Femtosecond Dynamics of Dissociation and Isomerization Reactions in Solutions

The main purpose of this work has been to investigate the fundamental dynamics of photodissociation and photo-induced isomerization reactions in the condensed phase. These "elementary" reactions have been chosen as model processes for exploring the influence of the solvent on a chemical event, as well as the factors that control this influence. Femtosecond laser spectroscopy has been employed as the main experimental tool in this work, as it allows monitoring the evolution of a chemical reaction directly on the time scale it occurs. Studies of the photodissociation of methylene iodide (CH2I2) indicate that this process occurs on slower time scale (700-900 fs) than previously reported. The dynamics of acetonitrile (CH3CN), carbon tetrachloride (CCl4), dichloro-methane (CH2Cl2), and hexane (C6H14), reveal that the major reaction channels following photolysis with high intensity UV pulses are dissociation and ionization by a two photon process. Various experimental methods (including ground state recovery (GSR), excited state absorption (ESA), stimulated emission (SE), and fluorescence upconversion) have been used to study the dynamics of the photo-induced excited state isomerization of 1,1´diethyl-4,4´-cyanine (1144-C). From these studies it is possible to conclude that the bond-twisting event can be associated with wave packet motion on the excited potential surface, and can be observed experimentally as a delay or a spectral shift depending on the method used. In addition, the isomerization rate of 1144-C has a distinct temperature dependence at different viscosities: at high viscosities, the reaction rate follows the Arrhenius relation (i.e. shows a positive temperature dependence), while at low viscosities the reaction is faster at lower temperatures (inverse temperature dependence). The experimental results are discussed in terms of the available theoretical models.