Victor Teboul

Properties of a confined molecular glass-forming liquid

We use molecular dynamics (MD) simulations to investigate the modification of the dynamic and static properties of liquid toluene when confined in cylindrical mesopores a few molecular diameters across. Due to the strong influence of the substrate on the dynamics of the confined liquid, we choose a procedure where no additional thermal interactions between the wall and the liquid are taken into account. We observe the characteristic oscillations of molecular density profiles (layering) when temperature and pore size are changed. Mean square displacements and intermediate incoherent scattering functions of the centre of mass of the molecules are calculated as functions of different distances from the wall along the principal axis of the pore z and along the perpendicular x- and y-directions. At 200 K the relaxations of the two correlation functions slow down by one order of magnitude as compared to the bulk, with a slightly more pronounced slowing down in the x-direction. This slowing down increases strongly when the wall is approached. However, we do not observe any layer-specific dependence of the dynamics, but instead a continuous change. When the molecules are arrested near the wall in the time window (1 ns) of the simulations, we find hopping processes.

An experimental and numerical study of high-frequency Raman scattering in argon gas

Abstract The depolarized binary collision induced spectrum of argon gas has been meaured, at room temperature, over an extended frequency domain not fully explored previously with comparable accuracy, and ranging from 2 to 400 cm -1 . In addition, theoretical spectra were calculated with different interaction models proposed in the literature. The comparison of experimental and theoretical spectra allows us to analyse the different theoretical models, in particular for short-distance interactions characterized through the high-frequency part of the spectra.

An isomerization-induced cage-breaking process in a molecular glass former below Tg

A recent experimental [P. Karageorgiev, D. Neher, B. Schulz, B. Stiller, U. Pietsch, M. Giersig, L. Brehmer, Nature Mater. 4, 699 (2005)] study has found liquidlike diffusion below the glass-transition temperature in azobenzene-containing materials under irradiation. This result suggests that the isomerization-induced massive mass transport that leads to surface relief gratings formation in these materials, is induced by this huge increase of the matrix diffusion coefficient around the probe. In order to investigate the microscopic origin of the increase of the diffusion, we use molecular dynamics simulations of the photoisomerization of probe dispersed red 1 molecules dispersed inside a glassy molecular matrix. Results show that the increased diffusion is due to an isomerization-induced cage-breaking process. A process that explains the induced cooperative motions recently observed in these photoactive materials.

Aging effects in supercooled silica. A molecular dynamics investigation

Abstract Using molecular dynamics simulations we investigate the presence of dynamical heterogeneities in supercooled silica, a ‘strong’ glass former, and the changes as a function of time of these heterogeneities during aging. We compare the radial distribution functions for the 10% most mobile silicon or oxygen atoms with the corresponding mean radial distribution functions. We measure changes of these radial distribution functions and the changes of the non-gaussian parameters with time after a quench. We find that dynamical heterogeneities are present in silica and that they increase during the aging process. We find that the oxygen and silicon atoms show similar heterogeneities but with a differing rate of change during aging.

High-frequency interaction-induced rototranslational wings of isotropic nitrogen spectra

The isotropic rototranslational scattering of nitrogen gas is studied in the frequency range 300–600 cm-1, both theoretically and experimentally, at a density of 41 and 169 amagat. The isotropic double differential cross-section for scattered light is calculated theoretically considering first- and second-order dipole-induced dipole, first-order dipole-dipole-quadrupole, and dipole-induced octopole light scattering mechanisms as well as their cross contributions. The irreducible spherical form for the induced operator of these light scattering mechanisms was determined. Good comparison is found in the frequency range 300–450 cm-1 between the theoretical and experimental double differential cross-sections. When an exponential contribution [exp(-ν/ν0)] with ν0 = 118 cm-1 is considered to model very short-range light scattering mechanisms good comparison is found over the whole frequency range.

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.

Collection Angle Dependence of the Depolarization Ratio in Light-Scattering Experiments

We have calculated, in a typical scattering experiment, the exact value of the intensity scattered by a sample for any value of the collection angle of the scattered beam and for different beam polarizations.

Polaron in a quasi 1D cylindrical quantum wire

Polaron states in a quasi 1D cylindrical quantum wire with a parabolic confinement potential are investigated applying the Feynman variational principle. The effect of the wire radius on the polaron ground state energy level, the mass and the Fröhlich electron-phonon-coupling constant are obtained for the case of a quasi 1D cylindrical quantum wire. The effect of anisotropy of the structure on the polaron ground state energy level and the mass are also investigated.

Isotropic and anisotropic interaction induced scattering in liquid argon

The collision induced scattering (CIS) spectra have been studied for liquid argon at 130 K and 615 amagat. For the first time, isotropic CIS intensities are measured and the depolarized CIS spectrum is obtained up to 370 cm−1. Molecular dynamics simulations are performed for several models of polarizabilities and intermolecular potentials. They show that theoretical polarizabilities deduced from self consistent field calculations are in agreement with both depolarized and isotropic CIS experimental spectral shape.

Time versus temperature rescaling for coarse grain molecular dynamics simulations

Coarse graining procedures are intended to well reproduce the structure of a material while increasing the simulations efficiency. However, the dynamics usually accelerates with coarse graining and a scaling procedure has to be used for dynamical data calculations. Most often a simple time-scaling coefficient is used for this purpose. However, for low temperature liquids this simple scaling procedure is questionable. Because supercooled liquids in their approach to the glass transition temperature do not follow a simple dynamics. In order to test if this scaling procedure is still pertinent at low temperature, we use molecular dynamics simulations of a coarse grain model of the methylmethacrylate molecule compared to simulations with the All atom model. We compare two different rescaling procedures, a time rescale and a temperature rescale procedure. Using these two procedures we compare the behaviors of the mean square displacements, the incoherent scattering functions, the self and distinct part of the Van Hove correlation functions and the non-Gaussian parameters. Results show that the temperature rescaling procedure reproduces well the All atom dynamical data at low temperatures, while the time rescaling procedure is correct only in the Brownian regime. We also find that the melting and the glass-transition temperatures are relatively well reproduced with the temperature rescaling procedure.