Nuclear-Magnetic-Resonance Line Narrowing by a Rotating rf Field

A nuclear-magnetic-resonance method is explored, which effectively attenuates the dipolar interaction in solids. The experimental technique corresponds to the observation of a free-induction decay in a frame of reference rotating with the frequency of an applied rf field. When the amplitude $H_1$ of this field is much greater than the local field in the solid, and when its frequency is appropriately chosen, the secular part of the dipolar interaction is removed. As a result the rotary saturation line is extremely narrowed. At smaller values of $H_1$, nonsecular terms in the dipolar interaction come into play and contribute to line broadening. These nonsecular effects are investigated both theoretically and experimentally. All the measurements were made in single crystals of calcium fluoride. The calculation of the nonsecular contribution to the line width utilizes the unitary transformation method of Jordhal and Pryce. Theory and experiment are in good agreement.