The dynamic structure factor, S(Q, ω), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4÷16.5 nm −1 and up to energies ω=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q≈5 nm −1 , varies from ≈6500 to ≈9000 m/s: this phenomenon was never experimentally observed in a glass.

The study of the effects of the density variations on the vibrational dynamics in vitreous silica is presented. A detailed analysis of the dynamical structure factor, as well as of the current spectra, allows the identification of a flattened transverse branch which is highly sensitive to the density variations. The experimental variations on the intensity and position of the Boson Peak (BP) in v-SiO2 as a function of density are reproduced and interpreted as being due to the shift and disappearance of the latter band. The BP itself is found to correspond to the lower energy tail of the excess states due to the piling up of vibrational modes at energies corresponding to the flattening of the transverse branch.

This paper compares the luminescence spectra of Eu3+ in sol-gel derived silica samples heated at different temperatures, following the densification process from wet gel to compact silica glass. Lifetimes, linewidths, and Stark splittings of the transitions were used to study the structural evolution of the gel network.

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.

We present structural and dynamical results of molecular dynamics simulation of vitreous silica undergoing a hydrostatic compression and decompression cycle at room temperature. Structural results as a function of density compare fairly well with experiments as well as with the longitudinal and transverse sound velocity pressure dependence. A shift of the boson peak (BP) toward higher energies and its simultaneous weakening is observed as in experiments. A detailed study of the dispersion of the glass vibration is presented at several densities and for the densified state. Evidence of phonon-like character with two distinct pseudo-periods is shown for longitudinal and transverse dynamics. The relationship between the BP vibrations and the correlation length scale indicated by the first sharp diffraction peak is discussed.

The high-frequency dynamics of v-GeO2 was investigated by inelastic neutron scattering. Neutron experiments were carried out by exploiting both three-axis and time-offlight techniques as most adequate to selectively access to a wider kinematics region or to higherresolution characteristics. Experimental evidences of a highly dispersing inelastic peak, with an associated velocity of 3650 m s −1 , and an almost dispersionless inelastic structure allowed us to definitely confirm the propagating nature of the high-frequency excitations. The measured data were compared with the results of a molecular-dynamics simulation.

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Single-crystal samples of L-histidine hydrochloride monohydrate (LHICL), C 6 H 9 N 3 O 2 ·HCl·H 2 O, were studied by Raman spectroscopy at temperatures ranging from 295 to 40 K over the spectral range 20-3400 cm −1 . A tentative assignment of the bands is given. The effect of temperature change on the vibrational spectrum is discussed. The behavior of the Raman spectra, in particular the emergence of new modes in the low-wavenumber region, indicates that LHICL undergoes a structural phase transition between 140 and 110 K. There are further indications that, possibly, a second change of structure occurs for temperatures between 80 and 60 K.