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It is demonstrated that it is possible to reverse the time evolution of polarization in a dipole-coupled nuclear spin system (by refocusing the ``spin diffusion'' process), leading to the appearance of a new type of echo, called polarization echoes. The polarization echoes for abundant I spins can be measured conveniently by a two-step polarization-transfer process from and to rare S spins that allows a local perturbation and local detection of the I-spin polarization. Polarization echoes have been recorded for a single crystal sample of ferrocene.
Zhang et al. (Mon,) studied this question.
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