Magnetic Resonance Line Shapes at the Onset of Saturation

Magnetic resonance absorption line shapes and widths in Li metal and Ca${\mathrm{F}}_2$ crystals have been measured as a function of radio-frequency power level. The measurements have been made on the ${\mathrm{Li}}^7$ resonance in Li at 77°K and 215°K and on the ${\mathrm{F}}^{19}$ resonance in Ca${\mathrm{F}}_2$ at room temperature. The measurements have been made from low rf power into the region of appreciable saturation, but not into the very high saturation region considered by Redfield. A decrease in $\delta H$, the peak-to-peak absorption derivative line width, was observed in both systems at sufficiently large values of $H_1$, the rf magnetic field. The changes of line width in Li at various values of $T_1$ and of characteristic motional frequency of the nuclei show good qualitative agreement with the criteria proposed in the recent theory of Tomita. Line-width data obtained by Redfield in aluminum are included for comparison. In the case of Ca${\mathrm{F}}_2$, where the spin-lattice relaxation proceeds via paramagnetic impurities, the decrease in $\delta H$ occurs at appreciably higher saturation levels than is the case in Li and Al.