Three-body interaction-induced light scattering in krypton gas: A computer simulation of the spectral line shapes

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

Controlled molecular motions in azobenzene containing materials

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Heterogeneities in the Glassy State

We study heterogeneities in a binary Lennard-Jones system below the glass transition using molecular dynamics simulations. We identify mobile and immobile particles and measure their distribution of vibrational amplitudes. For temperatures near the glass transition the distribution of vibrational amplitudes obeys scaling and compares reasonably well with a mean field theory for the amorphous solid state. To investigate correlations among the immobile and mobile particles we identify clusters and analyze their size and shape. For a fixed number of immobile particles we observe that the immobile particles cluster more strongly together as the temperature is increased which allows the particles to block each other more effectively, and to therefore stay immobile. For the mobile particles, on the other hand, the clustering is most pronounced at small temperatures, indicating that mobility at low temperatures can only be sustained in cooperative motion.Comment: 9 pages, 15 figure

A link between two unsolved problems: Macroscopic transport in azobenzene containing materials and the glass transition

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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

Induced cooperative motions in a medium driven at the nanoscale: Searching for an optimum excitation period

Recent results have shown the appearance of induced cooperative motions called dynamic heterogeneity during the isomerization of diluted azobenzene molecules in a host glass-former. In this paper, we raise the issue of the coupling between these “artificial” heterogeneities and the isomerization period. How do these induced heterogeneities differ in the saturation regime and in the linear response regime? Is there a maximum of the heterogeneous motion versus the isomerization rate, and why? Is the heterogeneity evolution with the isomerization rate connected with the diffusion or relaxation time evolution? We use out-of-equilibrium molecular dynamics simulations to answer these questions. We find that the heterogeneity increases in the linear response regime for large isomerization periods and small perturbations. In contrast, the heterogeneity decreases in the saturation regime, i.e., when the isomerization half-period (τp /2) is smaller than the relaxation time of the material (τα). This result enables a test of the effect of cooperative motions on the dynamics using the chromophores as Maxwell demons that destroy or stimulate the cooperative motions. Because the heterogeneities increase in the linear regime and then decrease in the saturation regime, we find a maximum for τp /2 ≈ τα . The induced excitation concentration follows a power-law evolution versus the isomerization rate and then saturates. As a consequence, the α relaxation time is related to the excitation concentration with a power law, a result in qualitative agreement with recent findings in constrained models. This result supports a common origin for the heterogeneities with constrained models and a similar relation to the excitation concentration