Temperature-Dependent Linewidth of Excited States in Crystals. I. Line Broadening due to Adiabatic Variation of the Local Fields

The effect of strains on the linewidth of the optical spectra of paramagnetic ions in ionic crystals is briefly discussed and applied to the ${}_{}{}^2{}_{}{}^{}E$ state of ruby. A mechanism for the broadening of spectral lines in crystals at elevated temperatures is introduced. This mechanism assumes that the linewidth is due to "slow" variation in the local electric field of an individual ion. This theory should hold for states which do not interact with the crystal field to first order and is applied to the ${}_{}{}^2{}_{}{}^{}E$ states of ruby. The agreement between theory and experiment is quite good.