Karl W. Krämer, Hans U. Güdel and Robert N. Schwartz
Infrared-to-visible upconversion in LaCl₃: 1% Er³⁺: Energy-level and line-strength calculations
Phys. Rev. B 56, 13830-13840 (1997)                 

Abstract: Single crystals of LaCl₃:1% Er³⁺ were grown by the Bridgman technique. Er³⁺ substitutes for La³⁺ and has a site symmetry of C_3h in LaCl₃. Seventy-three energy levels of Er³⁺ were determined up to 45000 cm^–1 from absorption, excitation, and luminescence spectra measured over the temperature range 4.2-293 K. A correlated crystal-field analysis was done by fitting 16 atomic, four crystal-field, and one correlated crystal-field parameters to the experimental Stark levels. The fit is excellent with a standard deviation of 9.0 cm^–1. On the basis of the wave functions thus obtained the oscillator strengths for the transitions from the ⁴I_15/2(1) ground state to all the levels up to 45000 cm^–1 were calculated by fitting seven complex A^l_t,p intensity parameters to 47 observed oscillator strengths. The intensity distribution among the crystal-field components was then calculated for the inter-excited-state transition ⁴I_11/2 ⁴F_7/2 which plays an important role in an excited-state-absorption upconversion process.