M. Nardone

Raman spectra of water in the translational and librational regions: I Study of the depolarization ratio

Polarized and depolarized Raman spectra of room temperature liquid H20 and D20 have been measured up to the bending region. Both spectral shapes and depolarization ratios are analysed in the translational and librational regions. An isotropic spectral component extending over all the investigated frequency range is derived and interpreted in terms of interaction induced mechanisms involving the polarizabilities of the isolated molecule.

Raman spectra of water in the translational and librational region II. Temperature evolution of the isotropic component

Polarized and depolarized Raman spectra in the translational and librational region of liquid water and heavy water have been measured as a function of temperature. A detailed analysis of the isotropic spectrum is performed. This component, which cannot arise from any allowed molecular contribution in this frequency region, shows two distinct spectral features which can be obtained from an analysis of temperature dependent lineshapes. The first one, which dominates the spectrum up to 500crn-1, shows on the basis of a H20-DzO comparison a translational behaviour at very low frequency and a librational one at higher frequencies. It is assigned on the basis of its frequency profile, of its absolute intensity and of its temperature dependence to rototranslational spectra due to long range induction mechanisms already found in many molecular liquids. The second one, which is peaked at roughly 800 cm-1 in water and extends up to the bending mode region suggests the existence of a short range interaction induced polarizability.

Isotropic induced scattering in liquid H2S

Polarization ratio measurements in liquid H2S as a function of frequency are reported from 0 to 1350 cm-1. An isotropic induced contribution to the Rayleigh wing is derived. Absolute intensities, obtained from a comparison with the polarized vibrational stretching, are also discussed.

Correlation functions from depolarized Raman and Rayleigh spectra of N2 at high density and 150 K

Orientational correlation functions derived from the rotational and rotovibrational Raman spectra of N2 at 150 K and 585 amagat are compared with the results of a molecular dynamics simulation available in the literature. The angular momentum correlation function is also estimated from the experimental memory function and compared with the one obtained by simulation. Mean square torques, intermolecular correlations and interaction induced contributions are also discussed.

Interaction induced light scattering

Several simplified models are discussed, more or less directly based upon Kirkwoods superposition approximation, currently employed in interpreting interaction induced light scattering spectra in high density simple fluids. Calculations are performed of the first two spectral moments for room temperature argon at moderate and high densities in Kirkwoods approximation using pair correlation functions from molecular dynamics and the dipole-induced-dipole pair polarizability model. Comparison with full molecular dynamics calculations indicates that at room temperature accurate zeroth and second spectral moments can be obtained using the superposition approximation and shows the limits of more simplified models in describing the density evolution of the spectral moments. The approximation is shown to work also for other induced polarizability models.

Collision induced light scattering in gaseous H2S

Rotational and interaction induced contributions in the light scattering spectrum of gaseous H2S are identified and discussed as a function of density. The intensity of the latter is shown to obey a density squared dependence and to be in agreement with that predicted by the first order dipoled induced dipole (DID) model for point polarizable scatterers.

Raman spectra of fluid nitrogen: intermolecular torques and orientational correlation times

A comparison is made between the rotovibrational Raman bands of fluid nitrogen at room temperature in the 180–355 amagat density range and the corresponding pure rotational bands reported previously. Accurate moment values are derived from the Raman spectra, accounting for the response of the entire experimental apparatus, after subtraction of the unwanted isotope band and of the leaking polarized Q branch. Correlation times are also calculated for both rotovibrational and pure rotational spectra accounting for the finite instrumental slitwidth. Intermolecular torques and orientational correlation times derived from the two spectra are in good agreement for almost the entire available density range. Small discrepancies, particularly evident in the low density correlation times, are discussed and related to the rotation-vibration interaction. Intermolecular correlations are found to be negligible.

Light scattering from disordered media

The Raman scattering cross-section in molecular systems is calculated with the aim of accounting for the low frequency spectrum of disordered media. It is shown that, as far as translational motions are concerned, the isotropic polarizability correlation function is not strongly affected by the structural disorder and is essentially determined by the density-density correlation function. In contrast the anisotropic term depends strongly on the disorder of the system. Its contribution to the scattered light spectrum is governed by single molecule dynamics, thus reflecting in some cases the entire density of translational states, while the intensity is determined by the rotational—translational coupling.

Rovibrational Raman Spectra and Polarizability Constants of the H2S Molecule

Rovibrational Raman spectra of low density H2S gas have been measured with the aim of extracting the components of the polarizability tensor derivatives with respect to the normal modes of the molecule. Absolute values are obtained by comparison with the intensity of the symmetric stretching polarized Q-branch available in the literature. A fit to the free rovibrational spectra is employed in order to separate overlapping bands and to determine the anisotropy ratio for the totally symmetric A 1 bands. The results are compared with those obtained with a bond polarizability model and with SCF calculation performed by Martin, Davidson and Eggers [5].

Interaction induced light scattering in orientationally disordered crystals: the translational phonon region

We have derived an expression for the light scattering spectrum of a crystal in which the mechanically regular sites are occupied by point polarizable orientationally disordered molecules when the polarizabilities are assumed to depend on the positions of the surrounding atoms (interaction induced polarizability fluctuations). Owing to the ‘electrical’ disorder properties of the system all phonons can contribute to the anisotropic scattering measured in all polarization configuration with wavevector, branch index and polarization dependent coefficients. Assuming short range interaction induced polarizabilities we show that the temperature reduced intensity I(ω)/[n(ω) + 1] is given by a superposition of the Brillouin zone centre symmetry ‘projected’ density of states with polarization dependent coefficients. These coefficients are found to be essentially frequency independent for all the projections, exception made for those corresponding to the acoustic phonons. For the acoustic branches the coefficients ...