Search results for "SPECTRA"
showing 10 items of 3542 documents
Amorphous polymorphis in ice investigated by inelastic neutron scattering
1997
Abstract High-density Ilda and low-density Ilda amorphous have been investigated by inelastic neutron scattering (INS) with emphasis on the energy window from 0.5 to 20 meV. At variance with earlier measurements the spectra in the Ilda phase show a simple ω2 behaviour in the acoustic region and the temperature dependence is found to be harmonic. Ilda converts with a strongly temperature-dependent rate towards Ilda ice. We have investigated in detail the time evolution of both the static and dynamic response functions at several temperatures. Elastic small-angle signals indicate the presence of strong heterogeneties at the early stages of the conversion process. At least two different time s…
Molecular dynamics and reverse Monte Carlo modeling of scheelite-type AWO4(A = Ca, Sr, Ba) WL3-edge EXAFS spectra
2016
Classical molecular dynamics (MD) and reverse Monte Carlo methods coupled with ab initio multiple-scattering extended x-ray absorption fine structure (EXAFS) calculations were used for modeling of scheelite-type AWO4 (A = Ca, Sr, Ba) W L 3-edge EXAFS spectra. The two theoretical approaches are complementary and allowed us to perform analysis of full EXAFS spectra. Both methods reproduce well the structure and dynamics of tungstates in the outer coordination shells, however the classical MD simulations underestimate the W–O bond MSRD due to a neglect of quantum zero-point-motion. The thermal vibration amplitudes, correlation effects and anisotropy of the tungstate structure were also estimat…
Thickness Inhomogeneity Effect in EXAFS Spectroscopy
2005
In many cases x-ray absorption spectra measured in transmission mode are degraded by an inhomogeneity in thickness of the samples. As a result, the EXAFS amplitude is decreased and information about the coordination numbers is distorted. To avoid this influence, it is necessary to prepare a homogeneous sample. But, for powder samples, thick inhomogeneous foils, and sputtered films this is not possible. Absorption spectra on these samples should be corrected for the thickness inhomogeneity effect.To correct an absorption spectrum it is necessary to know the sample thickness distribution function. We propose a method of solving an integral equation for a distribution function as an inverse pr…
Local Structure of Multiferroic MnWO4 and Mn0.7Co0.3WO4 Revealed by the Evolutionary Algorithm
2015
A novel reverse Monte Carlo/evolutionary algorithm scheme was applied to the analysis of the W L3-edge and Mn(Co) K-edges EXAFS spectra from multiferroic MnWO4 and Mn0.7Co0.3WO4. A 3D structural model, consistent with the experimental data, was obtained, and the influence of composition and temperature on the local structure of tungstates is discussed.
Reverse Monte Carlo modeling of thermal disorder in crystalline materials from EXAFS spectra
2012
Abstract In this work we present the Reverse Monte Carlo (RMC) modeling scheme, designed to probe the local structural and thermal disorder in crystalline materials by fitting the wavelet transform (WT) of the EXAFS signal. Application of the method to the analysis of the Ge K-edge and Re L3-edge EXAFS signals in crystalline germanium and rhenium trioxide, respectively, is presented with special attention to the problem of thermal disorder and related phenomena.
Isotopic Effect In Extended X-Ray-Absorption Fine Structure of Germanium
2008
Extended x-ray absorption fine structure has been measured on two powdered samples of 70Ge and 76Ge as a function of temperature from 20 to 300 K. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller factors. The isotopic effect is also detected on the difference of nearest-neighbor average ineratomic distances, thanks to a resolution better than 10 fm. (DOI: 10.1103/PhysRevLett.100.055901)
Analysis of extended x-ray absorption fine structure data from copper tungstate by the reverse Monte Carlo method
2014
The static disorder and lattice dynamics of crystalline materials can be efficiently studied using reverse Monte Carlo simulations of extended x-ray absorption fine structure spectra (EXAFS). In this work we demonstrate the potentiality of this method on an example of copper tungstate CuWO4. The simultaneous analysis of the Cu K and W L3 edges EXAFS spectra allowed us to follow local structure distortion as a function of temperature.
Number of observable features in the acoustic-Raman spectra of nanocrystals
2012
Low-frequency Raman scattering spectra are presented for gold nanocrystals with diameters 3.5 and 13 nm. The frequencies of the Raman peaks but also their number are shown to vary with the nanocrystal size. These results are analyzed using both the continuous elastic medium approximation and an atomistic approach. We show that the number of atoms in the nanocrystal determines an upper limit of the number of observable Raman features. The frequency range in which the continuous elastic medium approximation is valid is defined by comparison with the calculations based on the atomistic approach.
Fabrication of long period fiber gratings of subnanometric bandwidth.
2017
This paper reports on the fabrication of long period fiber gratings having subnanometric bandwidth in the 1500 nm spectral region. Large gratings have been photo-inscribed in a high NA fiber, the grating pitch and the order of the HE cladding mode are optimized to produce gratings with a large number of periods and preventing the coupling to TE, TM or EH modes. Resonances with a FWHM of 0.83 nm and 0.68 nm have been achieved for gratings 15 and 20 cm long respectively, the free spectral range between transmission notches is 125 nm. The polarization effects and the sensitivity of the gratings to temperature and to strain variations are presented as well. © 2015 Optical Society of America Thi…
Optical Spectra and Direct Optical Transitions in Amorphous and Crystalline ZnO Thin Films and Powders
2010
Comparative studies of ZnO crystalline and amorphous thin films and nanocrystalline powders are reported. The UV-visible optical spectra were analyzed with special attention paid to the direct optical bandgap. Atmospheric radio-frequency barrier torch discharge and pulsed hollow cathode sputtering techniques for the film fabrication were used. For the crystalline films, similar values of the direct optical bandgap were found independent of the growth method used. The analysis of the amorphous films and powders revealed a pronounced Urbach-like exponential absorption tail approaching the bandedge. For the powders, the bandgap energies were larger than those for the crystalline and amorphous …