Double Nuclear Magnetic Resonance and the Dipole Interaction in Solids

The nuclear magnetic dipole interactions in a polycrystalline sample of sodium fluoride have been investigated using an extension of the magnetic resonance technique. The resonance absorption of Na²³ was observed with a variable-frequency spectrometer in a fixed external magnetic field, and the mean square width of the line shape computed.This width has two main sources: (a) the interaction of Na²³ nuclei with other Na²³ nuclei through the dipole fields of the magnetic moments, and (b) the interaction of Na²³ nuclei with neighboring F¹⁹ nuclei by the same mechanism. The application of a strong rf field satisfying the resonance condition for F¹⁹ nuclei in the same external field caused rapid transitions between the fluorine Zeeman energy levels, and thus altered the average field produced at the position of sodium nuclei by the nuclear magnetic moments of fluorine. By varying the strength of this rf field and also by varying the deviation from precise resonance of the F¹⁹ spins, one could selectively alter the contributions of this source of line broadening.The available rf field was not sufficiently intense to eliminate entirely the broadening caused by source (b). However, the detailed behavior of this contribution to the width as a function of the deviation from resonance agrees well with a theory of Bloch for the case of the rf field strong compared to the line widths involved.