Search results for "Fine structure"
showing 10 items of 656 documents
Defects in yttrium aluminium perovskite and garnet crystals: atomistic study
2000
Native and impurity point defects in both yttrium aluminium perovskite (YAP) and garnet (YAG) crystals are studied in the framework of the pair-potential approximation coupled with the shell model description of the lattice ions. The calculated formation energies for native defects suggest that the antisite disorder is preferred over the Frenkel and Schottky-like disorder in both YAP and YAG. The calculated values of the distortion caused by the antisite YAl x in the lattice turn out to be in an excellent agreement with the EXAFS measurements. In non-stoichiometric compounds, the calculated reaction energies indicate that excess Y2 O3 or Al2 O3 is most likely to be accommodated by the forma…
Hyperfine interaction in the Autler-Townes effect: The formation of bright, dark, and chameleon states
2017
This paper is devoted to clarifying the implications of hyperfine (HF) interaction in the formation of adiabatic (i.e., ``laser-dressed'') states and their expression in the Autler-Townes (AT) spectra. We first use the Morris-Shore model [J. R. Morris and B. W. Shore, Phys. Rev. A 27, 906 (1983)] to illustrate how bright and dark states are formed in a simple reference system where closely spaced energy levels are coupled to a single state with a strong laser field with the respective Rabi frequency ${\mathrm{\ensuremath{\Omega}}}_{S}$. We then expand the simulations to realistic hyperfine level systems in Na atoms for a more general case when non-negligible HF interaction can be treated as…
New Regularization Method for EXAFS Analysis
2007
As an alternative to the analysis of EXAFS spectra by conventional shell fitting, the Tikhonov regularization method has been proposed. An improved algorithm that utilizes a priori information about the sample has been developed and applied to the analysis of U L3‐edge spectra of soddyite, (UO2)2SiO4⋅2H2O, and of U(VI) sorbed onto kaolinite. The partial radial distribution functions g1(UU), g2(USi), and g3(UO) of soddyite agree with crystallographic values and previous EXAFS results.
Ni61Mössbauer study of the hyperfine magnetic field near the Ni surface
1987
$^{61}\mathrm{Ni}$ M\"ossbauer measurements have been performed at 4.2 K on spherical Ni particles covered with a protective layer of SiO, with average diameter of 500 and 50 \AA{}. The hyperfine magnetic field at $^{61}\mathrm{Ni}$ nuclei for 500-\AA{} particles has been found to be 78.3(4) kOe, compared with the field for Ni foil of 75.0(2) kOe. The small difference is due to the demagnetization and dipolar fields in 500-\AA{} particles. The spectrum of 50-\AA{} particles has a surface component with the corresponding value of the hyperfine magnetic field of 40.3(5.4) kOe. This strongly indicates that, in accordance with recent theoretical studies, there is a decrease of the hyperfine mag…
Combining EXAFS and XRay Powder Diffraction to Solve Structures Containing Heavy Atoms
2005
Determination of structures using x-ray powder diffraction is complicated if the reflection intensities are mainly influenced by the scattering from heavy atoms and the atomic coordinates of light atoms remain uncertain. A method like EXAFS, which is sensitive to short range order, gives reliable atomic distances in the surroundings of heavy atoms with a precision of ±0.02 A. The probability for obtaining the complete structure from x-ray powder diffraction increases if one includes parameters derived from EXAFS measurements as restraints during the procedure of structure solving. We demonstrate the potential of combining EXAFS and x-ray powder diffraction by solving the structure UO2[H2AsO…
Cooperative Spin‐Crossover Behaviour in Polymeric 1D Fe II Coordination Compounds: [{Fe(tba) 3 }X 2 ]· n H 2 O
2007
A new family of 1D cooperative spin-crossover polymers with general formula [{Fe(tba)3}X2]·nH2O [tba = N-(4H-1,2,4-triazol-4-yl)benzamide; X = CF3SO3–, n = 2 (1), n = 0 (4); BF4–,n = 3 (2), n = 0 (5); 4-CH3C6H4SO3–, n = 3 (3), n = 0 (6)] has been synthesised and characterised using a series of spectroscopic methods, X-ray powder diffraction, magnetic susceptibility measurements and differential scanning calorimetry. The copper analogue of 1, [{Cu(tba)3}(CF3SO3)2]·3H2O (7), has also been synthesised and its crystal structure solved at 293 K. Compound 7 crystallises in the P space group. The bidentate N-(4H-1,2,4-triazol-4-yl)benzamide ligand bridges the copper ions through the 1,2-nitrogen p…
Electric hyperfine interaction in199Hg fluorides
1985
With the current integration technique, the Mossbauer effect on the 158 keV, 5/2−→1/2− transition in199Hg was studied using the cubic HgF2 and the tetragonal Hg2F2 as absorbers; in the latter, a quadrupole splittingeQVzz=}- 9.0(2) mm/s was observed. Electron densities were calculated with the self-consistent-charge-extended-Hueckel-molecular-orbital method and δ〈r2〉=(3.26±0.11) fm2 derived. From the quadrupole splitting and the calculation, an effective antishielding factor almost twice that of the neutral Hg atom is derived.
Interplay between local structure, vibrational and electronic properties on CuO under pressure
2020
The electronic and local structural properties of CuO under pressure have been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge and ab initio calculations, up to 17 GPa. The crystal structure of CuO consists of Cu motifs within CuO4 square planar units and two elongated apical Cu-O bonds. The CuO4 square planar units are stable in the studied pressure range, with Cu-O distances that are approximately constant up to 5 GPa, and then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical distances decrease continuously with pressure in the studied range. An anomalous increase of the mean square relative displacement (EXAFS Debye-Waller, s2) of the elong…
Model independent determination of the spin of theTa180naturally occurring isomer
2006
The hyperfine structures of the 33715.27 ${\mathrm{cm}}^{\ensuremath{-}1}$ and 33706.47 ${\mathrm{cm}}^{\ensuremath{-}1}$ transitions from the ground state of singly ionized Ta have been measured by collinear laser spectroscopy. The structures were found to contain a large second order contribution. From fitting the observed hyperfine components for both $^{181}\mathrm{Ta}$ and the $^{180}\mathrm{Ta}$ naturally occurring isomer it was possible to determine the first and second order hyperfine structure coefficients. As no model independent determination of the nuclear spin of the $^{180}\mathrm{Ta}$ isomer has been performed, fitting was attempted for a range of spins. A clear chi-squared m…
Systematic measurements of the bohr-weisskpf effect at isolde
1992
The hyperfine anomaly gives an insight into the coupling of spin and orbital magnetic moments in the nucleus. More precisely, the nuclear magnetization is expressible through the nuclear wave functions with which is tested not only the magnetic moment operator, but also the tensor product [s×C2]1. The experiment can then be expected to be of value in testing the nuclear structure theory. The greatest value of these measurements is gained when these are made systematically over a large number of isotopes. We propose to initiate a program at ISOLDE to measure the hyperfine anomaly systematically in the heavy alkali elements. The experimental setup to achieve, in particular, a precise measurem…