This is the main core project that we have employed as a test bed for the femtosecond optical spectroscopy development. The dynamics is that of a model for the visual primary process. The reaction is ultrafast and pushes our instrumentation to the limit. We have had to develop ways of measuring changes in electronic structure, vibrational and rotational energy content and reaction pathways all on the femtosecond timescale. An important goal of condensed matter chemical dynamics is to characterize reactive coordinates and understand how interactions with the solvent influence reaction pathways and energy flow. Isomerization reactions in which a significant structural change occurs without severing chemical bonds present the opportunity to study energy reorganization directly. Femtosecond laser studies have been performed on the photoisomerization reactions of cis-stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted in the 150 fs timescale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments completed in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for using transient fluorescence and transient absorption, confirming that the reaction generates hot product states and that the Franck Condon modes are largely spectators in the reaction; the anisotropy (alignment) of trans product molecules illustrating the effect of friction coupling overall motion to the reaction coordinate; and a theoretical treatment of three pulse anisotropy experiments. Current activity involves prototype investigations of coherence transfer in photoreactions.
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