Polarization echoes in NMR

Abstract

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.