Field-driven ONP

Field-driven ONP was first demonstrated by Vieth et who reported the use of rf... 

Fig. 3. Jablonski energy-level diagram typical for chromophores involved in ONP processes (specifically for pentacene in naphthalene, after Ref. [39]), showing the respective roles of optical excitation, ISC, OEP, electron-nuclear polarization transfer (here driven by mw radiation), and subsequent decay to the diamagnetic ground state. The relative populations of the triplet magnetic sublevels are given at the far right (assuming the crystal is oriented such that the long molecular axis of pentacene is parallel to the external magnetic field ).

Finally, ONP may be achieved with the aid of applied electromagnetic (i.e., radiofrequency, rf or microwave, mw) fields (c), and thus can most simply be considered as extensions of conventional DNP methods these approaches are referred to as radio-frequency induced ONP (RFONP) and microwave-induced ONP (MIONP), respectively. In conventional DNP, magnetization from thermally polarized electron spins (residing, for example, in permanent paramagnetic centers) is driven via hyperfine interactions to surrounding nuclei using resonant or near-resonant AC fields. More specifically, this process is governed by the conventional Overhauser effect in the case of dynamic hyperfine interactions (e.g., in metals or semiconductors),... 

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