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Puzzling anomalies previously observed in multipulse NMR experiments innatural abundance 29Si [A.E. Dementyev, D. Li, K. MacLean, and S.E. Barrett,Phys. Rev. B 68, 153302 (2003)] such as long-lived spin echoes and even-oddasymmetries, are also found in polycrystalline C60. Further experimentscontrolling the phases and tilting angles of the pulse trains, as well asanalytical and numerical calculations allowed us to explain the origin of theseanomalies. We prove that the observation of long magnetization tails requirestwo conditions: i) an rf field inhomogeneity able to produce different tiltingangles in different sites of the sample and ii) the absence of spin diffusion(non-effective flip-flop interactions). The last requirement is easilysatisfied in diluted dipolar solids, where the frequency differences betweensites, caused by disorder or other sources, are usually at least one order ofmagnitude larger than the dipolar couplings. Both conditions lead to thegeneration of stimulated echoes in Carr-Purcell (CP) andCarr-Purcell-Meiboom-Gill (CPMG) pulse trains. We show, both experimentally andtheoretically, that the stimulated echoes interfere constructively ordestructively with the normal (Hahn) echoes depending on the alternation or notof the pi pulse phases in the CP and the CPMG sequences. Constructiveinterferences occur for the CP and CPMG sequences with and without phasealternation respectively, which are the cases where long magnetization tailsare observed. Sequences with two, three and four pi pulses after the pi/2 pulseallow us to disentangle the contributions of the different echoes and show howthe stimulated echoes originate the even-odd asymmetry observed in both 29Siand C60 polycrystalline samples.Comment: 6 pages, 4 figure

Manifestations of the absence of spin diffusion in multipulse NMR experiments on diluted dipolar solids