A three centre Lennard-Jones model of CS2 predicts time dependent properties of the liquid, along the orthobaric curve, in good agreement with experiment. The molecules diffuse more easily parallel to their original orientation. The angular velocity autocorrelation function (acf) is an excellent approximation to the first memory function of the reorientational acf. Simple models of the orientational motion fail because the torque and angular velocity acf's relax on similar time scales. Collective correlations of the orientational and centre of mass density fluctuations are calculated with increased accuracy at a number of small non-zero wavevectors.

Time correlation functions for a model of liquid carbon disulphide

Measurements of the anisotropic Rayleigh and the allowed Raman spectra of carbon disulphide over the normal liquid range are described. The Raman reorientation times agree well with N.M.R. values and are qualitatively well predicted by the Stokes-Einstein-Debye formula with slip boundary conditions, although significant departures are found. The Hubbard relationship is obeyed. A careful comparison of the Rayleigh and anisotropic Raman intensities indicates that the orientational correlation parameter (g 2) decreases with increasing temperature. Using values of g 2 obtained in this way the ratio of the Rayleigh and Raman reorientation times is shown to be consistent with a small value for the correlation in angular velocity between molecules. The vibrational dephasing time is found to be roughly constant with temperature in contrast with the predictions of the theories for this quantity. An attempt is made to correlate its behaviour with that of the collision induced Raman spectra of CS2.

Properties of liquid CS2 from the allowed light scattering spectra

Intermolecular vibrational motion in CS2 liquid at 165 ⩽ T⩾ 300 K observed by femtosecond time-resolved impulsive stimulated scattering

The laser light scattering intensity fluctuations of supercooled liquid o‐terphenyl at temperatures between 12 and −15 °C have been studied using the photon‐correlation technique. The time correlation functions obtained using the VV and VH scattering configurations at 90° scattering angle show a wide distribution of relaxation times. Angular‐dependent measurements at 45°, 90° and 135° scattering angles show that the time correlation functions for VH and for VV are independent of scattering angles. The VV and VH correlation functions cannot be fit to a bimodal distribution function; neither can they be fit to a cumulant expansion expression with less than four cumulants, but they can be satisfactorily fit to the William–Watts distribution function. The mean relaxation times derived for both isotropic and anisotropic scattering processes from the VV and VH correlation functions are nearly equal. The temperature dependence of the relaxation times cannot be described by an Arrhenius equation with a constant activation energy. The average orientational relaxation times change by more than ten orders of magnitude in the temperature range between 147 and −16 °C but the temperature dependence of the orientational relaxation times over this wide dynamic range can be excellently correlated with the modified Debye–Stokes–Einstein equation. The closeness of the mean orientational relaxation time to the mean structural relaxation time is interpreted as due to strong rotation–translation coupling in the viscoelastic state. It is found that within the dynamic range of the autocorrelator, the collective shear wave plays no role in the time dependence of the VV and VH correlation functions. Quentrec’s theory of viscoelasticity based on the coupling of the shear wave to local translational order and orientational order is found to disagree with the experimental result. Comparison with the dielectric relation time data shows that in o‐terphenyl the dielectric relaxation and the present light beating techniques probe different types of motions.

Homodyne light beating spectroscopy of o-terphenyl in the supercooled liquid state

Molecular motions in simple liquids are observed on the femtosecond time scale by means of impulsive simulated scattering. In carbon disulphide and benzene liquids, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of molecular orientational motion which at short times is vibrational (i.e. librational) in character due to intermolecular interactions. In CS/sub 2/, the dephasing of the oscillations is primarily inhomogeneous. Temperature-dependent values of the configuration-averaged librational frequency and the extent of its inhomogeneity are determined. >

Molecular dynamics in liquids from femtosecond time-resolved impulsive stimulated scattering

The depolarized doublet found in the Brillouin spectra of many molecular liquids has been observed in liquid CS2. This doublet is best resolved at 162°K (near the freezing point); it broadens rapidly with increasing temperature and is unresolved at temperatures higher than 192°K. In this temperture range, the half‐widths increase from 11 to 19 GHz, yielding orientational relaxation times of 14×10−12 to 8×10−12 sec, respectively. The observed spectra are in good agreement with the predictions of hydrodynamic (Rytov) and microscopic (Keyes and Kivelson) theories.

B. P. Stoicheff

Papers

Depolarized Brillouin spectrum of liquid CS2