13)C nuclear spin T(1), the actual gain in detection sensitivity with DNP is much greater.