Vibrational dephasing and frequency shifts of polyatomic molecules in solution

A theory for the effects of repulsive and spatially slowly varying attractive forces on the vibrational frequency and dephasing of polyatomic molecular liquids is developed. Thermodynamic state dependence of these features is of particular interest because of the competition between the two types of forces. Solvent shifts of vibrational lines are computed. Proper separation of the rapidly and slowly varying branches of the intermolecular potentials leads to a separation of time scales that allows the dephasing relaxation to be computed using a combination of binary collision and mean field ideas. The question of homogeneous versus inhomogeneous broadening of the isotropic Raman line is addressed. The theory is applied to several polyatomic fluids with special emphasis on the isothermal density dependence of the spectral features. Agreement with experiment is good. The slowly varying attractive forces are found to play a significant role in determining the Raman linewidth for all the systems studied. Comparison with previous theories is made.