Dynamic nuclear polarization of diamond. II. Nuclear orientation via electron spin-locking

The polarization of 13C nuclei by means of nuclear orientation via electron spin-locking (Hartmann–Hahn cross-polarization between paramagnetic electrons and 13C nuclei) in a suite of natural diamonds has been investigated at 2.4 GHz and 9.6 GHz. The 13C polarization rate has been found to be independent of the microwave frequency, in agreement with theory. It is shown that since T1ρ(e)≪T1(e) for diamond, the effective polarization rate of 13C nuclei is relatively low. At low paramagnetic impurities (Ce<5 ppm) 13C nuclei in diamond are polarized faster by employing continuous wave microwave radiation to drive the forbidden transitions of the 13C-electron spin system, the so-called solid effect.