Theoretical Study Of Site-Selective Effects On The 4f-5d Spectra Of Lanthanide Ions And Spontaneous Lifetime Of Luminescent Ions

This thesis presents the study of site-selective effect of charge-compensated ion on the 4f→5d one-photon spectra of lanthanide ions,and of the spontaneous radiative lifetime(or decay rates)of the luminescent ion in media.My work mainly consists of three parts as follows:a)calculations for 4f→5d excitation spectra of C_4v site-selectrive F^-/H^-/D^- charge-compensated CaF₂:Pr³⁺and CaF₂:Ce³⁺;b)a many-body perturbation calculation of 4f^N←→4f^N-15d electric dipole moments for lanthanide ions;c)a microscopic simulation of the local-field-effect models of the luminescent ions in dielectric media by using Ewald-Kornfeld approach.In the first part,a general introduction to the parameterized Hamiltonian theory is presented.Based on this theory,we calculate the 4f→5d excitation spectra of C_4v site-selective F^-/H^-/D^- charge-compensated CaF₂:Pr³⁺and CaF₂:Ce³⁺by using the extended f-shell program written by M.Reid,and a good match with experiments is obtained.With the calculation results,the peaks in the measured spectra are reassigned,for example,the bands located at 206nm and 105nm are interpreted as a C_4v-site excitation instead of a L-site excitation in the spectra of F^--compensated CaF₂:Pr³⁺.Large energy splittings of E_g and T_2gbands due to the lower of the symmetry from O_h to C_4vare found.Especially for the Eg band the energy splitting is larger,since the E_g orbital lies along the C₄ axis.For the H^-/D^- charge-compensated crystals,the energy splittings of both E_g and T_2gbands become 1.5 times larger than the ones for the F^- compensated crystals,because of larger ions and stronger repulsion with surrounded ions.Moreover,the lattice distortion is semi-quantitatively discussed using the fitting values of the crystal-field parameters for the d electron based on the superposition model of crystal field.In the second part,we introduce the many-body perturbation theory and show how to construct the effective electric-dipole transition operator(extended to the first-order) for free ions.The first-order operator includes:a strong one-body correction term (proportional to the zero-order operator,with a coefficient-δ)and a weak two-body correction term.The one-body part of the effective operator is characterized by an effective radial integral <5d|r|4f>_eff=(1-δ)<5d|r|4f>.Via Relativistic Hartree-Fock ab initial calculation,we provided the values of <5d|r|4f>_effand the two-body correction factors for divalent and trivalent ions across the lanthanide series. The corrected radial integrals are reduced by 35%and 25%or so for trivalent and divalent lanthanide ions,respectively,and the calculated lifetimes by using <5d|r|4f>_effgenerally agree with previous experiments.The most important contribution to the reduction of the radial integral comes from the configuration mixings,i.e.the mixing of 5p⁵4f^N5d¹ into 4f^N.and the mixing of 5p⁵4f^N-15d² into 4f^N-15d¹.The last part mainly presents the microscopic simulation of the local-field effects on the luminescent ions embedded in dielectric media with periodic structure by using Ewald-Kornfeld approach.Under externally applied field,the pure media can be regarded as an periodic structured electric-dipole array.The solution of the local electric field at a certain lattice site involves an infinite summation of long-range interaction.A fast convergence can be achieved as long as a proper coefficientηis chosen in the Ewald-Kornfeld approach.Consequently,the ratios of the local electric field to the macroscopic electric field,f(depended on the reflective index n of the media),in the cases of substitution in a media with a single luminlescent ion and a nano-particle containing luminlescent ions,can be obtained by an iterated procedure. Along with the two famous local-field-effect models,virtual-cavity and real-cavity models,our simulation result is presented and analyzed.We have the following conclusions:a)for the case of single-ion substitution,the curves of f(n)located between the ones of virtual- and real-cavity models.If the substitution induces apparent changes on the polarizability of sun'ounded media ions,f(n)is close to the one of the real-cavity model;or on the contrary,f(n)tends to follow the one of the virtual-cavity model,b)if the luminescent ion is located in a empty cavity,f(n)tends continuously to the one of the real-cavity model when the cavity grows larger and larger.The difference between them is about 10%when the radius of the empty cavity is about 8 times of the lattice constant,c)for the case of nano-particle substitution,the calculation results generally accords with the ones of the so-called modified real-cavity model.Larger size of the nano-particle makes better consistency.