Band Shape and Phonon Broadening ofUBands in Alkali Halides

The remarkable similarity of the $U$ band originating from the first electronic transition of the ${\mathrm{H}}^{-}$ or ${\mathrm{D}}^{-}$ centers in an alkali halide to the $F$ band in the corresponding alkali halide is pointed out. The band shape and phonon broadening of $U$ bands in alkali halides of both NaCl and CsCl structures have been investigated by the method of moments employing a configurational coordinate model. It is found that the broadening is caused primarily by band-mode optic phonons. The interacting effective phonon frequencies for the ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ centers in a given alkali halide are thus given by ${\left({\omega }_e\right)}_{\mathrm{H}}\gtrsim {\left({\omega }_e\right)}_{\mathrm{D}}$. The Huang-Rhys factor is found to be two to three times larger than the corresponding quantity for the $F$ band. The isotope shift upon deuteration can be explained by the present model without invoking the participation of localized vibrational modes. The positions of the $U$ emission bands have also been predicted.