J. P. McTague

Molecular dynamics studies of orientational and collision‐induced light scattering in molecular fluids

Molecular dynamics calculations of the Rayleigh and Raman light scattering properties of atom–atom Lennard‐Jones fluids (’’N2’’, ’’O2’’, ’’Cl2’’, and ’’CO2’’) have been made, including collision‐induced (CI) effects to lowest order in the dipole‐induced dipole approximation. CI effects are shown to result in deviations from the Lorentz–Lorenz (Claus–Mosotti) relation, changes in the spectral moments of both Rayleigh an Raman bands and in their depolarization ratios. Where possible, the predicted magnitudes are compared with experiment. Calculated CI influences on the effective molecular polarizability are compared with theoretical estimates based on the superposition approximation. The importance of isotropic CI light scattering in molecular fluids is indicated. Finally, we discuss the separability of orientational and CI contributions to depolarized spectra.

Molecular dynamics study of an amorphous Lennard‐Jones system at low temperature

Using molecular dynamics techniques, low temperature properties of a 500‐particle amorphous system of Lennard‐Jones particles have been studied. The system was prepared by rapid quenching of an originally liquid sample. The structural properties are similar to those of random close‐packed hard spheres. Propagating longitudinal phonons are observed even at fairly short wavelengths and well defined transverse phonons are found at the longest wavelengths available. Some preliminary results for the ’’glass transition’’ region are reported.

Theory of k‐Independent Depolarized Rayleigh Wing Scattering in Liquids Composed of Anisotropic Molecules

A discussion is presented of the k‐independent factors that contribute to the depolarized Rayleigh wing scattering in liquids composed of anisotropic molecules. The spectrum consists of a sharp line (several tenths cm−1) and a broad line (several cm−1 or tens of cm−1). It is suggested that the spectrum arises from at least two separate processes, each of which induces fluctuations in off diagonal elements of the dielectric tensor: molecular reorientation, which is a relatively slow process, and some form of intermolecular interaction, which is a rapid process. The autocorrelation functions for the slow and rapid processes account in large part for the sharp and broad lines, respectively; however, the heretofore neglected cross correlation function between the slow and rapid processes may contribute significantly to the spectrum, particularly to the sharp line which may be appreciably reduced in intensity due to the cross correlation. This reduction in sharp line intensity suggests that the difference betw...

Crystal nucleation in a three‐dimensional Lennard‐Jones system: A molecular dynamics study

We have observed crystal nucleation in a Lennard‐Jones fluid by molecular dynamics. In a 108‐particle system cooled slowly below its triple point, crystallization occurs at kT/e=0.50 and 0.57 for densities Nσ3/V =0.90 and 0.91, respectively. For larger systems (256 and 500 particles) crystallization has also been achieved. The supercooling limit decreases with increasing system size. These results are compatible with classical nucleation theory.

Crystal nucleation in a three‐dimensional Lennard‐Jones system. II. Nucleation kinetics for 256 and 500 particles

We have, by molecular dynamics computer simulation, observed homogeneous, spontaneous crystal nucleation in the 256‐particle Lennard‐Jones fluid at ρ*=0.91, 0.92, and 0.93, and in the 500‐particle fluid at ρ*=0.92, 0.93, and 0.94. For the 500‐particle system a nuclear region may be identified, and the nucleation and growth processes are found to be temporally separate. In all these events the nucleated phase was a body‐centered (nearly cubic) structure. We believe this to be a manifestation of the Ostwald step rule.

Phase Diagram of Hard Ellipsoids of Revolution

Abstract We present the results of Monte Carlo simulations of the equation of state of hard ellipsoids of revolution with axial ratios a/b = 3, 2.75, 2, 1.25, 0.8, 0.5, 0.3636 and 0.3333. We identify four distinct phases, viz. isotropic fluid, nematic fluid, ordered solid and plastic solid. In all cases the thermodynamic phase transitions are located by free energy computation. We find nematic phases only for a/b ≥ 2.75 and a/b ≤ 1/2.75. A plastic solid phase is observed for 1.25 ≥ a/b / 0.8. It is found that the phase diagram is surprisingly symmetric under interchange of the major and minor axes of the ellipsoids.

A Monte Carlo simulation study of the two‐dimensional melting mechanism

We report here a Monte Carlo study of the thermodynamic and structural properties of a two‐dimensional system of 2500 particles interacting by a repulsive inverse sixth power potential. Particular effort was made in the melting region, both to identify the defect structures and to ascertain the thermodynamic changes associated with the transition. Significant precursor fluctuations occurred on both the solid and fluid sides, and the unbinding of dislocation pairs was observed on melting. More complex defect patterns, including grain boundary loops, were also generated near melting. The Kosterlitz–Thouless, Halperin–Nelson, and Young dimensionless elastic constant approached the universal value 16π in the solid, and the orientational correlation range tended to diverge in the fluid near freezing. However, we were unable to distinguish between a very weak (ΔV/V≂0.7%) first order and a continuous transition, since both spatial structures and temporal relaxations became exceedingly long.

Raman studies of the orientational motions of small diatomics dissolved in argon

We report a study of the orientational motions of oxygen and nitrogen dissolved in argon, using Raman spectroscopy. First, second, and fourth moments are reported for the rotation and vibration–rotation bands of these molecules at various concentrations along the liquid–vapor coexistence curve. It was found that less mean square torque is exerted on an oxygen molecule in argon solution than in the pure liquid at similar temperatures. The nitrogen data are probably in accord with this, although the experimental uncertainty is greater. Experimental values of the second moments are significantly in excess of the predicted classical values, even including important quantum corrections. This is probably due to ’’collision‐induced’’ effects which were not accounted for in the moment analysis. Molecular dynamics simulations of rigid rotor ’’nitrogen’’ were made and the first order DID contributions to the collision‐induced vibration–rotation spectrum were calculated. Even leaving out any high frequency overlap m...

Liquid Cl2 dynamics studied by depolarized Raman and Rayleigh scattering

Polarized and depolarized Raman and Rayleigh spectra of liquid chlorine have been measured along the coexistence curve from 184 to 238 K. Results are compared with predictions of current simple models for rotational diffusion and vibrational dephasing in liquids. In accord with recent investigations for polyatomic species, it has been found that the temperature dependence of the single particle reorientational time τsp is proportional to the viscosity/temperature ratio, η/T. Despite evidence for specific angle‐dependent valencelike forces between Cl2 molecules, comparison of the observed τsp with that predicted by the hydrodynamic ’’slip’’ and ’’stick’’ models shows clearly that the former provides much better agreement with experiment. The temperature dependence of the vibrational dephasing time is controlled by motional narrowing. A number of caveats associated with the extraction of reliable information concerning collective reorientational motions from depolarized Rayleigh scattering are briefly discu...

Raman spectrum of a Lennard‐Jones glass

A normal mode analysis of Lennard‐Jones glass, generated by molecular dynamics methods, has been made. The first order Raman spectrum of such a system of polarizable atoms is then calculated. Results agree well with the observed collision‐induced light scattering spectra of liquid rare gases. Comparison is also made with a model which assumes a single mode‐independent scattering matrix element, and significant differences are found.