Concentration Study Determination of the 6P, 6I, and 6D Energy Levels of Gd3+ in SrF2 at a C4υ Site

A correlation of changes in the absorption coefficients of the spectra of SrF₂:Gd³⁺ containing from 0.1 to 0.5 mole% doping is used to determine some of the 51 lowest energy levels of Gd³⁺ at a site of $C_{\text{4υ}}$ symmetry. The concentration study was necessary to separate the spectral lines of the tetragonal site from transitions due to ions in other sites. The various sites are the result of the charge compensation that occurs when Gd³⁺ substitutes for Sr²⁺. Previously unreported ${}^{6}I$ and ${}^{6}D$ levels are tabulated and prior work on the ${}^{6}P$ multiplets is supported. The details of a “concentration study” in which the concentrations are unknown are given.