511 research outputs found
X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4
The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in
MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge
structure spectroscopy (XANES) and compared to calculations based on density
functional theory (DFT) and ligand field multiplet theory (LFM). We also
present an efficient method, based on symmetry considerations, to compute the
dichroism of the cubic crystal starting from the dichroism of a single
substitutional site. DFT shows that the electric dipole transitions do not
contribute to the features visible in the pre-edge and provides a clear vision
of the assignment of the 1s-->3d transitions. However, DFT is unable to
reproduce quantitatively the angular dependence of the pre-edge, which is, on
the other side, well reproduced by LFM calculations. The most relevant factors
determining the dichroism of Cr K pre-edge are identified as the site
distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach
is concluded that when the pre-edge features are more intense than 4 % of the
edge jump, pure quadrupole transitions cannot explain alone the origin of the
pre-edge. Finally, the shape of the dichroic signal is more sensitive than the
isotropic spectrum to the trigonal distortion of the substitutional site. This
suggests the possibility to obtain quantitative information on site distortion
from the x-ray linear dichroism by performing angular dependent measurements on
single crystals
Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge
After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools
High-pressure study of X-ray diffuse scattering in ferroelectric perovskites
We present a high-pressure x-ray diffuse scattering study of the ABO
ferroelectric perovskites BaTiO_3 and KNbO_3. The well-known diffuse lines are
observed in all the phases studied. In KNbO_3, we show that the lines are
present up to 21.8 GPa, with constant width and a slightly decreasing
intensity. At variance, the intensity of the diffuse lines observed in the
cubic phase of BaTiO_3 linearly decreases to zero at GPa. These
results are discussed with respect to x-ray absorption measurements, which
leads to the conclusion that the diffuse lines are only observed when the B
atom is off the center of the oxygen tetrahedron. The role of such disorder on
the ferroelectric instability of perovskites is discussed.Comment: 4 pages, Accepted in PR
Assessing temperature effects on multipole contributions and angular dependence in core-level spectroscopies
This study aims at assessing the thermal nuclei motion effects on the multipole transition channels involved in two core-level spectroscopies, x-ray absorption spectroscopy (XAS) and x-ray Raman scattering (XRS). Temperature effects on the 1s -> s monopole, 1s -> p dipole, and 1s -> d quadrupole transitions are investigated using two reference systems for which we present original experimental data: alpha-Al2O3 at the Al K edge probed by XRS at room temperature and rutile TiO2 at the Ti K pre-edge probed by XAS at temperatures ranging from 6 to 700 K. Through the latter, this work enlightens the part of the pre-edge peak enhancement due to temperature in the K pre-edge region of 3d transition metal, which is known to be routinely used to determine the concentration, valence or symmetry of the probed element in a given sample. Nuclear thermal fluctuations are taken into account using a method based on density functional theory that consists in averaging spectra over atomic configurations, generated within the harmonic approximation and obeying quantum statistics at finite temperature. Since only a finite number of such configurations are used, the numerically averaged spectra generally lose the symmetry of the equilibrium crystal positions. In this paper, we demonstrate that the physical average has to be symmetric and propose a method to restore the physical angular dependence of the spectra. The approach is successfully applied to investigate the angular dependent XAS spectra in rutile as a function of temperature. The two systems under study allow to draw general conclusions regarding the effect of nuclear quantum fluctuations on the different transition channels available to both core-level spectroscopies.Peer reviewe
Study of the Galactic Interstellar Medium from High Resolution X-Ray Spectroscopy: X-Ray Absorption Fine Structure and Abundances of O, Mg, Si, S, and Fe
We study the composition of the Galactic interstellar medium (ISM) toward the
Galactic center region (5 < |l| < 20 degree) by utilizing X-ray absorption
features of three bright low-mass X-ray binaries (LMXBs), GX 13+1, GX 5-1, and
GX 340+0, observed with the Chandra HETGS. We detect X-ray absorption fine
structure (XAFS) of the Si K-edge, characterized by a narrow and a broad
absorption feature at 1846 and ~1865 eV, respectively. Comparison with ground
experimental data indicates that most of the ISM Si exists in the form of
silicates, although a composition of "pure" forsterite is ruled out. The XAFS
spectra of the sulfur K-edge indicate that a significant fraction of S exists
in the gas phase. From each source, we derive the column densities of Mg, S,
Si, and Fe from the K-edge depth and that of O (or H) from the absorption of
the continuum. The elemental abundance ratios are found to be consistent
between the three targets: the mean values of O/Si, Mg/Si, S/Si, and Fe/Si are
determined to be 0.55+-0.17, 1.14+-0.13, 1.03+-0.12, and 0.97+-0.31 solar,
respectively (90% error in the mean value). We discuss the origins of the
overabundances of the heavy metals relative to O in the Galactic ISM by
comparison with the abundance pattern of the intracluster medium in clusters of
galaxies. Assuming that most of the Mg and Si atoms are depleted into silicates
of either the proxine or olivine family, we estimate that the number ratio of
Mg to Fe in olivine is >~1.2 and that 17%-43% of the total O atoms in the ISM
must be contained in silicate grains.Comment: 31 pages, 15 figures, accepted for publication in ApJ, vol. 620,
2005. Proof corrections are reflected (column densities of O and H were
overestimated in the previous version.). Figures 1(a)-(c) are revise
Influence of the 6^1S_0-6^3P_1 Resonance on Continuous Lyman-alpha Generation in Mercury
Continuous coherent radiation in the vacuum-ultraviolet at 122 nm
(Lyman-alpha) can be generated using sum-frequency mixing of three fundamental
laser beams in mercury vapour. One of the fundamental beams is at 254 nm
wavelength, which is close to the 6^1S_0-6^3P_1 resonance in mercury.
Experiments have been performed to investigate the effect of this one-photon
resonance on phasematching, absorption and the nonlinear yield. The efficiency
of continuous Lyman-alpha generation has been improved by a factor of 4.5.Comment: 8 pages, 7 figure
Risk-based inspection as a cost-effective strategy to reduce human exposure to cysticerci of Taenia saginata in low-prevalence settings
Taenia saginata cysticercus is the larval stage of the zoonotic parasite Taenia saginata, with a life-cycle involving both cattle and humans. The public health impact is considered low. The current surveillance system, based on post-mortem inspection of carcasses has low sensitivity and leads to considerable economic burden. Therefore, in the interests of public health and food production efficiency, this study aims to explore the potential of risk-based and cost-effective meat inspection activities for the detection and control of T. saginata cysticercus in low prevalence settings
AEGIS at CERN: Measuring Antihydrogen Fall
The main goal of the AEGIS experiment at the CERN Antiproton Decelerator is
the test of fundamental laws such as the Weak Equivalence Principle (WEP) and
CPT symmetry. In the first phase of AEGIS, a beam of antihydrogen will be
formed whose fall in the gravitational field is measured in a Moire'
deflectometer; this will constitute the first test of the WEP with antimatter.Comment: Presented at the Fifth Meeting on CPT and Lorentz Symmetry,
Bloomington, Indiana, June 28-July 2, 201
X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz
We present a reciprocal-space pseudopotential scheme for calculating X-ray
absorption near-edge structure (XANES) spectra. The scheme incorporates a
recursive method to compute absorption cross section as a continued fraction.
The continued fraction formulation of absorption is advantageous in that it
permits the treatment of core-hole interaction through large supercells
(hundreds of atoms). The method is compared with recently developed
Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond
and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES
spectra were measured. Core-hole effects are investigated by varying the size
of the supercell, thus leading to information similar to that obtained from
cluster size analysis usually performed within multiple scattering
calculations.Comment: 11 pages, 4 figure
Quadrupole moment of the 6â isomeric state in 66Cu: Interplay between different nuclear deformation driving forces
AbstractWe have measured the spectroscopic quadrupole moment of the 6â isomeric state in 66Cu to be |Qs|=18.6(12) efm2. This state results from a weak coupling of the Ïp3/2 and the Îœg9/2 orbitals, which lead to sizable deformation at oblate and prolate shapes, correspondingly, in the 68Ni region. The interplay between these two different deformation-driving orbitals is observed at N=37 for the 6â state resulting in a most probable oblate shape
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