Optical spectra and energy levels of divalent ytterbium and samarium ions in some alkali halides

This study is concerned with the measurement and analysis of the optical absorption spectra of divalent samarium (Sm2+) and ytterbium (Yb2+) ions in potassium and rubidium halide crystals. To incorporate the ions into the crystals, the latter were heated in (the appropriate) metal vapour. The optical absorption spectra of the impurity-doped crystals were measured and are ascribed to 4fN+1 → 4fN5d electronic transitions. For the first time, the optical spectra of Sm2+ and Yb2+ ions in RbBr and RbI crystals are reported. In the case of Yb2+ ions in RbX, (X = Cl,Br,I) crystals, intermediate coupling calculations using J1j- coupled basis functions give a satisfactory agreement between experimental and theoretical energy levels of the ions, by assuming that the effective site symmetry of the substitutional impurity ions is octahedral. However, the Slater-Condon and spin-orbit parameters that give the best fit between theory and experiment are reduced by up to 44% from the free-ion values. In the case of Sm2+ ions in KX and RbX crystals, theoretical calculation of the energy levels (assuming octahedral approximation) was based on a model in which the 4f5 core electron states, best described in terms of LS-coupled basis functions, are coupled to the 5d electron states which are strongly perturbed by the crystal field. In this model, the agreement between theoretical and. experimental energy levels is also quite satisfactory. Also found, are sets of absorption bands whose spectral positions and behaviour can best be explained by assuming electronic transitions from the ground states of the ions to the bonding molecular orbitals formed by the overlap of the 6s atomic orbitals of the impurity ions and the (n + 1)s atomic orbitals of the ligand ions. Preliminary investigations of the effect of thermal ageing on the optical spectra of RbBr : Yb2+ crystals suggest that the Yb2+ ions exist in three separate phases (in the crystals) among which is the Suzuki phase