Infrared Absorption byUCenters in CsCl, CsBr, and CsI

The infrared absorption by the localized vibrational modes of ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ centers in CsCl, CsBr, and CsI has been measured in the temperature range 20 to 300°K. Above 100°K, the temperature dependence of the half-widths of the local-mode absorption bands in CsBr and CsI follows a $T^n$ dependence, where $2>n>\mathrm{}1\mathrm{}$ (instead of $n\cong 2$, as is usually the case with the other alkali halides). Side-band structures are observed in CsCl and CsBr. These can be explained as arising from the coupling of the local-mode phonon with band phonons through anharmonic interactions. In the case of CsI, no such side-band structure is noted in the temperature range studied, except for a weak shoulder occurring very close to the main local-mode band.