Time-Reversal Experiments in Dipolar-Coupled Spin Systems

By applying a suitable sequence of strong rf fields, a system of dipolar-coupled nuclear spins can be made to behave as though the sign of the dipolar Hamiltonian had been reversed. The system then appears to develop backward in time, and states of nonequilibrium magnetization can be recovered in systems which would superficially appear to have decayed to equilibrium. This behavior is consistent with dynamical and thermodynamical principles, but shows that the spin-temperature hypothesis must be employed with caution. The theory of the timereversal phenomenon is discussed, including the practical limitations on the accuracy with which it can be achieved. Various echo experiments in the laboratory and in the rotating frame are reported. The application of repeated time reversals to the problem of high-resolution NMR in solids is discussed.