Molecular structure and orientation: Concepts from femtosecond dynamics

In this article, we are concerned with the basic concepts of molecular orientation, its dynamics on the femtosecond and picosecond time scales, and the direct relationship of the foregoing to determination of molecular structures. Our aim is to present a unified picture for the dynamics in different phases: coherent inertial in isolated molecules, partially coherent in dense fluids, and reaching the diffusive limit in liquids. We compare theory and experiment for reactive and nonreactive systems, with examples of studies from this laboratory in molecular beams, dense fluids, liquids? proteins, and micelles. We discuss the high precision achieved in molecular structural determination for large isolated molecules. Important to molecular orientation in space is the time scale, and we present here the two limits: quantum-state orientation (long time or continuous wave experiment), and classical-like orientation achieved with femtosecond resolution.