Model for vibrational dephasing of diatomic molecules in liquids Liquid N2 and O2

Abstract

The characteristics of spontaneous Raman scattering from solutions of diatomic molecules are reviewed, especially with regard to what they may tell one about the dynamics of molecular motion. In the limit of extreme motional narrowing the polarized component of the Q branch is a lrentzian with a width which depends upon the rate (τ^-1) of dephasing of the molecular vibrations. An approximate, closed, quantum-mechanical expression for the contribution of ‘translational-translational’ quasielastic scattering to τ^-1, called τ_ph ^-1(T-T), is derived. The translational states of the liquid are determined by invoking the Lennard-Jones-Devonshire (LJD) cell model. The full LJD cell potential is replaced by an harmonic approximation thereto. The dependence of the τ_ph(T-T) upon thermodynamic variables and microscopic properties is discussed. The derived expression for τ_ph ^-1 (T-T) is then used to compute upper bounds on τ for liquid N₂ and O₂, for which experimental data are available. It is concluded that the T-T quasielastic scattering mechanism is essentially negligible and that other mechanisms must be dominant in determining τ. In particular, it is suggested that solvent-induced vibrational-reorientational coupling may be quite important.