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In spectroscopy, it is conventional to treat pulses much stronger than thelinewidth as delta-functions. In NMR, this assumption leads to the predictionthat pi pulses do not refocus the dipolar coupling. However, NMR spin echomeasurements in dipolar solids defy these conventional expectations when morethan one pi pulse is used. Observed effects include a long tail in the CPMGecho train for short delays between pi pulses, an even-odd asymmetry in theecho amplitudes for long delays, an unusual fingerprint pattern forintermediate delays, and a strong sensitivity to pi-pulse phase. Experimentsthat set limits on possible extrinsic causes for the phenomena are reported. Wefind that the action of the system's internal Hamiltonian during any real pulseis sufficient to cause the effects. Exact numerical calculations, combined withaverage Hamiltonian theory, identify novel terms that are sensitive toparameters such as pulse phase, dipolar coupling, and system size.Visualization of the entire density matrix shows a unique flow of quantumcoherence from non-observable to observable channels when applying repeated pipulses.

The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses