COMPUTER-SIMULATION STUDY OF RAYLEIGH AND RAMAN-SPECTRA AND SPECTRAL MOMENTS OF FLUID C2H6

Depolarized Rayleigh and Raman scattering from fluids is composed of orientational (OR), collision induced (CI), and cross (X) contributions. In order to investigate the influence of the interaction induced terms (CI and X) on allowed spectra (OR), C2H6, a molecule with a small ratio of the anisotropic part gamma to the isotropic part gamma of the polarizability is considered. Both Rayleigh and nu(3) Raman correlation functions were determined by molecular dynamics (MD) computer simulation and compared with experiment. The investigations extend over a wide range of densities (0.67 rho(c)-2.85 rho(c), where rho(c) is the critical density, rho(c)=0.206 g/cm(3)). The different contributions to the time correlation functions (TCFs) were simulated within the frame of a first-order center-center dipole-induced-dipole polarizability model. The potential model used was a standard two-site Lennard-Jones potential. Details concerning partial OR, CI, and X correlation functions and second moments for Rayleigh and Raman scattering are presented. At the lower densities a large CI and a smaller X contribution to the total Rayleigh TCF is found. At higher densities the X and CI contributions increase but tend to cancel each other so that the OR contribution becomes more important. Concerning the Rayleigh second spectral moments an explanation of the minimum of the experimental second moment observed at intermediate densities and of the increase of the second moment with density for higher densities is presented. Both the Raman TCFs and second moments are essentially orientational due to the large polarizability anisotropy derivative gamma'. (C) 1995 American Institute of Physics.