4fn→4fn−15dSpectra of Rare-Earth Ions in Crystals

The $4{f}^{n}\ensuremath{\rightarrow}4{f}^{n\ensuremath{-}1}5d$ absorption spectra of rare-earth ions, R${\mathrm{E}}^{2+}$ or R${\mathrm{E}}^{3+}$, in crystals can be fitted with energy gaps in the ground multiplets of the $4{f}^{n\ensuremath{-}1}$ core. Unlike the shielded $4{f}^{n}$ configuration, the $4{f}^{n\ensuremath{-}1}5d$ levels of RE in crystals are therefore very different from those of free ions, and are formed through the interaction between the electron in $5d$ orbitals with the $4{f}^{n\ensuremath{-}1}$ core.