Certain photochemical reactions such as cis-trans isomerization, proton transfer, and electron transfer are central to both chemistry and biology. Observations made on these reactions in simple chemical systems are of great value in modeling the analogous reactions in proteins. Many of these processes are light driven meaning that we can use lasers both to initiate the chemistry and to probe the reacting species. It is now known, based on kinetic measurements using ultrafast lasers, that numerous photochemical and photobiological reactions occur in as fast as 200 femtoseconds (200 x 10^15 seconds). In other words, the reaction occurs on the timescale of the vibrational motion of the molecule. In this context, it becomes relevant to describe the reaction coordinate for a chemical transformation in terms of the vibrational modes of the reacting species. By using resonance Raman spectroscopy to study a photochemically active molecule, we can construct a picture of the excited state surface on which the reaction takes place in terms of the vibrational coordinates of the ground state. This in tum allows us to visualize the trajectory which is followed as reactant is transformed to product giving insight into the reaction mechanism. Furthemmore, Raman spectroscopy can be obtained of molecules in a variety of environments yielding infommation about reactions in the solid, liquid, and vapor phases. Solvent effects on reaction mechanisms can therefore be probed as well.
Additional infommation about the properties of the photochemically active excited state surface can be obtained via Stark spectroscopy. By applying an extemal electric field to the sample, we can determine the degree of charge separation in a molecule as it evolves from reactant to product on the excited state surface. An applied electric field can also be used to tune the relative spacings of molecular electronic energy levels having different electronic dipoles or polarizabilities. As a result, the extent to which these states interact with one another is altered. These interactions can, in tum, affect interesting molecular properties including reaction rates, product yields, and nonlinear optical responses. Systematic measurement and analysis of the dependence of these quantities on the strength of the applied field will allow us to probe the relationship between the characteristics of the molecular electronic states and the resulting photochemical and photophysical properties.