Spectrum of Er3+ in LaCl3

The energy levels of the Er³⁺ ion in LaCl₃ are calculated by finding the eigenvalues of the matrices for the combined Coulomb, spin—orbit, and crystal-field interactions within the f¹¹ configuration. For suitably chosen values of the three Slater integrals and the spin—orbit coupling constant the root-mean-square deviation between the calculated and the experimental positions of 20 terms is 140 cm^-1. Crystal-field parameters can be chosen which reduce the root-mean-square deviation between the calculated and the measured splittings of 72 levels to 3.84 cm^-1. Zeeman splitting factors in the direction parallel to the crystal axis are also calculated. The results for Er³⁺ in LaCl₃ are compared with those for Nd³⁺ in LaCl₃.