Gareth J. Evans

Studies of molecular motions and vibrational relaxation in acetonitrile

Far-infra-red spectra in the 2–200 cm-1 region of dilute solutions of CH3CN in CCl4 solution have been employed to test the various analytical models available for the description of rotational motions of a symmetric top dipole in the liquid phase. It is found that none of the various approximants of the Mori continued fraction description of reorientational correlation function fit the experimental data satisfactorily. This implies that, although the data (from this and other techniques) may appear to be properly described by a simple rotational diffusion model at long times, the short time dynamics are too complicated to be reproduced even by much more sophisticated models. The data are able to clearly discriminate between the various models tried and, in addition, give valuable information about the nature and extent of the mean square torques acting on the probe molecule.

The dynamics of liquid water: Simulation and submillimeter spectroscopy

Abstract A combination of classical molecular dynamics computer simulation and submillimeter laser and interferometric spectroscopy is used to investigate the dynamical and structural properties of liquid water at 300 K, ϱ = 0.997 gm cm −3 . A new approach to the empirical pair potential, based on the ST2 of Raman and Stillinger has been used to match the low frequency part of the submillimeter spectrum and to generate a range of auto and cross correlation functions. The results from this new atom-atom potential have been compared with those from the work of Stillinger and Rahman and Clementi et al. Broad agreement was obtained and the new potential used to investigate by computer simulation the statistical interdependence of the water molecules rotation and translation in liquid water at 300 K. This investigation has resulted in the detection of a.c.f.s of the classical Coriolis, centripetal, and “non uniform” accelerations of the water molecule, together with some of the numerous cross correlations suggested by the rotating frame theory of Langevin diffusion.

Quantised rotation of HBr in liquid SF6

Abstract Absorption data on HBr in liquid SF6 at 296 K are presented in the frequency range 20–200 cm−1. The broadened J → J + 1 transitions are clearly resolved, more so at the higher frequencies. Although noisy, the data are good enough to discriminate between two models of liquid phase rotational dynamics. It is found that the modified M and J diffusion model of Frenkel and Wegdam rests on sounder hypotheses than a formalism, based on a truncated Mori expansion, that does not predict preferential broadening of the lower frequency J → J + 1 lines.

The Poley absorption in liquid crystals

Abstract The far infrared torsional oscillatory absorption of a nematic liquid crystal has been isolated for the first time free of higher frequency proper modes. This has been achieved using a molecule specially synthesised for this purpose with a dipole moment engineered perpendicular to the long axis so that torsional oscillation about this axis modulates this component only. The far infra-red power absorption of the liquid crystal in the isotropic, nematic, solid and supercooled conditions is therefore a Poley absorption, i.e. the torsional oscillation about the rigid long axis of the liquid crystal molecule itself. The effect of phase changes on a macroscopic level is to sharpen and intensify this absorption. This is clear evidence to the effect that phase changes in liquid crystals are cooperative phenomena, the molecular librational dynamics about the long axis are little different from those in a regular molecular liquid.

The itinerant oscillator model II analysis for dielectric relaxation and incoherent neutron scattering from atoms

A model of brownian motion in a fluid or disordered solid is developed whereby cooperative motions are accounted for in the shell of nearest neighbours of a given molecule. This is mathematically tractable for the angular fluctuations in a plane of a dipole vector embedded in the asymmetric top. The Fokker-Planck-Kramers equations of motion are developed from the initial stochastic differential equations and full solutions derived in terms of multivariate probability density functions. An integro-differential representation of the same system is solved in terms of a generalized Fokker-Planck equation derived by Davies and Evans [24] and valid for a column vector of n linearly independent dynamical variables.

Molecular dynamics simulation and review of some spectral properties of liquid methyl iodide

Abstract The molecular dynamics of liquid methyl iodide have been simulated with a 5 × 5 site-site model potential and the results compared with a variety of spectral data. A review of the results indicates that non pair-additive collision induction is an important factor affecting the spectral features in liquid CH3I. This is seen most clearly in the far infra-red. The computer simulation can therefore only be partially effective for CH3I because it is based on pair-additivity and excludes polarisability effects. Nevertheless the numerical method reveals some details of the molecular motion that cannot be obtained otherwise. These are exemplified by rotation-translation coupling.

The properties of some derivative autocorrelation functions computed with the atom-atom potential

(1977). The properties of some derivative autocorrelation functions computed with the atom-atom potential. Molecular Physics: Vol. 33, No. 6, pp. 1805-1811.

Molecular dynamics in Ch2Cl2/decalin glass

Abstract The dielectric loss in a glassy solution of CH 2 Cl 2 in decalin has been measured in the kHz and THz frequency ranges at 107–148 K. The low frequency part of the loss curve exhibits a peak which shifts upwards by about two decades with a 4 K increase in temperature, and at the glass to liquid transition temperature moves almost immediately out of the audio frequency range into the microwave. The far infrared peak in the loss is displaced by 30 cm −1 to 90 cm − in the glass as compared with the liquid solution at 293 K. These results are interpreted with a new version of itinerant libration whereby collective reorientations are treated as essential in explaining the complete loss profile in CH 2 Cl 2 /decalin glass which covers at least twelve decades of frequency. It is argued that in addition to the well documented primary and secondary losses in glasses and polymers, there exists in general a tertiary process at far infrared frequencies analogous to the Poley absorption in liquids. This is part of the dynamical evolution in a wide range of disordered solids.

The Mori formalism, a critical discussion of its use in representing far infrared and microwave absorptions

Abstract The three variable Mori formalism for molecular dynamics in the fluid state is evaluated using some extensive data of Gerschels on three strongly dipolar symmetric tops in the microwave and far infrared region of the electromagnetic spectrum. It is found that the use of loss and power absorption coefficients can lead to misleading results when theory and experiment are compared and it is essential in general to get the data over the whole frequency range of interest. Using this procedure it is found that the second memory function of the Mori continued fraction is difficult to approximate satisfactorily with a simple decay (such as an exponential) and it is unlikely that successive memory functions of the total orientational correlation function are simpler functions of time than the latter.

Collective modes in a nematic liquid crystal

Evidence is presented for the existence of collective modes in the nematic phase of two liquid crystals by far-infrared spectroscopy. The intensity and frequency of these modes are controlled by externally applied electric and magnetic fields. Their existence suggests that this spectroscopy may serve as a convenient tool for the study of liquid crystals.