A. A. Kornienko

Modified theory of f-f transition intensities and crystal field for systems with anomalously strong configuration interaction

A new theory explaining the intensity of f-f transitions and the crystal field using an approximation of a strong configuration interaction is proposed. The theory enables the anomalous influence of excited configurations with charge transfer on some multiplets of the f shell to be taken into account. With the help of this theory, a satisfactory description of the absorption transitions and luminescence branching ratios from 1D2 and 3P0 multiplets for the Pr3+ ion in double molybdates has been achieved for the first time. For further validation the theory, was used to provide a description of Stark splitting of Pr3+ — multiplets in elpasolites and determine the covalence parameters; these parameters were found to be in good agreement with values obtained by the other methods.

Influence of excited configurations on the intensities of electric-dipole transitions of rare-earth ions

The theory of induced electric-dipole transitions of rare-earth ions in crystals and glasses is improved by taking into account the third-order effects of perturbation theory with respect to the energies of virtual excitations of 4f electrons to the 5d states. Since the energy regions of excited 4fN − 15d states are usually superimposed with the charge-transfer bands, the effects caused by a virtual transfer of an electron from the outer shells of ions of the surroundings (ligands) to the unfilled 4fN shells are also considered. The Pr3+, Sm3+, and Eu3+ ions are considered as examples. It is found that some difficulties inherent in the Judd-Ofelt calculation scheme are successfully overcome. The agreement of the calculated results with the experimental data improves.

Transformation of optical properties of crystal media (KGW, YVO4) exposed to quasi-continuous laser radiation in the range of the transmission band of the medium

We present data from an investigation of the properties of the blue (475 nm) and green (520–560 nm) emissions that arise from KGW and YVO4 crystals upon excitation into the transmission band of the medium by continuous infrared laser radiation the power density of which is in the kilowatt range. As distinct from standard luminescence, the two emissions are narrowband, have a structure, and show a nonlinear character. In the case of blue emission, it is noted that, simultaneously with laser radiation, the crystal experiences a significant action of Raman generation. We demonstrate the excitation of the blue emission by the laser radiation alone and present its dynamic characteristics, showing that its intrinsic time differs from the corresponding times of the Raman and laser generations. Our results show that the green emission has an up-conversion nature with the several photons excitation of residual low-concentration erbium ions. We report that the band intensity ratio of the upconversion luminescence of erbium sharply changes with increasing excitation intensity. The nonlinear interaction that is developed in KGW is accompanied by transformation of the transmission. It is concluded that the investigated processes develop in undoped KGW and YVO4 crystals as a result of a prolonged exposure of the medium to the laser radiation, which leads to a change in its properties.

Blue luminescence in YVO4 and KGd(WO4)2 crystals excited by CW 1064-nm radiation

Upconversion luminescence near λ = 475 nm is studied in KGd(WO4)2 and YVO4 crystals excited by cw radiation at a wavelength of 1064 nm. The luminescence spectra for intracavity and extracavity excitation are compared. The upconversion emission of a YVO4 crystal excited outside the cavity is studied in detail. The spectrum recorded in the visible and near-IR regions exhibits luminescence bands around 475, 650, and 800 nm, which coincide with the 1G4 → 3H6 and 1G4 → 3F4 transitions, as well as with the 1G4 → 3H5 and 3H4 → 3H6 transitions of Tm3+ ions. Blue upconversion slope n was measured to be 1.3–1.6, which is considerably lower than that for the three-photon excitation process. The reasons for the anomalous value of n are discussed.

Determination of odd-symmetry crystal-field parameters from optical spectra

We have obtained analytical expressions for effective parameters of the crystal field that acts on spin-orbit multiplets of 4fN configurations taking into account admixture to them of 4fN−15d excited states and ligand-to-metal charge-transfer states. As an example, we analyze splittings of the ground and excited multiplets of Pr3+ and Tm2+ ions in some crystals without an inversion center. The effect of mixing of states of different configurations is most strongly pronounced for the 1G4 and 1D2 excited multiplets. The interconfigurational contribution to splittings is different for different multiplets. This circumstance makes it possible to estimate the values of the parameters of the odd-symmetry crystal field, which causes mixing of the 4fN and 4fN−15d states, and the covalence parameters of rare-earth ion-ligand bonds.

Crystal-field splitting of Pr3+ ion multiplets in YPO4 with allowance for interconfigurational interaction

The crystal-field splitting of the Pr3+ ion multiplets is analyzed with regard to the influence of excited opposite-parity configurations 4f(N − 1)5d and a configuration with charge transfer. Such an approach makes it possible to (i) refine the characterization of the Stark structure of multiplets by 39% compared with the approximation of a weak configurational interaction and (ii) determine the covalence parameters and the parameters of an odd-symmetry crystal field from experimental data for the Stark structure. The covalence parameters thus determined coincide in order of magnitude with the respective parameters calculated for other ligands using microscopic models.

Self-consistent description of the Stark structure of multiplets and intensities of absorption transitions in Pr3+ ions in Y3Al5O12

The Stark structure of multiplets and the oscillator strengths of absorption transitions in Pr3+ ions in a Y3Al5O12 crystal have been described with allowance for the effect of the excited opposite-parity configuration and the charge-transfer configuration. To take into account this effect, a modified crystal-field Hamiltonian and an effective line-strength operator for intermultiplet electric-dipole transitions are proposed. Their use in description of experimental data makes it possible to decrease the standard deviation by 37 and 20%, respectively, in comparison with the standard theories. The even-and odd-crystal-field parameters and the covalence parameters have been obtained from the description of Stark levels. The intensity parameters calculated on the basis of the covalence parameters and odd-crystal-field parameters are in satisfactory agreement with the parameters obtained from the description of the experimental oscillator strengths.

Effect of annealing temperature on the structural reorganization of Eu3+ optical centers in Al2O3-Eu2O3-BiOF gel films

The dependence of the structural reorganization of Eu3+ optical centers in Al2O3-Eu2O3-BiOF films on the annealing temperature has been investigated. It is shown by the methods of crystal field theory and computer simulation that the increase in the annealing temperature from 700 to 1100 °C leads to removal of bismuth from Eu-O-Bi complex centers with the C3V symmetry in the Al2O3 structure and the change in symmetry from D3 to Oh for a large fraction of EuAlO3 centers.

Simulation of U4+ ion optical properties in crystalline ZrSiO4

The applicability of crystal field models to describing the spectral properties of uranium is studied. Effective Hamiltonians and operators for adequate simulation are suggested. As a result of description of the energy spectrum of uranium, the parameters of crystal fields of even and odd symmetry, as well as covalence parameters, are obtained. From the covalence parameters and the parameters of the odd crystal field, the intensity parameters for the U4+ ion in ZrSiO4 are calculated.