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Five models for the site-site intermolecular pair interactions of methane are compared in some detail and used to investigate both structural and dynamical properties of the dense liquid deuteromethane by means of molecular dynamics (MD) simulations. The orientational distribution probabilities of molecular pairs are carefully analyzed for each anisotropic potential model. We propose a revision of existing classification methods used to group the innumerable relative orientations of methane-methane pairs into six basic geometries. With this new approach, our results for the probability of the six basic categories as a function of the intermolecular distance are different from the ones present in the literature, where the role of the angular spread on the anisotropic interaction energy is not taken in full consideration and certain configurations with no significant change in the pair-potential are assigned to different categories. The analysis of the static orientational correlations in liquid methane and the prevalence of certain configurations in different ranges guide the subsequent discussion of the MD model-dependent results for the dynamic structure factor. Comparison with our inelastic neutron scattering results for liquid CD4 at the nanometer and picosecond space and time scales allows us to confirm the full adequacy of the Tsuzuki, Uchimaru and Tanabe model of 1998 with respect to more recent potentials. [ABSTRACT FROM AUTHOR]

We discuss the microscopic dynamics and structure of liquid hydrogen and deuterium, as probed by inelastic x-ray scattering measurements. Samples are kept in corresponding thermodynamic conditions, at which classical systems are expected to exhibit the same dynamic and static responses. On the contrary, we observe clear differences revealing the onset of quantum deviations, both in the broadening of inelastic excitations and in the position of the first sharp diffraction peak. These features are discussed, compared to path-integral Monte Carlo simulations, and finally associated with the different de Broglie wavelengths of the two isotopes. [ABSTRACT FROM AUTHOR]

The high-frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous inelastic neutron scattering (INS) experiments. We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by ≈30%, is the higher limit of a speedup, located in the momentum region 1-1, which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of “instantaneous” disorder, rather than to the crossover from a liquid to solidlike response. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Features of the energy landscape sampled by supercooled liquids are numerically analyzed for several Lennard-Jones-like model systems. The properties of quasisaddles (minima of the square gradient of potential energy W = ∇V²), are shown to have a direct relationship with the dynamical behavior, confirming that the quasisaddle order extrapolates to zero at the mode-coupling temperature T[sub MCT]. The same result is obtained either analyzing all the minima of W or the saddles (absolute minima of W), supporting the conjectured similarity between quasisaddles and saddles, as far as the temperature dependence of the properties influencing the slow dynamics is concerned. We find evidence of universality in the shape of the landscape: plots for different systems superimpose into master curves, once energies and temperatures are scaled by T[sub MCT]. This allows to establish a quantitative relationship between T[sub MCT] and potential energy barriers for Lennard-Jones-like systems, and suggests a possible generalization to different model liquids. [ABSTRACT FROM AUTHOR]

The depolarized light scattering spectra of the glassforming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Rayleigh spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Rayleigh spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time scale and are of comparable intensity. Moreover, the cross correlation terms are always found to be an important contribution to the scattering intensity. [ABSTRACT FROM AUTHOR]

Using a realistic flexible molecule model of the fragile glass former orthoterphenyl, we calculate via molecular dynamics simulation the collective dynamic structure factor S(Q,ω), recently measured in this system by inelastic x-ray scattering. The comparison of the simulated and measured dynamic structure factor, and the study of the S(Q, ω) in an extended momentum (Q), frequency (ω), and temperature (T) range allows us (i) to conclude that the utilized molecular model gives rise to S(Q, ω) in agreement with the experimental data, for those thermodynamic states and Q values where the latter are available; (ii) to confirm the existence of a slope discontinuity on the T dependence of the sound velocity that, at finite Q's, takes place at a temperature Tx higher than the calorimetric glass transition temperature T[sub g]; (iii) to find that the value of T[sub x] is Q-dependent and that its Q→0 limit is consistent with T[sub g]. The latter finding is interpreted within the framework of the current description of the dynamics of supercooled liquids in terms of exploration of the potential energy landscape. [ABSTRACT FROM AUTHOR]

Detailed computer simulation results of several static and dynamical properties of water, obtained by using a realistic potential model proposed by Jorgensen et al., in the supercooled region, at densities well below the coexistence curve, are reported. We have analyzed the structural properties by evaluating the volume distributions of Voronoi polyhedra as well as angular and radial distributions of molecular clusters. In particular, the homogeneity of the system has carefully been checked. The investigated dynamical properties mainly concern the density and temperature dependence of the diffusion coefficient. The results are compared both with previous simulations, performed with different models, and with experimental findings. Some differences stress the fact that the conclusions drawn on the physical process underlying density and temperature behavior, can strongly be influenced by the use of different potential models. [ABSTRACT FROM AUTHOR]

The analysis of Voronoi polyhedra for liquid water and hydrogen sulphide, at different temperatures, has been performed by using the molecular configurations generated by computer simulation of the liquids with realistic potential models. Some topological and metric properties of the Voronoi polyhedra have been calculated and their distributions are studied. In addition, the cross correlation between pairs of metric quantities are also investigated. The latter correlations are found to be more relevant for a clear distinction between the two systems examined here. In particular, the cross correlation between the potential energy of a molecule and the volume of the corresponding Voronoi polyhedron makes it clear that the interpretation of the anomalous physical properties of water in terms of local volume has to be revised. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of liquid water at various temperatures is reported. The autocorrelation functions of linear and angular velocities and of the first and second rank rotation matrices are studied and compared with previous data obtained using different potential models. Comparisons with the results of molecular dynamics simulations of heavy water and hydrogen sulfide are also shown. [ABSTRACT FROM AUTHOR]

The recently developed technique of Brillouin ultraviolet scattering is used to measure phonon dispersion and damping in the prototypical strong glass v-SiO2. Results on the low temperature (T ∼ 10-100 K) sound attenuation at a frequency of ∼66 GHz are presented. The comparison between a model, which takes into account dynamical processes (thermally activated relaxations and the interaction with the bath of thermal vibrations), and the experimental data indicates the presence of a strong static contribution in the region explored by ultraviolet radiation, supporting the presence of a transition between static and dynamic attenuation mechanisms at a frequency of ∼100 GHz. [ABSTRACT FROM AUTHOR]

A comparison of models traditionally used to describe the dynamic structure factor S(Q,ω) obtained from earlier and more recent inelastic scattering studies is briefly presented. The case of monatomic systems, mostly addressed so far, is extended to the more complex case of molecular liquids. As a significant test case, the results of a recent neutron experiment on liquid methane is discussed. The relevance of neutron investigations on molecular fluids, though at the cost of some extra complication, is confirmed by the available results. [ABSTRACT FROM AUTHOR]

We report an optical heterodyne-detected transient grating experiment of glass-forming 3-methylpentane at different temperatures and wave vectors in the supercooled liquid range near the glass transition. We extract sound velocities, structural times and thermal diffusivities from fitting the spectra with a hydrodynamic model parametrizing the memory function of the viscosity with a stretched exponential. [ABSTRACT FROM AUTHOR]

We performed large scale molecular dynamics simulations of liquid gallium at 320 and 970 K, exploiting a phenomenological model of the ion-pair potential depending only on the electron density. The simulations provided a description of the structure and of several dynamic properties of the system, in quantitative agreement with the experimental findings. The analysis of the intermediate self-correlation function indicated the presence of a second process, other than the diffusive process, driving the dynamics of liquid gallium at short times. This fast process, having a characterizing decay time of the order of a fraction of a picosecond, is here identified with a cage motion. [ABSTRACT FROM AUTHOR]

The mechanisms responsible for phonon attenuation in glasses and in porous systems have been investigated. The acoustic attenuation has been measured by Brillouin light scattering using a Fabry-Pérot apparatus and a new ultraviolet spectrometer. In particular, melt-quenched vitreous silica (ρ = 2200 kg m-3), silica xerogels with different densities (ρ = 510 ± 50, 770 ± 80, 1380 ± 140 and 2190 ± 200 kg m-3), and silica aerogel (ρ = 670 ± 80 kg m-3) have been studied. The porosities of the samples have been measured by nitrogen adsorption-desorption techniques. The Brillouin linewidth is found to increase with increasing pore size, being related to the effect of the growing structural disorder. Comparison between the acoustic attenuation obtained by the two wavelengths shows the existence of a crossover length a* which identifies the wave-vector values above which the disorder induced by the pore size starts to be the dominant cause of absorption. The position of the crossover is found to be frequency dependent. [ABSTRACT FROM AUTHOR]

Focuses on the measurement of the static structure factors S(Q) of liquid normal H[sub 2] and D[sub 2] in the wave-vector region through x-ray scattering. Spectral-moment sum rule used to obtain directly absolute static structure factor values; Comparison with quantum and classical Monte Carlo simulations.

The low-momentum dynamics of liquid mercury has been investigated by means of inelastic neutron scattering and molecular dynamics (MD) simulations. Because of the rather high incoherent cross-section and the high mass of mercury, the measured dynamic structure factor is dominated by the self-correlation function, although the low-momentum collective dynamics are made accessible by the very good energy resolution of the experiment. Collective modes are clearly visible against the incoherent scattering up to a momentum transfer of 0.4 Å[sup -1] and the associated velocity is found to be 2200=200 m s[sup -1], well in excess of the sound velocity which is equal to 1470 m s[sup -1]. The self-dynamics, as resulting from either the experiment or the MD simulations, turn out to be characterized by two time scales. [ABSTRACT FROM AUTHOR]

The peculiar shape of the dynamic structure factor of glasses has been investigated thoroughly by light scattering and computer simulation techniques. The experimental spectra show a pronounced asymmetry with respect to their peak frequencies. Similar asymmetries have already been observed by simulation on argon glass in a range of exchanged momentum about ten times higher than that probed by light scattering. New simulation on a large sample of silica glass also shows similar features. Moreover the calculated dynamic structure factors appear as the straight continuation at higher momentum values of the experimental light scattering data. The qualitative independence of this phenomenon from the glass type together with the analysis of the eigenvector patterns in argon glass suggest that this asymmetric shape is a mark of the topological disorder. [ABSTRACT FROM AUTHOR]

New temperature-dependent inelastic X-ray scattering (IXS) and Raman scattering (RS) data are compared with each other and with existing inelastic neutron scattering (INS) data in vitreous silica, in the 300-1775K region. The IXS data show collective propagating excitations up to Q = 3:5 nm-[sup 1]. The temperature behaviour of the excitations at Q = 1:6nm-[sup 1] matches that of the boson peak found in INS and RS. This supports the fact that the acoustic origin of the excess of vibrational states gives rise to the boson peak in this glass. [ABSTRACT FROM AUTHOR]

We present the state of the art in the realization of a new spectrometer especially designed for high resolution and high-contrast scattering experiments in the ultra-violet (UV) spectral region. Different scattering techniques, employing neutrons, X-rays or visible light are compared with each other for transferred energies and momenta. The importance of high-resolution measurements in the UV range is stressed with particular attention devoted to the study of disordered materials. [ABSTRACT FROM AUTHOR]

Results of extensive computer simulations performed in liquid water are reported in order to clarify the main features of dynamical density correlations in this system. Puzzling aspects, such as the anomalous sound wave propagation at finite wave vectors, are related to the peculiar structural aspects of this hydrogen-bonded liquid. Additional features (such as the evolution of the relevant memory functions at increasing wave vectors and the emergency of a second mode in the longitudinal current spectra) are also discussed in some detail. [ABSTRACT FROM AUTHOR]

We present a computer simulation study of a monatomic glass model. The investigated system consists of up to 32 000 'argon' atoms, interacting via a Lennard-Jones potential. Its dynamical properties have been investigated both by the time evolution of the atomic trajectories during molecular dynamics runs at low temperature and by the normal mode analysis (NMA) in the inherent configuration. In the NMA, we used both the direct diagonalization of the dynamical matrix and the spectral moment method. The vibrational density of states (DOS), the dynamic structure factors, the Raman spectrum and the spatial patterns of the eigenmodes have been calculated. The Raman spectrum shows the characteristic bump, that is the so-called 'boson peak', at frequencies where the DOS presents an excess of states with respect to the Debye behaviour. By the analysis of the previous dynamical quantities and from the direct inspection of the pattern of the eigenvectors, we derive a consistent picture of the atomic dynamics in the boson peak energy region. [ABSTRACT FROM AUTHOR]

The presence of a fast-sound mode in the microscopic dynamics of the rare-gas mixture He-Ne, predicted by theoretical studies and molecular-dynamics simulations, was demonstrated by an inelastic neutron scattering experiment. In order to study the transition between the fast and the normal acoustic modes in the hydrodynamic regime, k values lower by about one order of magnitude than in the usual experiments have to be probed. We describe here the results of the first neutron Brillouin scattering experiment performed with this purpose on the same system already investigated at larger k. The results of both experiments, together with those of a new molecular-dynamics simulation, provide a complete and consistent description, still missing so far, of the onset of fast-sound propagation in a binary mixture. [ABSTRACT FROM AUTHOR]