The inhomogeneous distribution of P1 centers in type 1b HPHT diamond samples allows multiple DNP mechanisms to occur within the same crystal, resulting in complex DNP spectra. At some crystal orientations, different DNP mechanisms can compete to drive hyperpolarization with different signs at the same applied microwave frequency. We perform microwave-irradiated DNP using both monochromatic and frequency-modulated microwave excitation to explore the competition between these DNP mechanisms in diamond at room temperature. We demonstrate that frequency-modulated DNP is a tool for suppressing certain DNP mechanisms while enhancing others in a single-crystal diamond sample. Frequency modulation also enables higher enhancement of the NMR signal beyond traditional monochromatic DNP under some conditions. In a powder sample, competing enhancement mechanisms can also arise from different crystallite orientations in the powder. We observe that at certain microwave frequencies the DNP signal changes sign during the polarization build-up, even with monochromatic microwave irradiation. We do not observe this phenomenon in any single-crystal spectrum. We discuss both methods of investigating competing mechanisms of DNP as a means of selectively enhancing different DNP mechanisms driving ^13C NMR signal enhancement.
Sheehan et al. (Wed,) studied this question.