NUCLEAR-MAGNETIC-RESONANCE POLARIZATION AND COHERENCE ECHOES IN STATIC AND ROTATING SOLIDS

The mechanisms of defocusing and refocusing of spin order in extended dipolar coupled nuclear spin systems are investigated by experiments o n static and on rotating solids. It is demonstrated that polarization or coherence echoes are possible also under magic-angle sample spinnin g. The dipolar interactions,: averaged by the spinning, are recovered by rotor-synchronized multiple-pulse sequences. By a simple modificati on of the pulse sequences, it is possible to reverse the sign of the e ffective dipolar Hamiltonian and to induce the refocusing of polarizat ion or coherence. The creation of multiple-spin order in the course of polarization evolution or free precession is monitored by a modified echo experiment. Experimental results for a polycrystalline sample of calcium formate are presented. (C) 1996 American Institute of Physics.