DNP via the Solid Effect

Soon after the discovery of DNP via the OE, Jeffries realized that the saturation of so-called forbidden transitions will produce a comparable nuclear polarization directly in the sense that the applied radiofrequency field itself flips the nuclei in solids [26]. He mentions that this had previously been observed by Pound [24] and theoretically considered by Abragam [25], who coined the name Peffet solide. Jeffries later described the SE theoretically [60], and reviewed the early work of DNP in 1964 [2]. 

The SE is observed at low temperatures in solids for radicals with narrow EPR lines (typically trityl radicals), where the EPR linewidth is much smaller than the nuclear Larmor frequency (fflei/2 n)- As pointed out by Jeffries, it arises from driving the forbidden Wq and as illustrated in Pig. 5. 

DNP in the solid state follows a different mechanism compared to DNP in liquids, in particular at low temperatures, where the spatial parts of and 7/ ° become time-independent. Therefore, the enhancement is not driven by changes in polarizations arising from and Wo relaxation, as is the case for O-DNP, but rather by driving these forbidden flip-flop (Wo) and flip-flip (W2) transitions using microwave irradiation at the corresponding resonance frequencies cOe + or ffle — fflN, as shown in Pig. 6. 

Por this it is useful to expand the Hamiltonian (2) to include a term for the interaction of the electrons with the microwave field  

The SE depends on non-secular terms of the electron-nuclear part of the Hamiltonian, leading to the mixing of states under the influence of microwave irradiation. This can easily be recognized by writing the Hamiltonian in a frame that points into the direction of the effective microwave field in which the z axis is defined by effective microwave frequency  

---