"Ultrafast laser methods will be employed to examine the dynamics of chemical and photochemical reactions in liquid solutions. By contrasting the solution phase dynamics with those observed for isolated collisions in the gas phase, the fundamental role of solvent on chemical pathways will be explored at a molecular level. The experimental studies will be complemented by computational simulations that explicitly include treatment of the effects of solvent on reaction energy pathways and reactant and product motions.The research addresses a major challenge in Chemistry to understand the role of solvent on the mechanisms of chemical reactions. Questions that will be examined include how the solvent modifies reaction barriers and other regions of the reaction potential energy surface (PESs), alters the couplings between PESs, most importantly at conical intersections between electronic states, influences and constrains the dynamical stereochemistry of passage through transition states, and dissipates excess product energy.The experimental strategy will be to obtain absorption spectra of transient species with lifetimes of ~100 fs – 1000 ps using broad bandwidth light sources in the infrared, visible and ultraviolet regions. Time-evolutions of such spectra reveal the formation and decay of short-lived species that might be highly reactive radicals or internally (vibrationally and electronically) excited molecules. The transient species decay by reaction or energy loss to the solvent. Statistical mechanical theories of reactions in solution treat such processes using linear response theory, but the experimental data will challenge this paradigm by seeking evidence for breakdown of the linear response interaction of solvent and solute on short timescales because of microscopic chemical dynamics that perturb the solvent structure. The work will build on our pioneering experiments at the Rutherford Appleton Laboratory that prove the feasilbility of the methods."