M. Sampoli

Raman spectra of fluid N2

The depolarized rotovibrational band in fluid N2 has been measured in a wide temperature and density range. The behaviour of normalized second moment versus temperature and density is analysed in order to clarify the role played by the various non-rotational contributions. Practically no dependence on temperature is found while a net increase with density is present. Induced effects are carefully analysed in the dipole-induced-dipole approximation. In particular exact formulas are derived for both the intensity and second moment of induced cross contributions. Good agreement with experimental data is obtained. The cross contributions are responsible for the observed second moment increase with density. The different second moment density behaviour of rotovibrational and rotational bands has been interpreted in terms of the different permanent anisotropies and different many-body contributions involved in the scattering process.

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.

Rayleigh spectra of N2 fluid

The normalized second moments of rotational Rayleigh bands of fluid N2 are reported versus density at various temperatures (296, 202 and 150 K). The reduced second moments fit quite well a universal curve which differs significantly from the pure rotational value in the overall density range investigated (50 < ρ < 700 amagat). The discrepancies are attributed to the induced contributions. In particular, the purely collision induced contributions are responsible for the low density decay, while the cross interference between permanent and induced anisotropy increases the second moment at increasing density. A ‘theoretically’ constructed second moment is reported which reproduces quite well the experimental data up to the critical density. Experimental torques derived from M 4/M 2 and M 4/Mo have been briefly discussed. Some conclusions on the spectral shape analysis and mean square torque values have been worked out.

Raman bands of fluid N2

Molecular dynamic simulations and experimental measurements on Raman bands of fluid N2 at about 150 K and two densities (1·05 and 1·8 times the critical density) are reported. Induced effects are accounted for by the dipole-induced-dipole mechanism at the first order. A satisfactory agreement between time correlation functions and spectral shapes derived from simulation and experiment is obtained. A detailed analysis of the different contributions to the total correlation function is reported, either performing or without performing the projection of the collision induced polarizability onto the orientational variables. The density dependence of static and dynamical properties is discussed together with the role played by the cross and pure collision induced contributions. The approximation of neglecting all orientational correlation is successfully used for the estimate of the pure collision induced terms.

Raman spectra of water in the translational region

Abstract Light-scattering spectra of room-temperature liquid water in the region 10–400 cm −1 are presented and the isotropic component is derived. A comparison between H 2 O and H 2 S spectral shapes shows that the isotropic spectra of water arise mainly from interaction-induced mechanisms involving the first few terms of a multipolar and polarizability series expansion for the isolated molecule.

Induced contributions in the rayleigh spectra of water: A molecular dynamics simulation

Abstract A molecular dynamics simulation of TIP4P water at T =280 K has been performed to calculate the depolarized and isotropic Raman spectra in the translational and librational region. Comparison of the isotropic spectra with experiment confirms the assignment of the low-frequency scattered light to collision-induced phenomena and allows identification of the spectral contributions arising from polarizability modulation due to short-range interactions.

Induced contributions in isotropic and anisotropic Rayleigh spectra of fluid H2S

Abstract Isotropic and anisotropic Rayleigh spectra of fluid H 2 S are reported. Collision-induced rotational scattering contributions are considered to explain the spectral features. It is argued that isotropic spectra are mainly due to the dipole-quadrupole polarizability at high frequency while the first hyperpolarizability is significant at low frequency.

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.

Density evolution of Rayleigh and Raman depolarized scattering in fluid N2

Spectra of gaseous N2 at densities ranging from 20 to 330 Amg at room temperature have been recorded both for the pure rotational Rayleigh bands and for the rotovibrational Raman bands. Fits to a suitable parametric function have been attempted in order to work out the density evolution of the orientational scattering, and the presence of an eventual contribution from other effects. Deviations from these fits have been analysed and it was possible to conclude that the main effect of collisions on molecular rotations is a reorientation mechanism. A moment analysis of all the spectra has been performed and a study of the behaviour of M 2, of M 4/M 2 and M 3/M 1, and of M 4/M 3 was accomplished in terms of rotations and collisions, of intermolecular torques and of intermolecular orientational correlations respectively. A theoretical survey of depolarized light scattering is introduced in order to outline the possible presence of different contributions and of cross correlations between them.

A light scattering study of rotational bands in fluid CO2

Depolarized light scattering spectra of fluid CO2 are studied in a wide range of temperature and density. At ρ = 501 amagat and T = 263 K, both time correlation function and spectral moments compare well with the molecular dynamics results obtained in the site-site model of induced interactions. The evolution of the normalized second moment is analysed. At low density induced and rotational distinct contributions have been calculated in the centre-centre model using different potential models. The inclusion of quadrupolar interactions in the potential model seems to be important in reproducing the experimental second moment behaviour. Mean square torques, derived from the fourth spectral moments, are discussed and compared with computer results.