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Starting with an m-diffusion model a matrix description is given of the rotational motion of a dipole molecule undergoing frequent collisions. This treatment gives rise to an analytical expression for the dipole correlation function and for the angular momentum correlation function in which a limited number of parameters from the model appear. It is argued that the collision distribution which determines the rotational diffusion process need not necessarily be a Poisson distribution. In liquids with strong interactions the distribution is governed by the frequency distribution of the medium. This leads to the inclusion of a librational motion in the rotational diffusion model. A comparison of simulations with different collision distributions and experimental data is given
Molecular dynamics calculations on the time dependence of the anisotropic potential experienced by a probe molecule in argon, are presented. The calculations were performed for Ar densities of 100, 300, 500 and 784 amagat and T = 160 K. Collective motions clearly manifest themselves at higher densities, in particular in the power-spectrum of the part of the anisotropic potential that transforms as an irreducible tensor of rank 1. The relation between time-dependent local anisotropy and a number of experiments is discussed.
Many molecular relaxation processes in fluids are sensitive to the time-dependence of local, anisotropic density fluctuations. The role played by anisotropic density fluctuations in the rotational relaxation of a linear, quantized rotor will be discussed in some detail. An expression for the dipolecorrelation function of a probe molecule, dissolved in a simple fluid, will be derived along these lines. This expression will be shown to account for the experimentally observed density-dependence of the rotational linewidths of HCl in argon. Some simple models for anisotropic density fluctuations or, more precisely, for the four-point density-correlation functions, will be discussed.
The rotational far-infrared spectra of HCl in argon at densities between 100 and 480 amagat and T = 162.5 K are presented. The observed density dependence of the width of the different rotational lines is non-linear and differs for high- and low-frequency lines. An Enskog correction to the collision frequency i11 the dense gas fails to account for the density dependence of the high rotational lines. It is argued that the many-body character of the relaxation mechanism should be taken into account. Comparison with the results of MD calculations on argon indicates that such an approach may explain the observed density dependence.
The dipole correlation functions determined from far infra-red absorption measurements of a variety of liquids are compared with correlation functions calculated with different models for the motion of the molecules. These models vary from simple rotational diffusion models to models derived from solid state theory. Experimental results are presented of far infra-red measurements of HCl dissolved in liquid argon and of solutions of CH3CN in some organic solvents. For these liquid systems good agreement can be obtained between experiment and model calculations. It is pointed out that the description of these systems in terms of a model has a strong phenomenological character.
Results of far infrared measurements on the system HCl–Ar at low density and temperature are presented. Distinct spectral features are observed that must be attributed to Ar–HCl van der Waals molecules. Possible explanations of the observed spectra in terms of a simple picture of the internal motion of the complex are discussed. An estimate of the enthalpy of formation is made. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.
From measurements of the pure rotational lines of a linear probe molecule dissolved in a simple fluid one can deduce the power spectra of the anisotropic density fluctuations of /=1 and /=2 symmetry of the fluid. The power spectra thus obtained for argon at various densities are compared with the results of molecular dynamics calculations and with the approximate power spectra which can be calculated starting from neutron-scattering data. The density dependence of the high frequency components of the /=1 power spectrum and of the zero frequency component of the /=2 power spectrum give a strong characteristic of the dense fluid.
The density dependence of the pressure induced shift of the HCl pure rotational lines, perturbed by argon, has been measured. The shifts were measured for the rotational transitions J → J + 1, with J from 1 to 9, at a temperature of 162.5 K and argon densities between 39 and 480 amagat. In this density regime, the HCl rotational linewidths are known to depend non-linearly on the density. It is observed that most of the rotational lineshifts show no significant deviation from a linear density dependence. It therefore seemed justified to determine lineshift cross sections from the slopes of the straight lines that fitted the measured shifts best. The magnitude of the cross sections thus determined is significantly larger than has been predicted. The relevance of the lineshift cross sections for the determination of the HCl---Ar intermole...
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