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 progressive effect of solvent friction on rotational motion for a well-defined solute in solvents from gas to liquid densities. A quantitative model, J-coherence, of the microscopic interaction of solvent and solute, based on binary-collision angular-momentum transfer, has been used over the entire density range to describe microscopic friction. The limiting behavior at high density of the friction coefficient is compared with the prediction of the stochastic Langevin equation and with the continuum hydrodynamic model. The clear resolution of the coherent, inertial and diffusive motion allows for a description of the transition from the gas to liquid density regime at the molecular level.