Nuclear Spin-Lattice Relaxation in the Rotating Frame

We have developed the formalism of spin-lattice relaxation theory in a nuclear spin system subjected to rf irradiation either close to or far from resonance. In the former case, when there is a single spin temperature in the rotating frame, one obtains simple formulas for relaxation through a spin-lattice coupling of correlation time τcT₂ or τcT₂, but not τc~T₂. In the latter case, when there are two different spin temperatures, a single formula describes Zeeman relaxation irrespective of the relative values of τc and T₂. Some applications of the theory to the study of slow motions in solids are described. Experiments on the proton relaxation in paradi-iodobenze confirm the main predictions of the theory.