Decay of multispin multiple-quantum coherent states in the NMR of a solid and the stabilization of their intensity profi
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Decay of Multispin MultipleQuantum Coherent States in the NMR of a Solid and the Stabilization of Their Intensity Profile with Time V. E. Zobova and A. A. Lundinb a
Kirensky Institute of Physics, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia email: [email protected] b Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 117977 Russia email: [email protected] Received March 24, 2011
Abstract—Variations, experimentally observed in [14], in the intensity profiles of multiplequantum (MQ) coherences in the presence of two special types of perturbations are explained on the basis of the theory, ear lier developed by the authors, of the growth of the effective size of correlated clusters (the number of corre lated spins) and the relaxation of MQ coherent states [23]. The intensity and the character of perturbation were controlled by the experimenters. It is shown that the observed stabilization of profiles with time is not associated with the stabilization of the cluster size. Quite the contrary, a cluster of correlated spins monoton ically grows, while the observed variations in the intensity profile and its stabilization with time are attributed to the dependence of the decay rate of an MQ coherence on its order (its position in the MQ spectrum). The results of the theory are in good agreement with the experimental data. DOI: 10.1134/S1063776111140111
1. INTRODUCTION The intensive development of experimental meth ods of multipulse NMR spectroscopy of condensed media has led to the formation of multiplequantum (MQ) NMR spectroscopy [1]. The physics behind this spectroscopy is the transformation, by means of sequences of highpower rf pulses, of the original Hamiltonian of internuclear spin–spin interactions into a new Hamiltonian (spin alchemy) under which the original magnetization is transformed into differ ent rather complicated multifrequency time correla tion functions [1–4]. It is these functions that indicate the emergence of MQ states in the spin system. The emerging coherences and their dynamics pro vide a powerful and often irreplaceable means to study the behavior of particles in different systems: their clusterization and the rise of local structures situated, for example, on surfaces, in liquid crystals, in nanosize voids, etc. [5–7]. Depending on the experimental pro gram, MQ coherences may or, generally speaking, may not (see below) be subjected to different addi tional (for example, relaxational) effects. At the final stage of the experiment, a time reversal is performed by means of a magic sandwich, owing to which the order is again transferred to the magnetization [8, 9]. Even if we set aside the abovementioned applied aspects of MQ NMR spectroscopy, which ensure its widest application (from superconductivity studies to medicine), to realize its fundamental value, it suffices
to note that the methods of MQ spectroscopy allow for the experimental study of the development of multi particle
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