Spin-lattice relaxation of the nuclear magnetic system of a solid with multispin interactions induced by a modulated strong rf field

摘要

When a NMR line of a solid narrows at the "magic" orientation of the effective field in the rotating frame, the linewidth is determined by a three-spin effective interaction, rather than by the original dipolar interaction. When the line is narrowed further by a second effective field acting in the doubly rotating reference frame, the spin dynamics are now determined by four- and five-spin effective interactions. The Hamiltonian for these interactions is found. An expression for the longitudinal nuclear spin-lattice relaxation time in the third effective field acting in the triply rotating reference frame is found. This expression consists of terms with two-, three-, four-, and five-particle correlation functions of products of the dipolar coupling constants modulated by the atomic and molecular motions. The dependence of this time on the correlation frequencies of the motions is calculated in the approximation of high-dimensionality lattices for different models of the motion and values of the rf field parameters. The high sensitivity of the spin-lattice relaxation time in the triply rotating frame to details of the ultraslow internal motions in the solid is demonstrated; measuring it can therefore provide an effective way to investigate such motions. A method is proposed for testing the correlation of the atomic motions directly by comparing experimental plots of the temperature dependence of two-and three-spin relaxations measured when the first and second effective fields, respectively, deviate from their "magic" orientations.