Ultrafast Studies of Reaction Dynamics.

The long range goal of this research is to understand the role of the solvent in condensed phase reactions. In the gas phase, great strides have been made towards understanding chemical reactivity at the level of individual quantum states. Liquid state reactions are not as well understood due to complicated many-body interactions with the solvent. Our approach has been to study simple reactions in solvents near their critical state. Here a small change in pressure, at constant temperature, allows a continuous change in solvent bulk density and viscosity, from near-liquid to gas-like properties. In this way reactions can be studied across the interface between gas phase behavior and the poorly understood liquid state. In addition to chemical reactivity, the research directly investigated solvent structure and dynamics in the supercritical state. The unique solvent properties of supercritical fluids (SCFs) are the basis for SCF extractions and chromatography. Microscopic details of the fluid structure and dynamics have a direct bearing on the solubility and reaction behavior of these fluids. The high-intensity of the generated ultrafast pulses (0.01 to 60 GW/cm 2) necessitated the need to investigate the phenomenon of nonlinear absorption. Initial studies were performed on one-dimensional polydiacetylene polymer films. Mechanisms involving two-photon absorption were developed for explanation the new finding. The available high power in the femtosecond experimental system allowed investigation of solvent molecular relaxation processes. Using the Optical Kerr Gate technique, the nonlinear index of refraction and femtosecond molecular relaxation processes for carbon dioxide in the vapor, liquid and supercritical states were determined. The reaction dynamics DODCI (3,3\u27-diethyloxadicarbocynine) isomerization were investigated. The pathway of the rapid (fs-ps) excited state isomerization followed was clarified. A Kramers turnover was observed for the slow (us-ms) ground state isomerization. These two widely different time scales each provide unique information on solvent effect