NMR line narrowing by means of rotary spin echoes

The dipolar broadened F19 resonance line in Teflon has been narrowed by irradiating the solid with strong continuous rf magnetic fields under magic-angle conditions. The transverse magnetization in the rotating frame lost under the action of rf and dc magnetic field inhomogeneities has been recovered by means of rotary spin echoes produced by dc magnetic field pulses. The variation of the longitudinal relaxation with direction and strength of the effective magnetic field in this compound has also been measured

Transverse-magnetization recovery in the rotating frame

The transverse relaxation of F19 nuclei in Teflon in the rotating frame at exact resonance has been studied by using rf fields large compared to the local field in this solid. Various pulse sequences are explored which serve to trace out the decay of the magnetization in the rotating frame and, further, to recover the magnetization lost under the action of secular dipolar terms and of the inhomogeneity of the rf magnetic field. It is found theoretically, and partially confirmed experimentally, that the rotary free-induction decay can be refocussed even after the spin system has presumably attained a steady state in the rotating frame, contrary to the assumption of the spin-temperature approximation