Vibronic Transitions of Trivalent Er and Ce in BaY2F8 Single Crystals

High resolution (0.02 cm^?1) Fourier transform spectroscopy was applied in the 9–300 K and 100–24,000cm^?1 ranges, respectively, to detect in Er^3? and Ce^3? doped BaY₂F₈ single crystals (1) the narrow line spectra due to the intraconfigurational 4f→4f transitions of the rare earths (RE) and (2) the possible vibronic tails accompanying the zero-phonon lines. The ²F_5/2→²F_7/2 transition was monitored for the Ce^3?-doping and the crystal field splitting of the initial and final manifold was determined. Weak vibronic spectra accompanying six among the nine investigated 4f→4f transitions of Er^3? and the ²F_5/2→²F_7/2 transition of Ce^3? were detected. The vibronic spectra amplitude was found to scale with the RE contents. On the basis of the IR- and Raman-active vibrational modes, either measured or quoted in the literature, most of the vibronic lines could be successfully assigned to the lattice modes. Violations of the selection rules were envisaged and discussed.