A simple model for $f\to d$ transition of heavy lanthanide and actinide   ions in crystals
  The $f\to d$ transition model by Duan and co-workers [Phys. Rev. B {\bf 66}, 155108 (2002); J. Solid State Chem. {\bf 171}, 299 (2003)] has been very useful in interpreting the $f\to d$ absorption, emission and nonradiative relaxation of light lanthanide ions in crystals. However, based on the assumption that the $f^{N-1}$ core spin-orbit interaction is weak compared to $f\to d$ exchange interaction, this model, in the original form, is not applicable to interpretation of the $f\to d$ transitions of heavy lanthanide ions or actinide ions in crystals. In this work the model is extended to cover the cases of heavy lanthanides and actinides, where the spin-orbit interaction of $f$ orbitals may be stronger than the $f-d$ exchange nteraction.   Keywords: lanthanide; f-d transition; model; spectrum; actinide; VUV; 
Chang-Kui Duan, Michael F. Reid and Gang Ruan