Ultrasonic attenuation measurements on crystals which display persistent nonphotochemical ir spectral hole burning

Temperature-dependent ultrasound-absorption measurements have been used to identify an ${\mathrm{ReO}}_4$${\mathrm{}}^{\mathrm{-}}$-induced loss peak in alkali halide crystals. The measured shape is consistent with an activation process with barrier energies of 60 meV for RbBr, 50 meV for KI, and 30 meV for RbI. The temperature dependence of the hopping time is compatible with the disappearance of the persistent ir spectral features of ${\mathrm{ReO}}_4$${\mathrm{}}^{\mathrm{-}}$ as the temperature is increased to 10 K.