Mechanisms of resonant low frequency Raman scattering from metallic nanoparticle Lamb modes.

The low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species.

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy.

Acoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nanoparticles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle "supermolecules", that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These observations were permitted only thanks to the resonant excitation with the coupling plasmon excitation, leading to an enhancement up to 10(4) of the scattering by these vibrations. This enhanced ultralow frequency Raman scattering thus opens a new route to probe the local elastic properties of the surrounding medium.

Elastic moduli of permanently densified silica glasses.

Modelling the mechanical response of silica glass is still challenging, due to the lack of knowledge concerning the elastic properties of intermediate states of densification. An extensive Brillouin Light Scattering study on permanently densified silica glasses after cold compression in diamond anvil cell has been carried out, in order to deduce the elastic properties of such glasses and to provide new insights concerning the densification process. From sound velocity measurements, we derive phenomenological laws linking the elastic moduli of silica glass as a function of its densification ratio. The found elastic moduli are in excellent agreement with the sparse data extracted from literature, and we show that they do not depend on the thermodynamic path taken during densification (room temperature or heating). We also demonstrate that the longitudinal sound velocity exhibits an anomalous behavior, displaying a minimum for a densification ratio of 5%, and highlight the fact that this anomaly has to be distinguished from the compressibility anomaly of a-SiO2 in the elastic domain.

Viruses as nanoparticles: structure versus collective dynamics.

In order to test the application of the "nanoparticle" concept to viruses in terms of low-frequency dynamics, large viruses (140-190 nm) were compared to similar-sized polymer colloids using ultra-small-angle x-ray scattering and very-low-frequency Raman or Brillouin scattering. While both viruses and polymer colloids show comparable highly defined morphologies, with comparable abilities of forming self-assembled structures, their respective abilities to confine detectable acoustic vibrations, as expected for such monodisperse systems, differed. Possible reasons for these different behaviors are discussed.

In situ Brillouin study of sodium alumino silicate glasses under pressure.

The in situ elastic and plastic behaviors of sodium aluminosilicate glasses with different degrees of depolymerization were analyzed using Brillouin spectroscopy. The observed elastic anomaly progressively vanished with depolymerization. The densification process appears to be different from that observed in pure silica glass. In the plastic regime of densified glasses hysteresis loops were observed and related to modification of the local silicon environment facilitated by the addition of sodium.

Permanent densification of compressed silica glass: a Raman-density calibration curve.

Raman scattering experiments have been carried out to study persistent densification in SiO(2) glass following hydrostatic compression at room temperature. A new relationship linking selective Raman parameters to the degree of densification in the glass has been developed here. This approach will allow quantification of the residual densification in silica following microindentation experiments, with the goal being the development of a constitutive law for amorphous silica.

Progressive transformations of silica glass upon densification.

The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

Non-Debye normalization of the glass vibrational density of states in mildly densified silicate glasses.

The evolution of the boson peak with densification at medium densification rates (up to 2.3%) in silicate glasses was followed through heat capacity measurements and low frequency Raman scattering. It is shown that the decrease of the boson peak induced by densification does not conform to that expected from a continuous medium; rather it follows a two step behaviour. The comparison of the heat capacity data with the Raman data shows that the light-vibration coupling coefficient is almost unaffected in this densification regime. These results are discussed in relation to the inhomogeneity of the glass elastic network at the nanometre scale.

Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations.

Control of nanocrystal (NC) crystallinity currently raises great interest because of its potential benefits in both physics modeling and technological applications. Advances in methods for synthesizing perfect single-crystalline NCs are recent, so that the effect of crystallinity on NC properties has received only limited study and still needs to be properly investigated. Here, we report that crystallinity of gold NCs dramatically modifies their vibrations. Using low-frequency Raman scattering, we clearly demonstrate that single-domain NCs vibrate differently than their multiply twinned counterparts, through the splitting of the quadrupolar vibrations, which is only observed for the former. Using the resonant ultrasound approach, we calculate the vibrational frequencies of a gold sphere and show that elastic anisotropy induces a lift of degeneracy of the quadrupolar mode in good agreement with our experimental measurements. These findings open up challenging perspectives on using Raman spectroscopy to characterize nanocrystallinity.

Elastic properties of viruses.

Viruses are compact biological nanoparticles whose elastic and dynamical properties are hardly known. Inelastic (Brillouin) light scattering was used to characterize these properties, from microcrystals of the Satellite Tobacco Mosaic Virus, a nearly spherical plant virus of 17-nm diameter. Longitudinal sound velocities in wet and dry Satellite Tobacco Mosaic Virus crystals were determined and compared to that of the well-known protein crystal, lysozyme. Localized vibrational modes of the viral particles (i.e., particle modes) were sought in the relevant frequency ranges, as derived assuming the viruses as full free nanospheres. Despite very favorable conditions, regarding virus concentration and expected low damping in dry microcrystals, no firm evidence of virus particle modes could be detected.

Phonon dispersion of oriented DNA by inelastic x-ray scattering.

Films of oriented deoxyribonucleic acid (DNA), prepared by the wet spinning method, have been studied using inelastic x-ray scattering. Spectra were recorded within the range of energy transfers -30< homega <30 meV at momentum transfers homegaQ ranging from 2.5 to 30 nm(-1) whereby the direction of Q essentially coincided with the helical axis. Measurements at ambient temperature cover samples in the A, B, C, and D conformations of DNA. Within the limits of the instrumental resolution, the spectra were analyzed by the response of a damped harmonic oscillator delivering dispersion and damping of modes having displacements with nonzero projections onto Q, i.e., essentially the compression waves traveling along the helical axis. The longitudinal speed of sound resulting from the sinusoidal dispersion varies only weakly with conformation. Our sound speed values are compared to results from Brillouin spectroscopy. The dispersion curves exhibit a minimum at about the inverse rise per residue, which -- together with strong elastic scattering -- reflect the large degree of disorder. Overdamping of the modes is observed for Q>5 nm(-1). The possibility that the observed large damping parameters are due to several contributing modes is discussed in terms of a simple model calculation for an idealized double helix. Whereas the quasicrystalline approximation for an effective disordered chain could well describe the sinusoidal dispersion, it fails to reproduce the observed damping by one order of magnitude. Our results indicate that the high-frequency dynamics of DNA is liquidlike and is most appropriately described by instantaneous normal modes of short correlation length.

Signatures of the hydrogen bonding in the infrared bands of water.

Infrared spectroscopy measurements have been completed over a wide range of frequencies allowing to measure the evolution of both intramolecular and intermolecular vibrational modes in water as a function of temperature. Emphasis is made on the high frequency OH stretching band and the so-called connectivity band that lies in the far infrared region. The substructures of the two infrared bands are analyzed in terms of different levels of connectivity of the water molecules, along the statements of the percolation model. Both band profiles appear to be related to the different degrees of connectivity of water molecules. Comparison of the data with the predictions of the percolation model shows good agreement as for the temperature evolution of liquid water. This work provides additional support to the interpretation of water bands substructures as signatures of its very specific connectivity pattern.

Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals.

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.

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.

Intrinsic vibrational coherence in face-centered cubic supra-crystals of silver nanocrystals: Raman scattering measurements.

The ordering of silver nanocrystals is tuned from amorphous aggregates to highly well-ordered, face-centered cubic supra-crystals, using various substrates and controlling their temperature to obtain this. Low-frequency Raman scattering, for the first time, demonstrates vibrational coherence in fcc supra-crystals of nanocrystals. This is shown by a narrowing of the peak corresponding to the quadrupolar modes of the nanocrystals. However, this is obtained when the supra-crystals are smaller than the excitation wavelength. When the supra-crystals are larger, the narrowing cannot be observed. Furthermore, for any size of the supra-crystals, a shift to low frequency of the Raman peak due to the Lorentz field effect is seen.

High-frequency dynamics of the glass former dibutylphthalate under pressure.

The high-frequency dynamics of a fragile molecular glass former (dibutylphthalate) was studied through inelastic x-ray scattering (IXS), as a function of pressure and temperature. The mesoscopic structural arrest associated with the glass transition process was tracked by following upon cooling the inelastic excitations at fixed Q points in the dispersion curves, at ambient pressure and 2 kbar. The application of pressure to this system induces an offset between the macroscopic glass transition temperature T(g) and the mesoscopic glass transition temperature, as determined from IXS. The concomitant fragility decrease of dibutylphthalate under pressure unveils that the stronger the glass former is, the more its mesoscopic dynamics differ from the macroscopic regime. This trend is interpreted as the signature of a nanoscopic inhomogeneous elastic network. Further aspects of this system are obtained when studying the temperature dependence of its nonergodicity factor f(Q)(T). The chemical specificity of the molecule is suggested to be responsible for the nonobservation of a critical temperature T(c) in dibutylphthalate up to approximately 300 K.

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).

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.

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.

Retrograde axonal transport after radioactive serotonin injections into the olfactory bulb: a biochemical analysis of transported radioactive material.

A biochemical analysis of radioactive compounds was performed in the olfactory bulb (OB) and raphe dorsalis (RD) after injection of radioactive [(3)H] or [(14)C]serotonin (5-HT ranging from 10(?2) M to 10(?7) M) into the OB of rats treated or not with a monoamine-oxidase inhibitor (MAOI). In the OB of untreated rats, radioactivity was associated with precipitated protein and soluble perchloric acid (PCA) fractions. High performance liquid chromatography (HPLC) analysis of the PCA-supernatant gave 4 radioactive peaks: one associated with endogenous 5-HT, another with endogenous 5-hydroxyindole acetic acid (5-HIAA) and two without any relationship with endogenous hydroxyindoles: a '5-HT derivative A' and a '5-HT derivative B'. The presence of these '5-HT derivatives' was significantly reduced after treatment with 5,6-dihydroxytryptamine. In the RD, radioactivity was associated with the protein fraction and with '5-HT derivative A'. The kinetic analysis (from 30 min to 46 h) of the '5-HT derivative A' was characterized by a disappearance in the OB and an accumulation in the RD corresponding to a rate of migration in a range of 0.7 to 2 mm/h. This compound was absent or negligible in other non-serotoninergic neurons (such as the Locus Coeruleus, Amygdala and Cortex piriformis). No clear evidence for retrograde transport of radioactive 5-hydroxytryptophan (5-HTP) or 5-HIAA was found. At lower concentration of 5-HT injected into the OB, the half lives and the times of maximal accumulation for 5-HIAA, '5-HT derivative A' and '5-HT derivative B' were increased. The specific activity of 5-HT and 5-HIAA was also increased. The selective radioactive accumulation in the cell bodies of RD neurons after injection of radioactive 5-HT into the OB is discussed as resulting from a selectivity in (a) the uptake by 5-HT nerve terminals; (b) the metabolism of 5-HT into '5-HT derivative A' in the OB; (c) the retrograde axonal transport of '5-HT derivative A'. This '5-HT derivative A' could represent a messenger between nerve terminals and cell bodies and could be involved in homeostatic mechanisms that maintain cellular dynamics. When a MAOI was used, '5-HT-derivative A' and [(3)H]5-HT were found in the OB and also in the RD cell bodies, and to a lesser extent, in the non-serotoninergic cell bodies. These results indicate that MAO inhibition produces a relative non-selectivity in the 'uptake-metabolism and retrograde axonal transport' systems.