Mass Independence ofU-Center-Induced Phonon Resonances in KI

Low-temperature thermal-conductivity measurements have been made on KI containing $U$ centers, with both ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ impurity ions being studied. A phonon resonance at 6×${10}^{12}$ rad/sec is observed for both impurities; thus, there is no observable impurity mass effect. This frequency is virtually identical with the splitting between the main infrared $U$-center peak and the high-frequency sidebands in KI. In order to fit the data it was necessary to introduce a temperature-dependent term into the resonance scattering cross section.