Coupling betweenH−Localized Modes and Rare-Earth Ion Electronic States in Rare-Earth Trifluorides

Abstract

Crystals of La${\mathrm{F}}_3$, Ce${\mathrm{F}}_3$, Pr${\mathrm{F}}_3$, and Nd${\mathrm{F}}_3$ doped with hydrogen and deuterium have been studied spectroscopically. Two strong polarized fundamentals and their combinations have been observed in the infrared for both ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$. The fundamentals appear also polarized in the vibronic spectrum of doped Nd${\mathrm{F}}_3$ coupled to several electronic transitions. Extra electronic lines appearing only in the doped crystals occur on the long-wavelength side of the usual rare-earth electronic transitions. From frequency differences, these extra levels are the parent states for the local mode vibronic transitions. The displacement of the extra electronic lines from the usual electronic transitions is mainly due to a changed crystalline field and covalency arising from the replacement of F by H, and is greater at higher levels. In addition the extra electronic levels have slightly different frequencies for hydrogenated and deuterated crystals. This isotope shift depends in both magnitude and sign on the particular electronic level and ranges from 0.5 ${\mathrm{cm}}^{-1\mathrm{}}$ (for transitions to ${}_{}{}^4{}_{}{}^{}\mathrm{F}_{3/2}^{}{}_{}{}^{}$) to -2.0 ${\mathrm{cm}}^{-1\mathrm{}}$ (for transitions to ${}_{}{}^4{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$). This effect is accounted for by a large difference in zero-point amplitude for ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ localized modes which, through the electron-vibration interaction, perturbs each electronic level to a different extent.