Simulation of ultrashort heat pulses on vitreous thin plates-application to the determination of dynamic constants

Date du colloque : 09/2010International audienc

Ageing effects in supercooled silica: a molecular dynamics investigation

he two-, three- and four-body effective collision induced scattering spectral line shapes are calculated for dense gaseous krypton using the pairwise additivity (PA) approximation and different polarizability models. These spectra and several interaction induced spectra calculated at various densities are compared with the experimental measurements of Barocchi et al. [1988, Europhys. Lett., 5, 607]. The potential effect on the spectrum is found to be weak. The results obtained with the Meinander et al. [1986, J. chem. Phys., 84, 3005] empirical polarizability model and molecular dynamics fit well the experimental two- and three-body spectral shapes. The irreducible contribution to the spectral shape is evaluated using the dipole induced dipole irreducible polarizability [buckingham, A. D., and Hands, I. D., 1991, Chem. Phys. Lett., 185, 544]. This contribution is found to be relatively weak for the anisotropic spectra in the frequency and density range studied, explaining the good agreement between the pairwise approximation calculations and the experimental data. The spectra radiated by the quasi-molecules Kr2, Kr3, and Kr4 (the total spectrum within the PA approximation) are also simulated

Screening dependence of the dynamical and structural properties of BKS silica

Molecular dynamics simulations of amorphous silica are carried out on a large temperature range using a modified version of the BKS inter-atomic potential. We investigate the dependence on the screening procedure of the structural and dynamical properties of amorphous silica. We show that an increased screening of the electrostatic interaction leads to a decrease of the diffusion constants and then to better agreement with experimental data, while structural properties are unchanged. We show that the Arrhenius dependence of the diffusion constants may be reproduced in this case up to a temperature of 4000 K with activation energies very similar to the experimental data

Molecular dynamics simulation of phase transitions in crystalline lead (II) fluoride

Using classical molecular dynamics applied to crystalline PbF2, we have simulated transitions between cubic and orthorhombic phases and reciprocally. These transitions were induced by pressure and temperature treatments. The orthorhombic phase was evidenced by analysis of different distribution functions and XRD spectra drawn from the ions positions in the simulated samples. (C) 2014 Elsevier B.V. All rights reserved

Molecular dynamics simulation of Eu3+ in aqueous solution comparison with experimental luminescence spectra

Using molecular dynamics simulations, we have studied the environment of Eu3+ salts in aqueous solution. In the case of hydration number of 8, the symmetry of the first hydration shell is found to be very close to the D4d. We have therefrom calculated the ligand field parameters according to the point charge model. The luminescence spectra were reconstructed from this set of computed Bkq, and finally compared with experimental spectra

Numerical Simulations of the Structure and Spectroscopic Properties of Rare-Earth Doped Glasses

A review of the research that has been devoted to the simulation of the structure and spectroscopic properties of rare-earth doped glasses is presented. Since the seminal papers of Brawer and Weber who have applied Monte-Carlo or molecular dynamics techniques, some other very important results have been reported concerning the local structure of the dopants. As a result, crystal field models have been applied and several pecularities of the optical spectra of rare earth ions in glasses have been understood

Molecular dynamics simulations of rare-earth-doped glasses

In the recent years the use of the molecular dynamics technique has become very common in the study of glass. The purpose of the present paper is to focus on recent advancements on the use of this method to investigate rare-earth-doped glasses. We report an overview of the use of simulations to study their specific structural features and luminescence properties

Phonon-Assisted Photoluminescence in a Spherical Nanocrystal

Using the matrix density in the representation of path integrals for an electron, the multiphoton nonlinear absorption light coefficient in the second order of interaction energy with polar optical phonons is derived. This coefficient describes any electron interaction mechanism with phonons. From the interaction mechanism, the main role is played by dimensional resonance when the electron continuously absorbs energy from the field as a result of synchronizing its oscillation with the field. This dimensional resonance is possible when the frequency characterizing the laser field is a multiple of the phonon frequency. Whether a photon is absorbed or emitted, the initial level from where the transition occurs defines the temperature dependence. The absorption spectrum has the form of stripes whose intensity depends on the resonance character. The most pronounced absorption is at the triple resonance, where values of radiation and oscillatory and optical phonon frequencies are equal

Shortening of the Short Refractory Periods in Short QT Syndrome.

BACKGROUND: Diagnosis of short QT syndrome (SQTS) remains difficult in case of borderline QT values as often found in normal populations. Whether some shortening of refractory periods (RP) may help in differentiating SQTS from normal subjects is unknown. METHODS AND RESULTS: Atrial and right ventricular RP at the apex and right ventricular outflow tract as determined during standard electrophysiological study were compared between 16 SQTS patients (QTc 324±24 ms) and 15 controls with similar clinical characteristics (QTc 417±32 ms). Atrial RP were significantly shorter in SQTS compared with controls at 600- and 500-ms basic cycle lengths. Baseline ventricular RP were significantly shorter in SQTS patients than in controls, both at the apex and right ventricular outflow tract and for any cycle length. Differences remained significant for RP of any subsequent extrastimulus at any cycle length and any pacing site. A cut-off value of baseline RP <200 ms at the right ventricular outflow tract either at 600- or 500-ms cycle length had a sensitivity of 86% and a specificity of 100% for the diagnosis of SQTS. CONCLUSIONS: Patients with SQTS have shorter ventricular RP than controls, both at baseline during various cycle lengths and after premature extrastimuli. A cut-off value of 200 ms at the right ventricular outflow tract during 600- and 500-ms basic cycle length may help in detecting true SQTS from normal subjects with borderline QT values