Zero‐Phonon Transitions of Eu⁺⁺ ‐ and Sm⁺⁺ ‐Doped Alkali Halides

The theory of the electron‐phonon coupling for impurity centres in ionic crystals is reviewed in order to point out the most significant parameters involved in the optical properties of the zero‐phonon lines. The absorption and emission spectra of alkali halides doped with Eu ++ and Sm ++ have been measured for temperatures ranging from 15 up to 175°K. In general the analysis of the experimental data agrees fairly well with the theory. The Huang‐Rhys factor has been determined and also the effective frequencies which are responsible for the thermal behaviour of the spectra. However the electron‐phonon interaction alone has been found not to be sufficient in order to explain the thermal dependence of the zero‐phonon line strength and an additional mechanism for its thermal de‐excitation has been proposed.