Phonon and Photon Paramagnetic-Response Line Shapes in Ruby

Acoustic-paramagnetic-resonance (APR) and electron-paramagnetic-resonance (EPR) line shapes have been measured in ruby for various ${\mathrm{Cr}}^{3+}$ concentrations. The second-moment theory of Pryce and Stevens is shown to be applicable to APR as well as EPR, predicting the same second moments $〈\Delta {\nu }^2〉$ for both APR and EPR transitions. Experimentally, the APR and EPR line shapes are indeed identical for the same transition; however, the theory predicts a larger $〈\Delta {\nu }^2〉$ than measured, because the larger Cr-Cr interactions do not appear in the observed line. A qualitative calculation of $〈\Delta {\nu }^2〉$ from the known interactions supports this conclusion.