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F Sette

Publications and source records attributed to F Sette.

At least 19 recordsLinked to original sources
PubMed2002

Structural and microscopic relaxation processes in liquid hydrogen fluoride.

The high frequency collective dynamics of liquid hydrogen fluoride is studied by inelastic x-ray scattering on the coexistence curve at T = 239 K. The comparison with existing molecular dynamics simulations shows the existence of two active relaxation processes with characteristic time scales in the subpicosecond range. The observed scenario is very similar to that found in liquid water. This suggests that hydrogen bonded liquids behave similarly to other very different systems as simple and glass forming liquids, thus indicating that these two relaxation processes are universal features of the liquid state.

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PubMed2002

Deep inelastic atomic scattering of x rays in liquid neon.

An inelastic x-ray scattering (IXS) experiment in liquid neon has been performed in the +/-100 meV exchanged energy range and at exchanged wave numbers, q, comprised between 1 and 16 A(-1). At the highest probed q's a deep inelastic scattering regime is reached where the Ne core electrons, after collision with the x rays, recoil almost freely with an effective mass equal to the Ne atomic mass. IXS in this high q regime is here shown to provide quantitative information on the atomic momentum distribution of liquid Ne, thus supplying a complementary technique to neutron scattering. There are several open problems in quantum and classical liquids which can benefit from this complementarity.

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PubMed2002

Anomalous dispersion of longitudinal optical phonons in Nd(1.86)Ce(0.14)CuO(4+delta) determined by inelastic x-ray scattering.

The phonon dispersions of Nd(1.86)Ce(0.14)CuO(4+delta) along the [xi,0,0] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about q = (0.2,0,0). These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.

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PubMed2002

Inelastic x-ray scattering study of the collective dynamics in liquid sodium.

Inelastic x-ray scattering data have been collected for liquid sodium at T=390 K, i.e., slightly above the melting point. Owing to the very high instrumental resolution, pushed up to 1.5 meV, it has been possible to determine accurately the dynamic structure factor S(Q,omega) in a wide wave-vector range, 1.5-15 nm(-1), and to investigate on the dynamical processes underlying the collective dynamics. A detailed analysis of the line shape of S(Q,omega), similarly to other liquid metals, reveals the coexistence of two different relaxation processes with slow and fast characteristic time scales. The present data lead to the conclusion that (i) the picture of the relaxation mechanism based on a simple viscoelastic model fails and (ii) although the comparison with other liquid metals reveals similar behavior, the data do not exhibit an exact scaling law as the principle of the corresponding state would predict.

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PubMed2001

Quantum effects in the dynamics of He probed by inelastic x-ray scattering.

Quantum effects in the teraherz dynamics of supercritical 4He have been studied as a function of both density rho and temperature T; they have been characterized through their effects on the second and third spectral moments of the dynamic structure factor S(Q, omega), measured by the inelastic x-ray scattering (IXS) technique. The IXS spectra were collected in the low-Q region below and around the position of the first diffraction peak Q(m), i.e., in a range relatively unusual in this kind of investigation. The measured spectral moments clearly show a departure from their high-T classical expected values. We observe, moreover, that the amplitude of quantum deviations increases slightly with increasing density. This experimental method allows us to extract, even in a region where the dynamics still maintains a collective character, such typical single particle properties as the mean atomic kinetic energy.

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PubMed2001

Collective dynamics in fully hydrated phospholipid bilayers studied by inelastic x-ray scattering.

The short wavelength density fluctuation of DLPC (dilaurylphosphatidylcholine) bilayers close to full hydration has been studied by the inelastic x-ray scattering technique below and above the main transition temperature. The analysis based on a generalized three effective eigenmode theory allows us to construct the dispersion relation of the high frequency sound mode for the first time. The marked softening of the excitation near k = 14 nm(-1), corresponding to the lipid chain-chain correlation peak in the structure factor, in the L(alpha) phase implies prevalent occurrences of short-wavelength in-plane motions of lipid chains that might be of importance for transportation of small molecules across membranes.

Lipid Bilayers↗
PubMed2000

Evidence of two viscous relaxation processes in the collective dynamics of liquid lithium.

New inelastic x-ray scattering experiments have been performed on liquid lithium in a wide wave vector range. With respect to the previous measurements, the instrumental resolution, improved up to 1. 5 meV, allows one to accurately investigate the dynamical processes determining the observed shape of the dynamic structure factor S(Q, omega). A detailed analysis of the line shapes shows the coexistence of relaxation processes with both slow and fast characteristic time scales, and therefore shows that pictures of the relaxation mechanisms based on a simple viscoelastic model must be abandoned.

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PubMed2000

Crystal-like high frequency phonons in the amorphous phases of solid water.

The high frequency dynamics of low-density (LDA) and high-density (HDA) amorphous ice and of cubic ice ( I(c)) has been measured by inelastic x-ray scattering in the 1-15 nm(-1) momentum transfer ( Q) range. Sharp phononlike excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8 nm(-1) in LDA and I(c) and up to 5 nm(-1) in HDA. The narrow width of these excitations is in sharp contrast to the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to soundlike excitations which is interpreted as a sign of low local disorder.

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PubMed2000

Nature of the short wavelength excitations in vitreous silica: An X-Ray brillouin scattering study

The dynamical structure factor [ S(Q,E)] of vitreous silica has been measured by inelastic x-ray scattering varying the exchanged wave vector ( Q) at fixed exchanged energy ( E)-an experimental procedure that, contrary to the usual one at constant Q, provides spectra with much better identified inelastic features. This allows us to obtain the first direct evidence of Brillouin peaks in the S(Q,E) of SiO2 at energies above the boson peak (BP) energy, a finding that excludes the possibility that the BP marks the transition from propagating to localized dynamics in glasses.

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PubMed2000

Experimental evidence of the acousticlike character of the high frequency excitations in glasses

The dynamic structure factor S(Q,E) of glassy glycerol has been measured by inelastic x-ray scattering as a function of momentum transfer Q and at constant energy transfer E. This allows one to establish independently from specific models of S(Q,E) the following: (i) Propagating collective excitations exist in glasses at high Q. (ii) Their dispersion up to E higher than E(BP) (the boson peak energy) confirms that E(BP) is not the onset of modes localization. (iii) The observation of an almost Q independent plateau on the high Q side of the Brillouin peak supports numerical simulations on glasses, describing the vibrational eigenvectors in terms of acousticlike and "random" components.

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PubMed2000

Relaxation processes in harmonic glasses?

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones glass. We propose to identify this process with the so-called microscopic (or, instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation and allows one to relate to each other (1) the characteristic time and strength of this process, (2) the low frequency limit of the dynamic structure factor of the glass, and (3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

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PubMed2000

Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-1). This, enabling S(Q,E) measurements as a function of Q at constant E transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with Q, to a high-Q regime, where it is no longer possible to observe a dominant excitation in the S(Q,E).

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PubMed2000

Analysis of inelastic x-ray scattering spectra of low-temperature water

We analyze a set of high-resolution inelastic x-ray scattering (IXS) spectra from H2O measured at T=259, 273, and 294 K using two different phenomenological models. Model I, called the "dynamic cage model," combines the short time in-cage dynamics described by a generalized Enskog kinetic theory with a long-time cage relaxation dynamics described by an alpha relaxation. This model is appropriate for supercooled water where the cage effect is dominant and the existence of an alpha relaxation is evident from molecular-dynamics (MD) simulation data of extended simple point charge (SPC/E) model water. Model II is essentially a generalized hydrodynamic theory called the "three effective eigenmode theory" by de Schepper et al. 11. This model is appropriate for normal liquid water where the cage effect is less prominent and there is no evidence of the alpha relaxation from the MD data. We use the model I to analyze IXS data at T=259 K (supercooled water). We successfully extract the Debye-Waller factor, the cage relaxation time from the long-time dynamics, and the dispersion relation of high-frequency sound from the short time dynamics. We then use the model II to analyze IXS data at all three temperatures, from which we are able to extract the relaxation rate of the central mode and the damping of the sound mode as well as the dispersion relation for the high-frequency sound. It turns out that the dispersion relations extracted from the two models at their respective temperatures agree with each other giving the high-frequency sound speed of 2900+/-300 m/s. This is to be compared with a slightly higher value reported previously, 3200+/-320 m/s, by analyzing similar IXS data with a phenomenological-damped harmonic oscillator model 22. This latter model has traditionally been used exclusively for the analysis of inelastic scattering spectra of water. The k-dependent sound damping and central mode relaxation rate extracted from our model analyses are compared with the known values in the hydrodynamic limit.

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PubMed1999

Viscoelastic behavior of water in the terahertz-frequency range: an inelastic x-ray scattering study.

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(q,omega) of liquid water in the THz frequency range have been performed as a function of wave vector q (1-7 nm(-1)) and temperature T (273-473 K), using pressure (0-1.5 kbar) to keep the density at rho approximately 1 g/cm(3). We show that, for q 2 nm(-1), we find that the viscoelastic contribution to the memory function becomes q dependent. Thus this work provides a view on the evolution of the collective dynamics of water across the q region where the continuum approximation inherent in the hydrodynamic formulation begins to fail. The physical consequences of such a result are discussed in some detail.

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PubMed1999

Brillouin and Umklapp scattering in polybutadiene: comparison of neutron and x-ray scattering.

We report a comparison of high resolution inelastic x-ray Brillouin scattering to coherent inelastic neutron scattering for amorphous deuterated polybutadiene, done for one temperature in the glass phase and another one in the melt. The x-ray scattering proves to be by far the better technique for such a polymer within its present resolution bounds. The neutron scattering allows one to extend these measurements to a much better resolution, showing an additional quasielastic signal in the melt. The results suggest x-ray measurements at higher momentum transfer, to see whether they are complementary to neutrons.

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