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). Jørgensenʹs 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.