Zero‐Phonon Lines of Color Centers in LiF and NaF

The optical absorption of LiF and NaF irradiated by electrons or neutrons is measured at liquid He temperature and above. In these crystals zero‐phonon lines are observed on the F‐aggregate centers. From the temperature dependence of the relative intensities of zero‐phonon lines and broad bands, Debye temperatures are deduced using the Debye approximation, and the semi‐local mode approximation based on the Debye model. Debye temperatures obtained by this method are different in each F‐aggregate center. The coupling between the trapped electrons in the lattice defects and the lattice waves is discussed and interpreted in terms of both the local and normal modes.