During the last couple of years, we have been trying to understand the various stages of the DNP process leading to nuclear signal enhancements in static solid solutions, containing free organic radicals. Despite the fact that almost everything is known already, we have made an effort to clarify the basic spin dynamics resulting in these enhancements. For these studies, we performed experiments on the electrons and the nuclei in a variety of amorphous solids. In particular, the combination of EPR and ELDOR spectroscopy together with NMR measurements on the same sample, has led us to formulate computational models for explaining the line shapes of EPR and DNP spectra during microwave (MW) irradiation.
In this presentation, our findings in static samples will be summarized, by comparing simulated spectra of small model spin system with experimentally obtained spectra. For our computations, we use a cross relaxation mechanism to describe electron depolarization and rely on the indirect Cross Effect (iCE) for deriving DNP spectra from EPR spectra. The interaction regimes for the appearance of the Thermal Mixing (TM) phenomena will of course be discussed as well. For the analysis of experimental results, we show how we derive the change of the EPR lineshapes following microwave (MW) irradiation from ELDOR spectra, using the electron spectral diffusion (eSD) and TM models, and again rely on iCE to interpret experimental DNP spectra. Experimental results corresponding to the TM process will also be discussed and the dependence of the enhancements as a function of MW power, MW modulation and radical concentration will be verified.
Note: All static DNP studies have been performed in full collaboration with Daniella Goldfarb and Akiva Feintuch, and our post-doctoral and graduate students.