Role of collective effects in rare earth ion VUV spectra formation in crystals

The evolution of VUV spectra in BaY2F8 crystals as far as yttrium ions are substituted by rare earth element ions of the yttrium subgroup has been investigated. Transformations into pair d-states corresponding to r.e.e. ions being the nearest neighbours in the yttrium Brave lattice take part in spectra formation with comparatively small concentrations of substituting ions. At higher concentrations the character of a spectrum is determined by the presence or absence of exciton-like f-d states splitting from the conductivity zone