Gaussian to exponential crossover in the attenuation of polarization echoes in NMR

An ingenious pulse sequence devised by Zhang, S., Meier, B. H., and Ernst, R. R., 1992, Phys. Rev. Lett., 69, 2149 reverses the time evolution ('spin diffusion') of the local polarization in a dipolar coupled H-1 spin system. This refocusing originates a polarization echo, whose amplitude attenuates by increasing the time t(R) elapsed until the dynamics are reversed. Diffe rent functional attenuations are found for a set of dipolar coupled systems : ferrocene, (C5H5)(2)Fe, cymantrene, (C5H5)Mn(CO)(3), and cobaltocene, (C5 H5)(2)Co. To control a relevant variable involved in this attenuation a pul se sequence has been devised to progressively reduce the dipolar dynamics. Since it reduces the evolution of the polarization echo it is referred to a s the REPE sequence. Two extreme behaviours were found while characterizing the materials. In systems with a strong source of relaxation and slow dyna mics the attenuation follows an exponential law (cymantrene). In systems wi th strong dipolar dynamics the attenuation is mainly Gaussian. By the appli cation of the REPE sequence the characteristic time of the Gaussian decay i s increased until the presence of an underlying dissipative mechanism is re vealed (cobaltocene). For ferrocene, however, the attenuation remains Gauss ian within the experimental timescale. These two types of behaviour suggest that the many-body quantum dynamics present an extreme intrinsic instabili ty which, in the presence of small perturbations, leads to the onset of irr eversibility. This experimental conclusion is consistent with the tendencie s displayed by the numerical solutions of model systems.