Multiple Spin Echoes in Dipolar Solids

The decay of transverse nuclear spin magnetization in a homogeneously broadened solid can be prolonged for a time T₂^T₂ by applying a train of 90^∘{} pulses in a modified Carr-Purcell sequence with pulse spacing τ<T₂. A time-expansion theory which treats the problem from the viewpoint of spin echoes is used to explain an observed proportionality between T₂^ and τ^-5, and predicts that T₂^ depends upon a modified sixth moment of the resonance. An alternative viewpoint, in which one considers the long-time development of the spin system in a periodically modulated rf field H₁(t), shows that the experiment is closely related to spin locking and explains the fact that T₂^→T_1ρ(H̄₁), the relaxation time parallel to the average rf field, as τ→0. The experiment offers some possibilities for the study of spin-lattice relaxation in weak fields and for double resonance in the rotating frame.