Optical Properties of SomeF-Aggregate Centers in LiF

Abstract

Detailed studies have been made of the emission and associated excitation and absorption spectra of x-irradiated LiF. By studies of changes in the spectra during periods in the dark and during subsequent reirradiation, and by use of the Mollwo-Ivey relationships, it has been possible to identify optical bands associated with the ${F_2}^{+}$ and the ${F_3}^{+}$ centers. The ${F_2}^{+}$ center has an absorption band at 645 mμ and an emission band at 910 mμ, while the ${F_3}^{+}$ center has an absorption band at 458 mμ and an emission band at 528 mμ. The ${F_3}^{+}$ absorption band (458 mμ) is one of several bands underlying the $M$ band (441.2 mμ). It has been possible to induce transformations from the ionized $F$-aggregate centers to the $F$-aggregate centers, but it has not been possible to bring about the reverse transformations. The vibrational frequency as determined from the temperature dependence of the half-width of the pure $M$ band was found to be close to the frequencies obtained from other bands. This result suggests that the centers associated with these bands are all interacting with the same normal lattice modes.