Picosecond Molecular Dynamics in Liquids and Electron Localization

Abstract

The confluence of picosecond spectroscopy and molecular dynamics in liquids is a natural outcome of the fact that the duration of the probe pulse and the phenomena share the same molecular space and time-scale. A 2 psec pulse is far less than, or comparable to, molecular reorientational times, while in 10 psec most molecules in translational diffusion move but a few angstroms. At subpicosecond times the fluid becomes frozen to the observer, who only senses the vibrations of the otherwise stationary molecules. Picosecond spectroscopy (1) thus allows us a direct view of this microscopic world for the first time and we can begin to test models of dynamical behaviour in liquids with a quantitative perspective that previously had been feasible only in the gas and crystalline phases. And yet, reducing the myriad forms of picosecond data into the correct molecular correlation functions, which describe the equilibrium and dynamical interactions of molecules in dense fluids, is not simple and offers a major challenge to contemporary theory of molecular motions in liquids (2,3).