Localized modes in crystals and sharp details of the optical absorption spectra

Abstract

A theory is put forward in qualitative terms to account for the sharp nibs frequently observed at the long wavelength end of the spectral lines of ionic crystals, in which optical absorption by a cation has taken place. The broad band is primarily due to excitation of the vibrational modes of the crystal by the Franck-Condon strain; nibs are supposed to be caused by localized modes introduced in the crystal either by the differing mass or by a change in the force constants of the absorbing ion. Experimental evidence in rocksalt-type crystal indicates the presence of T_2g -modes of localized vibrations.