Knowledge Bank

Phase-coherent molecular motion is initiated whenever a sufficiently short (i.e. femtosecond) laser pulse passes through most media. Optical absorption leads to coherent wavepacket propagation on excited state surfaces, while impulsive stimulated scattering (ISS) leads to coherent motion in electronic ground states. This motion can then be monitored (or altered) with subsequent femtosecond pulses. Collective vibrational motion in solids and liquids, local intermolecular vibrations, and even many intramolecular vibrations have been time-resolved. If the motion under observation is involved in a chemical or structural rearrangement, then the rearrangement process may be observed in real $time.1,2$ In crystalline solids, structural phase transitions and oriented bimolecular reactions have been examined. Molecular dynamics in simple liquids and structural relaxation in viscoelastic fluids have also been characterized. Unimolecular dissociation in the liquid phase is under study. In simple liquids, elementary molecular orientational and translational motions are vibrational (not diffusional) in character. Through ISS, time-domain characterization of librational frequencies and their inhomogeneities has been carried out in pure and mixed liquids at various temperatures. As in any vibrational spectroscopy, the results yield information about dynamics (in this case, molecular orientational dynamics) and also (effective) potentials. Configuration-averaged intermolecular ``force constants'' are determined. In excimer-forming molecular crystals, the process of excimer formation (an oriented bimolecular reaction) can be initiated phase-coherently and then time resolved. Results in pyrene and a-perylene crystals, in which the excited-state reaction path is closely related to a single lattice phonon coordinate, will be presented.