Biomedical subjects

F Sette

Publications and source records attributed to F Sette.

At least 19 recordsLinked to original sources

PubMed2006

Evidence for a crossover in the frequency dependence of the acoustic attenuation in vitreous silica.

We report measurements of the sound attenuation coefficient in vitreous silica, for sound waves of wavelength between 50 and 80 nm, performed with the new inelastic UV light scattering technique. These data indicate that in silica glass a crossover between a temperature-dependent (at low frequency) and a temperature-independent (at high frequency) acoustic attenuation mechanism occurs at Q approximately equal to 0.15 nm(-1). The absence of any signature in the static structure factor at this Q value suggests that the observed crossover should be associated with local elastic constant fluctuations.

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PubMed2005

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

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PubMed2005

Relaxation dynamics in (HF)x(H2O)1-x solutions.

The high-frequency dynamics of (HF)(x)(H(2)O)(1-x) solutions has been investigated by inelastic x-ray scattering. The measurements have been performed as a function of the concentration in the range x = 0.20-0.73 at fixed temperature T = 283 K. The results have been compared with similar data in pure water (x = 0) and pure hydrogen fluoride (x = 1). A viscoelastic analysis of the data highlights the presence of a relaxation process characterized by a relaxation time and a strength directly related to the presence of a hydrogen-bond network in the system. The comparison with the data on water and hydrogen fluoride shows that the structural relaxation time continuously decreases at increasing concentration of hydrogen fluoride passing from the value for water to the one for hydrogen fluoride tau(alphaHF), which is three times smaller. This is the consequence of a gradual decreasing number of constraints of the hydrogen-bond networks in passing from one liquid to the other.

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PubMed2005

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

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PubMed2005

Clocking femtosecond X rays.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

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PubMed2005

High-frequency longitudinal and transverse dynamics in water.

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S (Q,omega) of liquid water have been performed for wave vectors Q between 4 and 30 nm(-1) in distinctly different thermodynamic conditions ( T=263-420 K ; at, or close to, ambient pressure and at P=2 kbar ). In agreement with previous inelastic x-ray and neutron studies, the presence of two inelastic contributions (one dispersing with Q and the other almost nondispersive) is confirmed. The study of their temperature and Q dependence provides strong support for a dynamics of liquid water controlled by the structural relaxation process. A viscoelastic analysis of the Q -dispersing mode, associated with the longitudinal dynamics, reveals that the sound velocity undergoes a complete transition from the adiabatic sound velocity ( c(0) ) (viscous limit) to the infinite-frequency sound velocity ( c(infinity) ) (elastic limit). On decreasing Q , as the transition regime is approached from the elastic side, we observe a decrease of the intensity of the second, weakly dispersing feature, which completely disappears when the viscous regime is reached. These findings unambiguously identify the second excitation to be a signature of the transverse dynamics with a longitudinal symmetry component, which becomes visible in S (Q,omega) as soon as the purely viscous regime is left.

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PubMed2004

Structural relaxation in liquid water by inelastic UV scattering.

Using the novel synchrotron radiation based inelastic ultraviolet scattering technique, the dynamic structure factor of normal and supercooled liquid water has been measured at a momentum transfer Q approximately equal to 0.1 nm(-1), in the temperature range 260-340 K. The structural (alpha) relaxation has been observed in the supercooled temperature region (T< or =273 K), where the inverse relaxation time matches the frequency of the probed sound modes. The T dependence of the relaxation time shows a diverging behavior with a critical temperature T approximately equal to 220 K. These results provide a unique experimental opportunity to frame the dynamics of water in the mode-coupling theory.

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PubMed2002

High-frequency acoustic modes in liquid gallium at the melting point.

The microscopic dynamics in liquid gallium at melting has been studied by inelastic x-ray scattering. We demonstrate the existence of acousticlike modes up to wave vectors above one-half of the first maximum of the static structure factor, at variance with earlier results from inelastic neutron scattering [F. J. Bermejo et al., Phys. Rev. E 49, 3133 (1994)]. Despite structural (extremely rich polymorphism) and electronic (mixed valence) peculiarities, the collective dynamics is strikingly similar to the one of van der Waals and metallic fluids. This result speaks in favor of the universality of the short time dynamics in monatomic liquids rather than of system-specific dynamics.

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PubMed2002

Evidence of short-time dynamical correlations in simple liquids.

We report a molecular dynamics study of the collective dynamics of a simple monatomic liquid--interacting through a two-body potential that mimics that of lithium--across the liquid-glass transition. In the glassy phase we find evidences of a fast relaxation process similar to that recently found in Lennard-Jones glasses. The origin of this process is ascribed to the topological disorder, i.e., to the dephasing of the different momentum Q Fourier components of the actual normal modes of vibration of the disordered structure. More important, we find that the fast relaxation persists in the liquid phase with almost no temperature dependence of its characteristic parameters (strength and relaxation time). We conclude, therefore, that in the liquid phase well above the melting point, at variance with the usual assumption of uncorrelated binary collisions, the short time particle motion is strongly correlated and can be described via a normal mode expansion of the atomic dynamics.

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PubMed2002

Pressure evolution of the high-frequency sound velocity in liquid water.

The high-frequency sound velocity v( infinity ) of liquid water has been determined to densities of 1.37 g/cm(3) by inelastic x-ray scattering. In comparison to the hydrodynamic sound velocity v(0), the increase of v( infinity ) with density is substantially less pronounced, indicating that, at high density, the hydrogen-bond network is decreasingly relevant to the physical properties of liquid water. Furthermore, we observe an anomaly in v( infinity ) at densities around 1.12 g/cm(3), contrasting the smooth density evolution of v(0).

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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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