Kuzma, N.N., et al., Lineshape-based polarimetry of dynamically-polarized in solid-state mixtures. J. Magn. Reson., 2013. 234(0): p. 90-94.
Dynamic nuclear polarization (DNP) of 15 N 2 O , known for its long-lived singlet-state order at low magnetic field, is demonstrated in organic solvent/trityl mixtures at ∼1.5 K and 5 T. Both 15 N polarization and intermolecular dipolar broadening are strongly affected by the sample’s thermal history, indicating spontaneous formation of N2O clusters. In situ 15 N NMR reveals four distinct powder-pattern spectra, attributed to the chemical-shift anisotropy (CSA) tensors of the two 15N nuclei, further split by the intramolecular dipolar coupling between their magnetic moments. 15 N polarization is estimated by fitting the free-induction decay (FID) signals to the analytical model of four single-quantum transitions. This analysis implies ( 10.2 ± 2.2 ) % polarization after 37 h of DNP, and provides a direct, instantaneous probe of the absolute 15 N polarization, without a need for time-consuming referencing to a thermal-equilibrium NMR signal.