Crystal-field analysis for RE3+ ions in laser materials: II. Absorption spectra and energy levels calculations for Nd3+ ions doped into SrLaGa3O7 and BaLaGa3O7 crystals and Tm3+ ions in SrGdGa3O7

Graphical abstract: In this paper we report a detailed analysis of spectroscopic data obtained from low temperature absorption spectra, which enabled assignment of energy levels, and subsequently their analysis in terms of the free-ion and crystal field (CF) parameters. Highlights: → Polarized absorption spectra measured for Nd3+ and Tm3+ ions in ABC3O7 crystals. → Energy levels analyzed in terms of the free-ion and crystal-field (CF) parameters. → The combined ADS/SPM procedure have been successfully applied. → The B-bark parameters and the power law exponents tk of SPM model are determined. → The energies of levels are important for evaluation of the emission cross-section. - Abstract: Low temperature polarized absorption spectra are analyzed to achieve assignments of energy levels for Nd3+ and Tm3+ ions at monoclinic Cs site symmetry in ABC3O7 crystals. Based on the concept of average optical center, the experimental energy levels for single crystals of SrLaGa3O7:Nd3+ (SLG:Nd), BaLaGa3O7:Nd3+ (BLG:Nd), and SrGdGa3O7:Tm3+ (SGG:Tm) were analyzed in terms of the free-ion parameters and the crystal field (CF) ones, Bkq. Assignments of the energy levels resolved in the spectra were done in stages applying the ascent/descent in symmetry method in CF analysis. The actual monoclinic Cs site symmetry at the metal centers in ABC3O7 crystals and the approximated orthorhombic C2v and tetragonal C4v symmetry were considered. The starting values of Bkq's for SLG:Nd and BLG:Nd crystals were obtained from superposition model (SPM) analysis. The final fitted crystal field parameters show high compatibility with the existing data for structurally similar ion-host systems. The obtained values of the intrinsic parameters provide basis for SPM analysis of CF parameters for rare earth ions in other similar systems, especially those exhibiting low-symmetry sites. The SPM parameters derived for SLG:Nd are used for simulation and assignment of the energy levels involved in the potential laser transitions at about 1800 nm due to Tm3+ ions in SGG crystals. The evaluated emission cross-section is about two times lower than that obtained previously.