4fn→4fn-15d transitions of the heavy lanthanides:: Experiment and theory -: art. no. 045114

The 4f(n)-->4f(n-1)5d(fd) excitation spectra of the heavy lanthanides (Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+) incorporated in LiYF4. CaF2, and YPO4 are investigated in the ultraviolet and vacuum-ultraviolet spectral region (100-275 nm). Spin-forbidden transitions as well as spin-allowed transitions are observed for all heavy lanthanides. In the excitation spectra the crystal-field splitting of the 5d electron can be clearly observed. Fine structure (zero-phonon lines and vibronic lines) is observed for the transition to the lowest 5d crystal-field component, for both the high-spin and low-spin fd bands, Energy-level and intensity calculations are performed by an extension of the commonly used model for energy-level calculations of 4f(n) states. A good agreement between experimental and simulated spectra is obtained, using parameters that describe the 5d crystal-field splitting (from the spectra of Ce3+). the parameters for the splitting or the 4f(n-1) core (from the literature on energy-level calculations for 4f(n) states) and parameters for the spin-orbit coupling of the 5d electron and the Coulomb interaction between 4f and 5d electrons (from atomic ab initio calculations using the computer code of Cowan). To improve the agreement between the model and experiment, the 5d crystal-field parameters were adjusted slightly to correct for the decreasing crystal-field strength for the heavier rare earths due to the lanthanide contraction. The f-d interaction parameters in the fluoride host lattices were reduced to about 67% of the calculated free-ion values in order to compensate for the nephelauxetic effect.