Calculation of the vibrational linewidth and line shape of Raman spectra using the relaxation function. II. Application to the mixture neon-nitrogen with inhomogeneous broadening due to concentration fluctuations

The Raman spectra of nitrogen in the mixture neon-nitrogen have been measur ed for neon mole fractions, x(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 increa ses as a function of x(M). The linewidth is strongly composition dependent, with a very large value for x(M)=0.65. Further, it is shown that the width decreases as a function of temperature. Molecular dynamics simulations hav e been performed in order to calculate the linewidth and shape, using the r elaxation function. The simulations are in good agreement with experiment. It is shown that the increase in linewidth in the intermediate concentratio n range is due to an increase of the correlation time, rather than an incre ase 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 simu lations also provide insight into the relatively large decrease of the widt h as a function of temperature in the intermediate concentration range. Whe n the temperature is increased from 296 to 408 K, the amplitude of modulati on increases, but the correlation time decreases more. (C) 2000 American In stitute of Physics. [S0021-9606(00)51602-X].