The absolute intensities of transitions occurring in the optical spectra of single crystals of ${\mathrm{Y}}_2$${\mathrm{O}}_3$ doped with small amounts of Pr, Nd, Eu, Er, and Tm; single crystals of La${\mathrm{F}}_3$ doped with small amounts of Pr and Nd; and single crystals of ${\mathrm{Er}}_2$${\mathrm{O}}_3$, ${\mathrm{Tm}}_2$${\mathrm{O}}_3$, and ${\mathrm{Yb}}_2$${\mathrm{O}}_3$ have been measured at room temperature. All observed transitions occur within the ground configurations of the trivalent rare-earth ions, and their intensities are accounted for using three phenomenological parameters for each system, whose values are determined by a a least-squares fitting calculation. A calculation of the relevant crystal-field terms is made, and theoretical values for the intensity parameters are calculated using free-ion radial wave functions and certain closure approximations. It is concluded that excited $4f^{n-1\mathrm{}}$ $g$ configurations contribute to observed intensities more than is indicated by free-ion calculations, and that the excited $4f^{n-1\mathrm{}}5d$ configuration contributes to the observed intensities an order of magnitude less than indicated by free-ion calculations. A hypersensitive transition in the neodymium systems is observed and its sensitivity is traced to a simple change in point symmetry of the host.

• Received 18 November 1965

DOI:https://doi.org/10.1103/PhysRev.145.325