Vacuum-ultraviolet absorption bands of trivalent lanthanides in LaF3

Abstract

The ultraviolet absorption bands of trivalent rare earths in La${\mathrm{F}}_3$ are located by the use of absorption and selective excitation spectra. These absorptions are quite strong arising from the electric dipole transition, which promotes a $4f$ electron from the rare-earth $4f^n$ shell to the $5d$ or some higher-lying orbital. The onsets of the $5d$ absorptions range from about 40 000 ${\mathrm{cm}}^{-1\mathrm{}}$ for cerium to beyond 80 000 ${\mathrm{cm}}^{-1\mathrm{}}$ for lutetium showing a general increase from cerium to gadolinium, a sudden drop from gadolinium to terbium, and another increase. The positions of the bands are compared with estimates by several workers of the $5d$ level position for trivalent free ions. In general the La${\mathrm{F}}_3$ crystal field lowers and broadens the $5d$ level with respect to its freeion counterpart. For the simplest rare earth, cerium, the splitting caused by the La${\mathrm{F}}_3$ crystal field is calculated using a point-charge model. Agreement with experiment is poor, so a two-parameter fit to the splitting is made which indicates that the point-charge model plus the approximation for the radial integrals involved overestimate the crystal-field perturbation on the $5d$ electron by roughly a factor of 3.