4fn→4fn−15dtransitions of the light lanthanides: Experiment and theory

The ${4f}^n\to {4f}^{n-1}5d\left(\mathrm{fd}\right)\mathrm{}$ excitation spectra of the light lanthanides $({\mathrm{Ce}}^{3+},$ ${\mathrm{Pr}}^{3+},$ ${\mathrm{Nd}}^{3+},$ ${\mathrm{Sm}}^{3+},$ and ${\mathrm{Eu}}^{3+})$ incorporated in ${\mathrm{LiYF}}_4,$ ${\mathrm{CaF}}_2,$ and ${\mathrm{YPO}}_4$ are investigated in the ultraviolet and vacuum-ultraviolet spectral region (100–250 nm). In these host lattices fine structure (zero-phonon lines and vibronic lines) is observed for fd transitions involving the lowest $5d$ crystal-field state. The observation of zero-phonon lines makes it possible to analyze the complicated structure in the fd spectra and to compare the experimentally observed spectra with energy-level calculations for the ${4f}^{n-1}5d$ states. Energy-level and intensity calculations were performed by an extension of the commonly used theory for energy-level calculations of ${4f}^n$ states. A good agreement between experiment and theory is obtained for the overall structure using the crystal-field splitting (from the spectra of ${\mathrm{Ce}}^{3+}),$ the parameters for the splitting of the ${4f}^{n-1}$ core (from the literature on energy level calculations for ${4f}^n$ states) and the spin-orbit coupling of the $5d$ electron and Coulomb interaction between $4f$ and $5d$ electrons (from atomic parameters using the computer code of Cowan).