Sharp resonance modes of cubic symmetry rare-earth ions in CaF2-type crystals

The inclusion of NaF or LIF into rare-earth (R3+) doped CaF2 and SrF2 incre ases the concentration of isolated R3+ ions to the point where these cubic symmetry centers dominate the defect site distribution. The optical spectra of these centers are noteworthy in revealing sharp vibronic lines that hav e intensities and line widths comparable to their parent zero-phonon transi tion. The vibronic frequency, of approximately 140 cm(-1), is observed to d ecrease along the rare-earth series. It is proposed that these vibronic tra nsitions are associated with low-frequency resonance modes of the heavy tri valent R3+ ions substituting for the divalent cations. The loose packing of the smaller radius R3+ ions contributes to the weak coupling to the host g iving rise to the sharp vibronic transitions. (C) 1999 Elsevier Science B.V . All rights reserved.