Concentration dependence of the vibrational band shape and frequency in binary mixtures.

We propose a stochastic model which describes vibrational broadening of Raman lines in binary mixtures. The line broadening by a dynamic environment is included by considering two-particle exchange processes in the nearest neighbor shell of the reference molecule. Different environment states influence the vibration of the reference molecule. A finite interaction time between the molecules leads to finite correlation times in the vibrational relaxation. As a result the symmetric band shapes in the neat liquids deviate from Lorentzian profiles. Experimentally this is observed for the symmetric C–H stretching vibration in neat CH2I2. The model represents an extension of the model proposed by Knapp and Fischer, which reproduces Lorentzian shapes in the neat liquids. A quantitative comparison of both models with experimental data for the C–H stretching vibration in CH2I2/CCl4 mixtures is performed at several mole fractions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]