FEMTOSECOND DYNAMICS OF MICROSCOPIC FRICTION - NATURE OF COHERENT VERSUS DIFFUSIVE MOTION FROM GAS TO LIQUID DENSITY

With femtosecond resolution, we have observed in real time the progres sive effect of solvent friction on rotational motion for a well-define d solute in solvents from gas to liquid densities. A quantitative mode l, J-coherence, of the microscopic interaction of solvent and solute, based on binary-collision angular-momentum transfer, has been used ove r the entire density range to describe microscopic friction. The limit ing behavior at high density of the friction coefficient is compared w ith the prediction of the stochastic Langevin equation and with the co ntinuum hydrodynamic model. The clear resolution of the coherent, iner tial and diffusive motion allows for a description of the transition f rom the gas to liquid density regime at the molecular level. (C) 1997 Elsevier Science B.V.