Time-reversal of the evolution of a dipole-coupled, many-spin system undercontinuous resonant irradiation

Time-reversal of the evolution of a dipole-coupled, many-spin system under continuous resonant excitation with a radio-frequency (rf) field of arbitra ry amplitude is demonstrated in solid-state H-1 nuclear magnetic resonance (NMR) experiments on polycrystalline adamantane. Time-reversed evolution is accomplished with an rf pulse sequence that generates an effective nuclear spin Hamiltonian that includes both dipole-dipole coupling and rf interact ion terms, with signs opposite to those in forward evolution. The amplitude of the effective continuous rf field is varied by varying the phases of rf pulses in the sequence. Experiments show echo-like NMR signals under time- reversed evolution after forward evolution to an apparent quasieguilibrium state under continuous rf excitation. Such echolike signals are inconsisten t with the hypothesis of spin temperature in the rotating frame, according to which the approach to quasiequilibrium under continuous rf excitation is an irreversible process. The use of this time-reversed evolution in multip le quantum (MQ) NMR spectroscopy is also demonstrated. MQ NMR spectra obtai ned with increasing excitation times exhibit a partial confinement of nucle ar spin order to zero- and one-quantum operators. This novel behavior is sh own to be a consequence of energy conservation.