Theory of isotopic effects in the optical spectra of lanthanide ions in crystals

Physical mechanisms contributing to the isotopic structure of spectra of the insulating activated crystals induced by the isotopic disorder in the host crystal or in a system of the impurity optical centers have been considered. The microscopic theory of the geometrical anharmonic isotopic effect is presented. We have shown that the local deformation due to a mass defect in the crystal lattice is dominant in the formation of the fme specffic spectral structure in crystals with the proper inhomogeneous isotopic composition (such as 6Li7LiiYF4:Ho with the natural abundance of c 7.42% of the 6Li isotope). It contributes essntially to the observed shifts of the spectral lines in the case of different masses of the active optical centers (such as 164Er, 166Er, '68Er and 170Er isotopes in the LiYF4 crystal) as well. Detailed calculations of crystal field parameters and of their first and second derivatives with respect to the lattice strains for the impurity lanthanide ions in lithium-yttrium double fluorides were performed within the framework of the exchange charge model and used in the estimations of the different contributions to the isotope-induced shifts of the Stark sublevels.