RESUMEN
Materials that luminesce after excitation with ionizing radiation are extensively applied in physics, medicine, security, and industry. Lanthanide dopants are known to trigger crystal scintillation through their fast d-f emissions; the same is true for other important applications as lasers or phosphors for lighting. However, this ability can be seriously compromised by unwanted anomalous emissions often found with the most common lanthanide activators. We report high-resolution X-ray-excited optical (IR to UV) luminescence spectra of CaF2:Yb and SrF2:Yb samples excited at 8949 eV and 80 K. Ionizing radiation excites the known anomalous emission of ytterbium in the CaF2 host but not in the SrF2 host. Wave function-based ab initio calculations of host-to-dopant electron transfer and Yb2+/Yb3+ intervalence charge transfer explain the difference. The model also explains the lack of anomalous emission in Yb-doped SrF2 excited by VUV radiation.
RESUMEN
Ytterbium-doped wide-bandgap fluoride crystals CaF2, SrF2 and NaMgF3 have been measured using x-ray absorption near edge structure (XANES) on the L3 edge to determine the ratio of trivalent to divalent Yb ions present in the crystals. This study improves upon previous XANES measurements of dopant ion valency by taking into account the x-ray emission transition probabilities for the divalent and trivalent species instead of simply assuming that the relative concentrations may be determined by the ratio of the x-ray excitation band areas. Trivalent to divalent ratios as high as 5 are inferred even at low total dopant ion concentrations of 0.05 mol% Yb.
RESUMEN
The electronic structure of a novel lanthanide-based single-ion magnet, {C(NH2)3}5[Er(CO3)4]·11H2O, was comprehensively studied by means of a large number of different spectroscopic techniques, including far-infrared, optical, and magnetic resonance spectroscopies. A thorough analysis, based on crystal field theory, allowed an unambiguous determination of all relevant free ion and crystal field parameters. We show that inclusion of methods sensitive to the nature of the lowest-energy states is essential to arrive at a correct description of the states that are most relevant for the static and dynamic magnetic properties. The spectroscopic investigations also allowed for a full understanding of the magnetic relaxation processes occurring in this system. Thus, the importance of spectroscopic studies for the improvement of single-molecule magnets is underlined.
RESUMEN
Calculated energy levels from recent ab initio studies of the electronic structure of SrCl2:Yb(2+) and CsCaBr3:Yb(2+) are fitted with a semi-empirical 'crystal-field' Hamiltonian, which acts within the model space 4f(14) + 4f(13)5d + 4f(13)6s. Parameters are obtained for the minima of the potential energy curves for each energy level and also for a range of anion-cation separations. The parameters are compared with published parameters fitted to experimental data and to atomic calculations. The states with significant 4f(13)6s character give a good approximation to the impurity-trapped exciton states that appear in the ab initio calculations.
Asunto(s)
Transferencia de Energía , Metales/química , Modelos Químicos , Modelos Moleculares , Simulación por ComputadorRESUMEN
A simple method for constructing effective Hamiltonians for the 4f(N) and 4f(N - 1)5d energy levels of lanthanide ions in crystals from quantum-chemical calculations is presented. The method is demonstrated by deriving crystal-field and spin-orbit parameters for Ce(3 + ) ions doped in LiYF(4), Cs(2)NaYCl(6), CaF(2), KY(3)F(10) and YAG host crystals from quantum-chemical calculations based on the DV-Xα method. Good agreement between calculated and fitted values of the crystal-field parameters is obtained. The method can be used to calculate parameters even for low-symmetry sites where there are more parameters than energy levels.
RESUMEN
Detailed analysis of the vibronic structure in the electronic absorption spectrum of Cs2NaYbF6 at temperatures between 10 and 300 K enables the crystal field energy level diagram of Yb3+ in this cubic host to be deduced. Ultraviolet and visible laser excitation of Cs2NaYbF6, Cs2NaY(0.9)Yb(0.1)F6, and Cs2NaHo(0.99)Yb(0.01)F6 give spectral features mainly due to Yb3+ being situated at a range of defect sites. The 4f13 crystal field analyses of octahedral YbX6(3-) (X = F, Cl, Br) systems show the expected trends in parameter values, but the energy level fits are poor. Inclusion of the interaction with the charge-transfer configuration 4f14np5 provides an exact fitting of energy levels for YbX6(3-), and a smooth variation of ff and fp crystal field parameters for Cs2NaLnCl6 (Ln = Er, Tm, Yb) is observed.
RESUMEN
The 4f-5d transition rates for rare-earth ions in crystals can be calculated with an effective transition operator acting between model 4f(N) and 4f(N-1)5d states calculated with effective Hamiltonian, such as semiempirical crystal Hamiltonian. The difference of the effective transition operator from the original transition operator is the corrections due to mixing in transition initial and final states of excited configurations from both the center ion and the ligand ions. These corrections are calculated using many-body perturbation theory. For free ions, there are important one-body and two-body corrections. The one-body correction is proportional to the original electric dipole operator with magnitude of approximately 40% of the uncorrected electric dipole moment. Its effect is equivalent to scaling down the radial integral (5d/r/4f) to about 60% of the uncorrected HF value. The two-body correction has magnitude of approximately 25% relative to the uncorrected electric dipole moment. For ions in crystals, there is an additional one-body correction due to ligand polarization, whose magnitude is shown to be about 10% of the uncorrected electric dipole moment.
RESUMEN
Low-temperature emission and polarized absorption spectra have been recorded for U(3+) ions diluted in Cs(3)Lu(2)Cl(9) and Cs(3)Y(2)I(9) host crystals. The experimental crystal-field levels were fitted to 13 parameters of a semiempirical Hamiltonian representing the combined atomic, one-electron crystal field (CF) as well as two-particle correlation crystal-field (CCF) operators. The red shift of the first f-d transitions from approximately 14,800 cm(-1) in the spectrum of U(3+):Cs(3)Lu(2)Cl(9) to as low as 11,790 cm(-1) in that of U(3+):Cs(3)Y(2)I(9) has been attributed to an increase in the covalence of the U(3+)-X(-) bonds. Comparison of the differences in the Coulomb repulsion strength between U(3+) and Er(3+) ions in Cs(3)Lu(2)Cl(9) and Cs(3)Y(2)I(9) crystals suggests that the 5f electrons of U(3+) ions are more 3d-like than 4f. The CF splitting of the (2)H(9/2) and (4)F(5/2) multiplets is unexpectedly larger for U(3+):Cs(3)Y(2)I(9) than for U(3+):Cs(3)Lu(2)Cl(9), which may be viewed as a result of the proximity of f-d states. For a correct description of the energy level structure of the (2)H(9/)(2) and (4)F(5/2) multiplets, the inclusion of CCF terms in the parametric Hamiltonian has proved to be essential. The larger f-f transition intensities for U(3+):Cs(3)Y(2)I(9) were also considered to be a consequence of the red shift of the first f-d states. The inadequacy in determination of the minor atomic parameters (other than parameters for Coulomb and spin-orbit interactions) and the insufficient inclusion of the influence of excited configuration in the applied CF Hamiltonian are assumed to be the main deficiencies preventing a better agreement between the experimental and calculated energies of CF levels.
RESUMEN
Perturbative contributions to single-beam two-photon transition rates may be divided into two types. The first, involving low-energy intermediate states, require a high-order perturbation treatment, or an exact diagonalization. The other, involving high-energy intermediate states, only require a low-order perturbation treatment. We show how to partition the effective transition operator into two terms, corresponding to these two types, in such a way that a many-body perturbation expansion may be generated that obeys the linked cluster theorem and has a simple diagrammatic representation.
RESUMEN
For a quantum many-body problem, effective Hamiltonians that give exact eigenvalues in reduced model space usually have different expressions, diagrams, and evaluation rules from effective transition operators that give exact transition matrix elements between effective eigenvectors in reduced model space. By modifying these diagrams slightly and considering the linked diagrams for all the terms of the same order, we find that the evaluation rules can be made the same for both effective Hamiltonian and effective transition operator diagrams, and in many cases it is possible to combine many diagrams into one modified diagram. We give the rules to evaluate these modified diagrams and show their validity.
RESUMEN
Effective interaction operators usually act on a restricted model space and give the same energies (for Hamiltonian) and matrix elements (for transition operators, etc.) as those of the original operators between the corresponding true eigenstates. Various types of effective operators are possible. Those well defined effective operators have been shown to be related to each other by similarity transformation. Some of the effective operators have been shown to have connected-diagram expansions. It is shown in this paper that under a class of very general similarity transformations, the connectivity is conserved. The similarity transformation between Hermitian and non-Hermitian Rayleigh-Schrodinger perturbative effective operators is one of such transformations and hence the connectivity can be deducted from each other.
