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Published June 4, 1998 | public
Journal Article

Femtosecond Dynamics of Solvation: Microscopic Friction and Coherent Motion in Dense Fluids


In this paper, we present detailed experimental and theoretical studies of the femtosecond dynamics of microscopic friction. The real-time rotational motion of a well-defined system of diatomic solute in monatomic solvent has been studied for two solvents ranging from gas to liquid densities. Both coherent inertial and diffusive limits of the motion and all stages in the transition between these two regimes are observed in detail. The transient anisotropies over the entire range of experimental densities and solvents are well-represented by the J-coherence bimolecular collision model presented here. This stochastic hard-sphere collision model explicitly relates the physical properties of the solvent to the anisotropy and the coefficient of rotational friction, permitting calculation of the transient anisotropy from the Enskog hard-sphere collision frequency. Friction coefficients obtained from J-coherence analysis of experimental anisotropies were compared with those from Gordon J-diffusion, and Langevin−Einstein analyses, and with the hydrodynamic range of friction. The density cutoff for applicability of diffusive or continuum treatments is such that the angular trajectory for average J in the angular velocity autocorrelation lifetime is ∼50°, while the microscopic, molecular picture of the friction can be applied from the gas to the liquid.

Additional Information

© 1998 American Chemical Society. Received: November 25, 1997; In Final Form: January 19, 1998. Publication Date (Web): March 19, 1998. This work was supported by a grant from the National Science Foundation and the Air Force Office of Scientific Research. We wish to thank Dr. C. Wan for setting up the femtosecond laser system and providing advice and assistance throughout the experimental stage of this work.

Additional details

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