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The spectral line shape of the fundamental vibration of nitrogen is calculated from molecular dynamics simulations by determining the Fourier transform of the relaxation function. It has been applied to the fluid phase at various pressures and temperatures, and to solid δ-N[sub 2]. The validity of the assumption that the spectrum at relatively high temperatures and pressures can be calculated by assuming that these systems are in the fast modulation regime (Δτ[sub c]<1), has been verified. A deviation of the vibrational line shape from the motional narrowing limit has been found for fluid nitrogen at low pressure, with a Kubo parameter, Δτ[sub c], equal to 0.23, and for the vibrational line of the molecules on the a sites in δ-N[sub 2], with Δτ[sub c] equal to 0.075. It is concluded that the value of the Kubo parameter is not an unambiguous criterion for the fast modulation regime. Moreover, a detailed comparison reveals a difference in the dynamical behavior of the molecules on the a and c sites. It is shown that this procedure can also be used if one does not know whether or not closely spaced lines are to be expected. The present procedure is suited to calculate line shapes in the intermediate Kubo regime, e.g., in concentrated mixtures, where no simple relations are available. Finally it is shown that in nitrogen at low density and 126 K the ratio of the correlation time of the frequency autocorrelation function and the dephasing time is smaller than in CH[sub 3]I, where the n dependence of the vibrational overtone is subquadratic. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Raman spectra of nitrogen in the mixture neon-nitrogen have been measured for neon mole fractions, x[sub M], of 0.10, 0.65, and 0.95 at 296 K and 408 K up to the solidification pressure. It was found that the frequency increases as a function of x[sub M]. The linewidth is strongly composition dependent, with a very large value for x[sub M]=0.65. Further, it is shown that the width decreases as a function of temperature. Molecular dynamics simulations have been performed in order to calculate the linewidth and shape, using the relaxation function. The simulations are in good agreement with experiment. It is shown that the increase in linewidth in the intermediate concentration range is due to an increase of the correlation time, rather than an increase of the amplitude of modulation. In this range inhomogeneous broadening due to concentration fluctuations occur: the vibrational line shape starts to deviate from a Lorentzian curve, and gets a Gaussian component. The simulations also provide insight into the relatively large decrease of the width as a function of temperature in the intermediate concentration range. When the temperature is increased from 296 to 408 K, the amplitude of modulation increases, but the correlation time decreases more. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]