Magnetic Resonance Line Shapes at the Onset of Saturation

Abstract

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\ifmmode^\circ\else\textdegree\fi{}K and 215\ifmmode^\circ\else\textdegree\fi{}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 $\ensuremath{\delta}H$, 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 $\ensuremath{\delta}H$ occurs at appreciably higher saturation levels than is the case in Li and Al.