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Modeling crystal-field interaction for f-elements in LaCl{sub 3}.

Description: The results of crystal field calculations in the framework of exchange charge model (ECM) are reported for trivalent lanthanide and actinide ions doped into LaCl{sub 3}. Whereas the scalar strength of the model crystal field parameters are consistent with that previously determined by fitting the experimental data, the sign of the second-order parameter is found to be negative, in contrast to previous reports. The contribution from long-range electrostatic interactions exceeds that from the nearest neighboring ligands and leads to the negative sign of the second-order crystal field parameter. Other interaction mechanisms including overlap, covalence, and charge exchange are less important to the second order parameter, but dominate the fourth- and sixth-order parameters. This work provides a consistent interpretation of the previously controversial experimental results for both lanthanide and actinide ions in this classical host.
Date: July 14, 1997
Creator: Liu, G. K. & Zhorin, V. V.
Partner: UNT Libraries Government Documents Department

Energy up-conversion and trapping: Dynamics of 5f states of Bk sup 4+ in CeF sub 4

Description: Energy transfer mechanisms of excited 5f states of Bk{sup 4+} (0.05 atom %) in CeF{sub 4} containing circa 0.95 atom % Cf{sup 4+} have been investigated at 4 K. The observed anti-Stokes fluorescence arises from both up-conversion and two-photon excitation. Nonexponential decays of Stokes fluorescence are ascribed to both excitation-excitation annihilation and site-dependent trapping processes and, therefore, are not describable by the Inokuti-Hirayama or exciton annihilation models. Based on the Forster-Dexter energy transfer theory, a new model is developed to fit the observed fluorescence decay data. 16 refs., 3 figs.
Date: January 1, 1991
Creator: Liu, G.K. & Beitz, J.V.
Partner: UNT Libraries Government Documents Department

Photon avalanche up-conversion in holmium doped fluoride glasses

Description: Photon avalanche green up-conversion emission centered at 545 nm has been observed in Ho{sup 3+} doped and Ho{sup 3+}, Tm{sup 3+} co-doped ZrF{sub 4}-based fluoride glasses when excited near 585 nm which is off resonance with any ground state absorption bands of either Ho{sup 3+} or Tm{sup 3+} ions. Detailed spectral measurements and analysis suggest that the 545 nm emission occurs from the {sup 5}S{sub 2},{sup 5}F{sub 4} states of Ho{sup 3+} that are populated by excited state absorption from the {sup 5}I{sub 7} state of Ho{sup 3+}. Strong cross-relaxation that efficiently populates the {sup 5}I{sub 7} state makes the photon avalanche process possible in this system.
Date: August 1, 1996
Creator: Chen, Y.H.; Liu, G.K.; Beitz, J.V. & Jie Wang
Partner: UNT Libraries Government Documents Department

Spectroscopy and dynamics of 5f states of Es{sup 3+} in LaF{sub 3}

Description: Using time- and wavelength-resolved laser-induced fluorescence methods, the 5f state spectroscopy and photodynamics of {sup 253}Es{sup 3+} in LaF{sub 3} have been investigated. Based on an effective operator Hamiltonian model and approximating the metal ion site symmetry as C{sub 2V}, a set of crystal field parameters has been obtained that fit the 56 assigned levels associated with the 7 states of Es{sup 3+} that were observed. The {sup 5}F{sub 5} emitting state of Es{sup 3+} exhibited a decay rate that approached the expected purely radiative decay of the state. This suggests that the shorter lifetime previously found for this state of Es{sup 3+} in LaCl{sub 3} arose from radiation damage induced by alpha decay of {sup 253}Es.
Date: November 1, 1997
Creator: Beitz, J.V.; Williams, C.W. & Liu, G.K.
Partner: UNT Libraries Government Documents Department

Modelling crystal-field interaction for f-elements in LaCl{sub 3}

Description: The results of crystal field calculations in the framework of exchange charge model (ECM) are reported for trivalent lanthanide and actinide ions doped into LaCl{sub 3}. Whereas the scalar strength of the model crystal field parameters are consistent with that previously determined by fitting the experimental data, the sign of the second-order parameter is found to be negative, in contrast to previous reports. The contribution from long-range electrostatic interactions exceeds that from the nearest neighboring ligands and leads to the negative sign of the second-order crystal field parameter. Other interaction mechanisms including overlap, covalence, and charge exchange are less important to the second order parameter, but dominate the fourth-and sixth-order parameters. This work provides a consistent interpretation of the previously controversial experimental results for both lanthanide and actinide ions in this classical host.
Date: September 1, 1997
Creator: Zhorin, V.V. & Liu, G.K.
Partner: UNT Libraries Government Documents Department

Molecular dynamics simulation of local structure and vibrational spectrum of uranyl (UO{sub 2}){sup 2+} in vitreous B{sub 2}O{sub 3}.

Description: Laser spectroscopic and extended X-ray absorption fine structure (EXAFS) spectra have shown that uranium in B{sub 2}O{sub 3} glass matrix forms uranyl in the electronic configuration of (UO{sub 2}){sup 2+},but its surrounding structure is not well known. Understanding of uranyl local structure, ion-ligand interaction, and chemical stability on the nanometer scale in glasses is essential in management of long-term performance of high-level nuclear wastes after disposal in a geologic repository. In the present work, the structure, phonon density of states, and vibrational spectrum of vitreous B{sub 2}O{sub 3} and the surrounding environment that contains a uranyl ion have been studied using a molecular dynamics (MD) simulation method that utilizes the Born-Mayer-Huggins and Coulomb pair potentials and the Stillinger-Weber three-body potential. A system of 406 ions was considered in our calculation. Simulation of a thermal quenching from 3000 K to 300 K was performed to generate a uniform and equilibrium model glass matrix before structure configuration and vibrational frequencies were obtained from the system. The structure of the simulated glass is in agreement with that reported by Krogh-Moe and Mozzi et al. The characteristic network of planar boroxol (B{sub 3}O{sub 6}) rings is evident in the simulated system. A configuration of a U{sup 6+} cation in the vitreous B{sub 2}O{sub 3} matrix is shown in Fig. 1. It is shown that a nearly linear (UO{sub 2}){sup 2+} uranyl ion is coordinated by four equatorial oxygen anions in an approximately planar arrangement. The U-O bond length is approximately 0.178 nm for the axial oxygen and 0.254 nm for the equatorial oxygen, which is in good agreement with the U-O distances obtained from fitting EXAFS spectra. Based on the simulated model structure, the uranyl vibrational spectrum is simulated and compared with experimental results obtained using site-selective fluorescence line narrowing (FLN) techniques.
Date: November 3, 2000
Creator: Zhuang, Z.-H.; Liu, G. K. & Beitz, J. V.
Partner: UNT Libraries Government Documents Department

A laser spectroscopic study of Nd-doped Zirconia

Description: High-surface-area rare-earth (RE) modified zirconia powders prepared by solution methods can be used as catalytic support of noble metals and as electrolyte oxygen sensors in automobile exhaust-emission- control systems. Previous neutron-scattering study showed that substituting Zr with trivalent RE ions not only stabilizes the cubic and tetrgonal phases over a wide range of temperatures but also creates oxygen vacancies in the RE-Zr oxide solid solution. This work focuses on Nd fluorescence in Nd{sub 0.1}Zr{sub 0.9}O{sub 1.95} powders under laser excitation of the Nd{sup 3+} ground state to the {sup 4}G{sub 7.2} states. Distinct features were found at 8K in the {sup 4}I{sub 9/2}{r_arrow}{sup 4}G{sub 7/2} excitation and {sup 4}F{sub 3.2}{r_arrow}{sup 4}I{sub 9/2} emission spectra using two sets of incident and emission frequencies, respectively. Results are discussed in terms of site-sensitive local structures surrounding the Nd ions in the two-phased oxide structure.
Date: June 1, 1996
Creator: Loong, C.-K.; Liu, G.K.; Ozawa, M. & Suzuki, S.
Partner: UNT Libraries Government Documents Department

Molecular dynamics simulation of lattice structure, radiation damage, and vibrational spectra of zircon (ZrSiO{sub 4}).

Description: The lattice structure, phonon density of states, and infrared spectrum for crystalline zircon, ZrSiO{sub 4}, have been studied using a molecular dynamics (MD) simulation method that utilizes the Born-Mayer-Huggins and Coulomb pair potentials and the Stillinger-Weber three-body potential. A lattice block of ZrSiO{sub 4}, which contains 343 unit cells with dimensions of 4.6249 x 4.6249 x 4.1874 nm{sub 3} and containing 8232 ions, was considered in our calculations. The simulated lattice structure agreed with that determined from x-ray and neutron diffraction experiments. The vibrational modes and absorption spectrum were calculated based on the simulated lattice and compared with infrared absorption spectra. Characteristic lines in infrared spectra obtained from previous experiments on natural and synthetic zircon were assigned to specific bond structures by interactive MD simulations with variation of selected potential parameters. It is shown that the O-Si-O three-body correlations in the SiO{sub 4} tetrahedron significantly influence the spectrum. It is demonstrated that the oxygen ions that are parallel and perpendicular to the c-axis in the SiO{sub 4} tetrahedron are inequivalent and make different contributions to the vibration spectrum. The energy distribution among 24 atoms in a unit cell in the 1011-cm{sup {minus}1} vibrational mode is shown in Fig. 1. Comparison between the simulated infrared absorption spectrum and that from experiments on synthetic zircon is shown in Fig. 2. The interactive method of fitting simulated results to those determined from experiments may be used as a tool for studying nanostructure and thermodynamics properties of materials. The model potentials for the ZrSiO{sub 4} lattice are refined and further applied to MD simulation of lattice disordering and line broadening that are induced by radiation damage processes and amorphization. We have further simulated alpha-decay-induced damage and dynamical recovery in the lattice of ZrSiO{sub 4}. The simulated lattice with structure damage is used for reproducing ...
Date: November 3, 2000
Creator: Liu, G. K.; Zhuang, H.-Z. & Beitz, J. V.
Partner: UNT Libraries Government Documents Department

Vibronic interaction and crystal structure distortion in Cm sup 4+ :CeF sub 4

Description: We have applied selective excitation to unravel the complex 5f state spectra of Cm{sup 4+} in CeF{sub 4} which arises from multiple metal ion sites and vibronic interactions. An unusual laser-induced site distortion on one of the two ion-sites has been studied. Distinct spectral lines of the distorted ion site were detected. The site distortion was found to be stable below 45 K and reverted, via a thermally activated process, at higher temperatures. Below 45 K, persistent spectral holes were burnt in the absorption lines of Cm{sup 4+} on distorted site. Site distortion and hole burning are interpreted based on excited-state vibronic coupling and the assumption of an asymmetric two-minimum potential surface. 15 refs., 2 figs.
Date: January 1, 1990
Creator: Liu, G.K.; Beitz, J.V. & Carnall, W.T.
Partner: UNT Libraries Government Documents Department

Ion-ion interaction and energy transfer of 4+ transuranium ions in cerium tetrafluoride

Description: Dynamics of excited 5f electron states of the transuranium ions Cm{sup 4+} and Bk{sup 4+} in CeF{sub 4} are compared. Based on time- and wavelength-resolved laser-induced fluorescence, excitation energy transfer processes have been probed. Depending on concentration and electronic energy level structure of the studied 4+ transuranium ion, the dominant energy transfer mechanisms were identified as cross relaxation, exciton-exciton annihilation, and trapping. Energy transfer rates derived from the fitting of the observed fluorescence decays to theoretical models, based on electric multipolar ion-ion interactions, are contrasted with prior studies of 4f states of 3+ lanthanide and 3d states of transition metal ions. 16 refs., 1 tab.
Date: January 1, 1990
Creator: Liu, G.K. & Beitz, J.V.
Partner: UNT Libraries Government Documents Department

Fluorescence dynamics and cross relaxation of 4+ curium in cerium tetrafluoride

Description: Fluorescence from Cm{sup 4+} has been observed for the first time. The dynamics of its emitting 5f state have been probed using time- and frequency-resolved laser-induced fluorescence. Nonexponential fluorescence decays were found from Cm{sub 0.001}Ce{sub 0.999}F{sub 4} and Cm{sub 0.05}Ce{sub 0.95}F{sub 4} at temperatures from 4.2 K to 300 K. A simple cross-relaxation model was found to fit the excited f-state dynamics of the Cm{sub 0.001}Ce{sub 0.999}F{sub 4} sample at low excitation intensity whereas an exciton-exciton annihilation model was necessary to model the observed fluorescence decay as exciton number density increased at higher excitation intensity. The annihilation model, modified to account for a small amount of trap site emission, was required to fit all the observed decays from the Cm{sub 0.05}Ce{sub 0.95}F{sub 4} sample. 15 refs., 2 figs.
Date: January 1, 1989
Creator: Liu, G.K. & Beitz, J.V.
Partner: UNT Libraries Government Documents Department

Effects of self-radiation damage on electronic properties of {sup 244}Cm{sup 3+} in an orthophosphate crystal of YPO{sub 4}

Description: Electronic energy level of alpha-emitting isotope {sup 244}Cm{sup 3+} (t{sub {1/2}}=18.1 y) doped into single crystals of YPO{sub 4} has been studied using site-selected laser spectroscopic methods. Electronic transitions between the nominal {sup 8}S{sub 7/2} state of Cm{sup 3+} were utilized to characterize the effects of alpha-decay induced structural damage. The total splitting of the four crystal field doublets in the ground multiplet is 12.5 cm{sup -1} and that of the excited multiplet is 611 cm{sup -1}. Due to radiation damage accumulated in 17 years since the crystals were grown, the inhomogeneous line width of the {sup 8}S{sub 7/2} {leftrightarrow} {sup 6}D{sub 7/2} transitions in broader than 50 cm{sup -1} as measured without site selection. The line width of resonant fluorescence line narrowing (RFLN) is less than 1 cm{sup -1} at 4 K. A total of 12 satellite lines were observed symmetrically spacing about the RFLN line. The position, width, and intensity of these satellite lines have been analyzed to gain information on the electronic and structural properties of the actinide ions in the metamict phases of Cm{sup 3+}:YPO{sub 4}.
Date: October 1, 1997
Creator: Liu, G.K.; Li, S.T.; Beitz, J.V. & Abraham, M.M.
Partner: UNT Libraries Government Documents Department