Gaussian to Exponential Crossover in the Attenuation of Polarization Echoes in NMR

GONZALO USAJ; PASTAWSKI, HORACIO M; R LEVSTEIN, PATRICIA

MOLECULAR PHYSICS

Año: 1998 vol. 95 p. 1229 - 1229

0026-8976

An ingenious pulse sequence devised by Zhang, S., Meier, B. H., and Ernst, R. R., 1992, Phys.Rev. L ett., 69, 2149 reverses the time evolution (`spin di usion’ ) of the local polarization in adipolar coupled 1H spin system. This refocusing originates a polarization echo, whose ampli-tude attenuates by increasing the time t R elapsed until the dynamics are reversed. Di erentfunctional attenuations are found for a set of dipolar coupled systems: ferrocene, (C5H5)2Fe,cymantrene, (C5 H5 )Mn(CO)3, and cobaltocene, (C5H5)2Co. To control a relevant variableinvolved in this attenuation a pulse sequence has been devised to progressively reduce thedipolar dynamics. Since it reduces the evolution of the polarization echo it is referred to as theREPE sequence. Two extreme behaviours were found while characterizing the materials. Insystems with a strong source of relaxation and slow dynamics the attenuation follows anexponential law (cymantrene). In systems with strong dipolar dynamics the attenuation ismainly Gaussian. By the application of the REPE sequence the characteristic time of theGaussian decay is increased until the presence of an underlying dissipative mechanism isrevealed (cobaltocene). For ferrocene, however, the attenuation remains Gaussian withinthe experimental timescale. These two types of behaviour suggest that the many-body quan-tum dynamics present an extreme intrinsic instability which, in the presence of small perturba-tions, leads to the onset of irreversibility. This experimental conclusion is consistent with thetendencies displayed by the numerical solutions of model systems.