Optical properties of single crystals of some rare-earth trifluorides, 5-34 eV

The reflectances of single crystals of the trifluorides of La, Ce, Pr, Nd, Dy, and of polycrystalline Gd${\mathrm{F}}_3$ were measured in the 10-34-eV region, along with 5-9-eV transmission measurements on films of these materials. Localized $4f\to 5d$, $6s$ transitions on the rare-earth ions give rise to the absorption below 10 or 11 eV. Strong interband absorption gives anisotropic reflectance peaks in the 10-15-eV region, presumably from ${\mathrm{F}}^{-}$ $2p$ valence bands to lanthanide $5d$, $6s$ (and other states) conduction bands, accounting for about one-third of the expected ${\mathrm{F}}^{-}$ $2p$ osillator strength. Transitions from lanthanide $5p$ levels cause two types of spectra beginning around 20 eV: transitions of the $5p\to 5d$ type, apparently localized on lanthanide ions, but with some an isotropy; and transitions of $5p$ electrons to the conduction band, giving a peak of very high reflectance at energies above 27 eV.