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Notes: At high densities intramolecular vibrations are strongly dependent on the interactions with the surrounding molecules. In this paper a study is made of the consequences of these interactions on the Raman Q-branch of nitrogen. In particular the difference between a disordered and an ordered surrounding is surveyed. For this purpose, high-resolution Raman spectroscopy has been performed at room temperature on pure nitrogen as well as on a dilute mixture of nitrogen in argon, around the fluid–solid phase transition of these systems, which occur at (approximate)2.5 GPa and at (approximate) 1.3 GPa, respectively. Going from the liquid to the solid phase, a positive jump in the line shift and a dramatical drop in the linewidth are seen in both systems at the transition pressure. For a better understanding of the underlying mechanisms, molecular dynamical simulations have been performed on corresponding model systems. The results of these calculations are in fair agreement with the experimental data and reveal the reasons for the discontinuities. Although the average distance of the nearest neighbor molecules around the nitrogen molecule increases, the distance to the nearest neighbor molecules in line with the molecular axis of the nitrogen decrease at the phase transition. This results in a positive jump in the frequency. Further, the time-autocorrelation function of the vibration frequency has a long persisting positive tail in the fluid phase. This behavior is absent in the solid phase. Even more important is that this function has negative values during a substantial time interval in the solid phase. As a result, the correlation time is greatly reduced at the phase transition, which results in an important reduction of the linewidth as well. Finally, it is proven that also in the solid phase the nitrogen is really dissolved in argon. © 1998 American Institute of Physics.

Notes: It is known from experimental evidence that the Raman shift of nitrogen and nitrogen in argon, measured as a function of the pressure at ambient temperature, reveals a positive jump at the transition from the liquid to the solid phase. Intuitively, this increase is sometimes attributed to the increase of density, but recently it has been shown that the change in order at the transition also plays a role. The present study deals with the behavior of nitrogen diluted in xenon. In this system, even a negative jump is found experimentally. Computer simulations on a model system for the N2–Xe mixture make clear that in this case the effect of the change in order is opposite. © 1999 American Institute of Physics.

Notes: A lattice gas model with nonadditive lateral interactions is used to describe Ag, Au and AgAu bilayer films on a Ru(0001) substrate. By Monte Carlo simulations of adsorbate configurations and thermal desorption spectra of the Ag, Au and AgAu films, the microscopic interaction parameters of the gold and silver adatoms in the alloy film on Ru(0001) are obtained. The model reproduces experimental thermal desorption spectra of Ag, Au and AgAu films on Ru(0001) and also shows segregation of silver in the second monolayer of the alloy in agreement with experimental results. Based on perturbation theory, the configurational Gibbs free energy G of the AgAu monolayer alloy is calculated as a function of temperature and composition. © 1998 American Institute of Physics.

Notes: We have performed Monte Carlo simulations in order to determine the γ–β, α–γ, and the α–β phase transition lines of nitrogen with a recently developed nitrogen–nitrogen potential, and to examine the driving forces for these transitions. We have shown that it is possible to obtain the α–γ phase transition line by starting, at higher temperature, with the hexagonal representation of the disordered fcc structure. The transition line was found about 0.4 GPa higher than the experimental line. The orientational order–disorder behavior of the γ–β and the α–β phase transitions could be observed, but the structural changes, fcc–hcp, did not occur, probably due to a potential barrier. It is also possible that the potential model causes the fcc structure to be stable with respect to the hcp structure. The orientational entropy of the various phases has been compared qualitatively by introducing the orientational order parameters. It is suggested that a small difference in translational entropy, due to a difference in the c/a ratio, stabilizes the hcp structure (β phase) with respect to the fcc structure at zero pressure. In contrast with previous work, our simulations reveal that not all the layers of the ordered hcp structure at low temperature have the same orientational order. This might be the reason that the ordered fcc structure is stable at low temperature. © 1997 American Institute of Physics.

Notes: A study on the Raman shift and width of nitrogen and nitrogen in helium has been performed as a function of pressure and temperature by means of experiments, molecular dynamics (MD) simulations and hard fluid (HF) theory. The experiments have been performed using Raman spectroscopy in a diamond anvil cell at pressures up to 10 GPa and temperatures between 250 and 400 K. Both the experimental shift and width results of pure nitrogen link up very well with accurate measurements at lower pressures and with less accurate measurements at higher pressures. For the first time the Raman shift and width have been determined as a function of temperature at an isobar, such that a sensitive test of theoretical models can be made. The MD calculations on the linewidth along an isobar show very good agreement with experiment. The influence on the linewidth of the bondlength dependence of the site–site interaction parameters (often called the attractive contribution) appears to be small, which indicates that this has a small anisotropy. For pure N2 the MD and the HF calculations of the repulsive contribution to the Raman shift are about the same. This shows that both ways of calculation are consistent. The experimental Raman shift of nitrogen diluted in helium appears to be much larger than that of pure nitrogen. In contrast, the linewidth is much smaller than that of pure nitrogen. HF calculations were also performed for the Raman shift of N2, infinitely diluted in He. The results for the bondlength independent (repulsive) contribution give clearly smaller values than those of the experiment, which means that the effect of the change of the potential parameters at excitation must be positive. This implies that that part of the intermolecular potential, which is due to the overlap of the molecular charge distributions has a dependence on the bondlength, that results in a positive contribution to the Raman shift. It will be shown that for N2 the good agreement with experiment of earlier HF calculations with an attractive contribution, based on a purely dispersive model, is due to a cancellation of errors. For nondiluted mixtures of He–N2 under noncritical conditions the plot of experimental FWHM values as a function of the volume fraction shows a broad maximum, which is indicative for inhomogeneous broadening. This behavior is described with the help of the Knapp–Fischer model. © 1996 American Institute of Physics.

Notes: The relaxation behavior of single linear and ring polymers, with and without excluded volume, is studied by Brownian dynamics. The velocity, angular momentum, and end-to-end vector autocorrelation functions are investigated. It is found that the velocity and angular momentum autocorrelation functions decay exponentially and that the time dependence of the vector end-to-end autocorrelation function is mainly determined by the time dependence of its direction.

Notes: Bead size effects on the excluded volume of two-dimensional linear and ring polymers are investigated with Brownian dynamics. It is found that the mean dimensions of the chains follow a scaling relation with scaling variable X=N(σ/l)d/φ, where N is the number of units on the chain, σ is the size of the unit, l is the link length, d is the dimension, and φ is the crossover exponent. The scaling law is 〈R2〉/〈R2〉0 or 〈S2〉/〈S2〉0∼X2ν−1 for X→∞. Here ν is the critical exponent for the mean dimensions of an isolated polymer chain and the subscript 0 denotes the nonexcluded volume case.

Notes: The shapes of two-dimensional linear and ring polymers, with and without excluded volume, are investigated by determining the ratios of the two principal orthogonal components of the mean-square radius of gyration. It is found that ring chains are more symmetrical than linear ones.

Notes: Ring and linear polymers, with an without excluded volume, have been studied by Brownian dynamics. The shape of the polymers was investigated by calculating the principal orthogonal components of the radius of gyration. Ring polymers are found to be more spherical than linear ones. (AIP)

Notes: We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N2 clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N2 molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densification is larger for the latter. In both cases, the transition is reversible. © 2001 American Institute of Physics.