Density dependence of vibrational energy relaxation rates in supercriticalsolution: A hydrodynamic model

An approximate expression describing the density dependence of vibrational energy relaxation rates in fluids in terms of thermodynamic and transport p arameters of the fluid is developed on the basis of a classical statistical mechanical theory of vibrational energy relaxation of highly excited molec ules in polyatomic solvents. The energy relaxation rate is expressed via th e friction coefficient, which describes the interaction between solute osci llator and solvent molecules. The corresponding force-force time correlatio n function is expressed in terms of the dynamic structure factor of the sol vent and the force of interaction between solute and solvent molecules. App roximating the dynamic structure factor appropriately leads to expressions for the density dependence of vibrational relaxation rates in terms of ther mophysical solvent parameters. Using these expressions the density dependen ce of vibrational relaxation rates in supercritical ethane and propane both in the vicinity of the critical point and far from it are evaluated and co mpared with measured relaxation rates obtained under the same physical cond itions. [S1063-651X(99)15710-6].