This is an excellent review and summary on different relaxation mechanisms observed in EPR spectroscopy. Understanding EPR relaxation is crucial to understand the DNP process.
Eaton, Sandra S., and Gareth R. Eaton. “Relaxation Mechanisms.” In EMagRes, edited by Robin K. Harris and Roderick L. Wasylishen, 1543–56. Chichester, UK: John Wiley & Sons, Ltd, 2016.
After a paramagnetic species absorbs energy, there are various relaxation processes by which the excitation energy is lost to the surroundings thereby enabling return to the ground state. The focus of this article is on relaxation of species with S= 1∕2 in magnetically dilute samples. The relative importance of various spin–lattice relaxation processes for each paramagnetic species is strongly dependent on temperature, electronic, and molecular structure. The Raman and local-mode processes make significant contributions to T 1 relaxation in rigid and semirigid lattices for a wide range of species at temperature above about 10 K. The Orbach process requires a low-lying excited state. The thermally activated process is significant when a stochastic process averages inequivalent environments on a timescale comparable to the Larmor frequency, as occurs by rotation of methyl groups or hopping of a hydrogen-bonded proton. Spin-echo dephasing at low temperatures is dominated by nuclear spin diffusion. It is enhanced by dynamic processes that average inequivalently coupled nuclei on the time scale of the hyperfine interaction and by motions that average g and A anisotropy. Analysis of the processes that contribute to relaxation as a function of temperature is shown for triarylmethyl radicals, semiquinones, nitroxides, Cu2+ complexes, iron–sulfur complexes, and radicals in irradiated solids. In fluid solution, motion provides additional relaxation mechanisms. Analysis of T2 in solution is a powerful tool to elucidate motion. Experiments as a function of both temperature and resonance frequency are key to distinguishing between relaxation mechanisms.