The lowest 4f-to-5d and charge-transfer transitions of rare earth ions in YPO4 hosts

In order to study the high-energy optical transitions i.e. 4f-to-5d (f-d) and charge-transfer (CT) spectra of trivalent rare earth (4fn-series, lanthanide) ions in crystal lattices, the luminescence excitation spectra of the series ions in YPO4 host lattices are measured in a VUV-UV spectral region of 120-350 nm. The f-d bands are observed with all of the series ions except for the ions with the transition energy higher than the host lattice absorption edge i.e. except for Gd3+ and Yb3+, and the CT bands are observed except for Ce3+, Gd3+ and Tb3+ (Pm is unavailable). Joergensen's formulation which has elucidated the systematic variation of the energies of the f-d and CT transitions through the 4fn-series is modified in a form of a single-electron scheme. The many-electron effects in the scheme are calculated and presented as a numerical table. The energies of the observed f-d and CT bands fit well with those calculated by the use of this table