Fabrizio Barocchi

Collision-induced raman scattering of hydrogen at various temperatures☆

The depolarized interaction-induced light scattering spectra of hydrogen have been measured at three different temperatures on an absolute scale. The experimental spectral moments have been compared with the theoretical ones evaluated up to the first (h2) quantum correction.

Dynamic structure factor of liquid mercury

Abstract The low momentum dynamics of liquid mercury has been investigated by means of inelastic neutron scattering and molecular dynamics (MD) simulations. Due to the rather high incoherent cross-section and the high mass of mercury, the measured dynamic structure factor is dominated by the self-correlation function which has been studied in details. The low momentum collective dynamics are also 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 0.6 A −1 . The self-dynamics, as resulting from either the experiment and MD simulations, turn out to be characterized by two time scales.

Dynamic structure of He-Ne mixtures by molecular dynamics simulation: From hydrodynamic to fast and slow sound modes

Molecular dynamics (MD) results for the dynamic structure of a He(0.77)Ne(0.23) gas mixture at two densities (15.8 and 36.1 nm(-3)) show a clear crossover from hydrodynamic modes to distinct excitations for the two species. The higher density dispension curve neatly shows high- and low-frequency branches setting on with a rather localized transition. The lower density results agree very well with existing neutron scattering data and, in particular, display hydrodynamic behavior up to k approximately 2 nm(-1), in contrast with the conclusions of previous simulation studies. A smooth transition to fast sound is shown to take place for 2<k/nm(-1)<5, where the present MD data fill the existing gap in the experimental results.

Propagation of acoustic excitations in a liquid at large wavevectors: a molecular-dynamics study

A two-decade development of dedicated high-performance instrumentation at major neutron and synchrotron radiation sources has stimulated significant growth in the number and quality of spectroscopic studies of the collective dynamics of fluids, making Brillouin scattering, in its neutron and x-ray versions, a fast growing field of research. However, in contrast with the large amount of work done for wavevector Q lower than the position Qp of the main peak of the static structure factor S(Q), very little is known about the behaviour of acoustic excitations at much larger Q. We present molecular-dynamics simulation results for the translational part of the dynamic structure factor of the molecular liquid CD4 up to high Q values (Q~4Qp), analysed through the fitting of the viscoelastic model line shape. The analysis, carried out by applying the concepts described in a recent paper, shows that underdamped sound excitations persist at least up to such high Q values, in agreement with the existence of the distinct part of S(Q), with the exception of a restricted interval around Qp where the collective oscillations become overdamped.

Neutron diffraction study of long-range interactions in gaseous krypton

The low-Q part of the static structure factor S(Q) of has been derived at five low densities and room temperature by a small-angle neutron diffraction measurement and a careful treatment of experimental corrections. From the data the small-Q dependence of the Fourier transform c(Q) of the direct correlation function c(r) has been obtained and has allowed the first accurate experimental determination of the long-range three-body Axilrod-Teller potential strength in a classical fluid.

Neutron diffraction on mercury : Density dependence of the static structure factor

We report on a new neutron diffraction determination of the static structure factor of liquid mercury at room temperature. By applying pressure on the sample up to 2 kbar (giving a density change of about 0.8%), it has been possible to measure, for the first time for mercury, the isothermal density derivative of the static structure factor. This quantity, in the case of simple insulating fluids, has already been shown to be more sensitive than the structure factor itself to the details of the interatomic potential.

DILS Spectral Moments of Argon At Room Temperature up to the Triple-Point Liquid Density

The DILS (Depolarized Interaction-induced Light Scattering) spectrum of argon has been measured along the Txa0=xa0298xa0K isotherm over a range of densities between the critical and the triple-point liquid density. From these spectra, which where extended down to frequencies as low as 15xa0GHz, absolute values of the zeroth and second moments have been derived. The results are reported as a function of density and compared to molecular-dynamics simulations for a realistic model of argon. It is shown that, within the experimental errors, the pair approximation for both the potential and the polarizability anisotropy is valid for this system.

The three-body correlation spectral moments in depolarized interaction-induced light scattering of H2 at 297 K

Abstract The three-body correlation spectral moments of depolarized interaction-induced light scattering have been measured in normal H 2 at 297 K by means of an analysis of the density behaviour of the spectra between 10 and 300 cm −1 . Comparison with classical calculation shows a major discrepancy for the second moment which is attributed to the anisotropic properties of the interacting molecules.

The Rayleigh-Brillouin Light Scattering Spectrum of Dense Argon Gas Between 70 and 260 MPa at T = 298 K: the Sound Velocity and the Specific Heats Ratio γ

Abstract Measurements of the Rayleigh–Brillouin spectrum in dense argon gas at T = 298 K and pressures between 70 and 260 MPa are reported. No dispersion in the sound velocity was found up to frequencies of the order of 4 GHz, and low and high frequency measurements agree within 0.5%. This permits the use of hypersonic velocity for deriving thermodynamic quantities for this system. From our measurements we have derived the density behavior of the specific heats ratio γ. Moreover, in absence of dispersion, we have shown that Brillouin scattering gives the sole possibility of measuring both the very low and high sound velocities in fluids, at the same time, therefore giving the direct possibility of checking either the absence or presence of dispersion.

Self-dynamics of hydrogen gas as probed by means of inelastic neutron scattering

The neutron double-differential cross-section of molecular hydrogen at low density has been measured at two rather low scattering angles and different final neutron energies by means of three-axis spectrometry. This first inelastic scattering determination of the single-particle roto-translational dynamics of room temperature H2 allows for a detailed test of the theoretical modelling of the spectral line-shapes of such a fundamental molecule, performed by referring both to a careful quantum-mechanical treatment and to a simpler semi-classical approximation. A comprehensive report on the neutron measurements and data analysis is presented, along with an overview of the theories used for comparison with the experimental results. An encouraging picture of the present capabilities in the calculation of the true dynamic response of hydrogen gas to slow and thermal neutrons is obtained, opening new perspectives for accurate data calibration in inelastic neutron spectroscopy, with special relevance for small-angle experiments.