The shape of NMR absorption and cross-relaxation spectra in a heteronuclear spin system

A dynamic theory of heteronuclear spin systems in solids at high temperatures is developed. A system of nonlinear integral equations for the spin time correlation functions is derived in the self-consistent fluctuating local field approximation taking into account corrections for the correlated local field fluctuations in real crystal lattices. The theory is applied to interpretation of the experimental data available for a LiF crystal representing a system with the nuclei of two types. The signals of free precession for Li and F nuclei, as well as the spectra of harmonic cross-relaxation, cross-polarization of the 6Li isotope, and depolarization of the 8Li isotope, were calculated for the magnetic field oriented along the principal crystallographic axes of LiF. The results of calculations show good agreement with experiment.