The role of collective effects in the formation of V ultraviolet spectra of rare-earth ions in crystals

The evolution of V ultraviolet spectra of $BaY_2F_8$ crystals as the yttrium ions are replaced by ions of rare-earth elements of the ytterbium subgroup is investigated. Even transitions into the paired d-states, which correspond to the rare-earth ions which are close neighbors in the Bravais lattic yttrium, take part in the formation of the spectrum at comparatively low concentrations of the substituting ions. At greater concentrations the nature of the spectrum is determined by the presence or absence of exciton-like f-d states which are split off from the conduction zone.