Towards a quantitative characterization photoisomerization rates
Photoisomerization rates of energy-selected isolated molecules, of molecules in supercritical fluids and clusters, and of molecules in liquids at normal and elevated pressures CM now be measured directly. The results can be analyzed by unimolecular rate theory, information of collisional energy transfer in gases and in compressed fluids, as well as knowledge on the solvent influence on the intramolecular motions, potentials and transport. By a combination of theory and empirical fitting of otherwise inaccessible quantities, a quantitative picture arises. In low pressure gases, reactions on electronically excited potential energy surfaces often compete with fast internal conversions to such an extent that the main reaction occurs in the electronic ground state. In this case, the main reaction is intimately related to thermally activated reactions in the ground state. When the density is increased, the ground state component may become completely collisionally quenched such that the reactions in the electronically excited states dominate. These again may be influenced by collisional relaxation and other solvent effects. Whether the excited state reaction approaches a thermal unimolecular reaction in the electronically excited state or fast proceeds "downhill" towards ranges of electronically nonadiabatic dynamics, depends on the properties of the excited state potential energy surface and its modification by clustering or by reagent-solvent interaction. Besides "static" solvent effects, "dynamic" solvent effects of the viscosity-type may influence the dynamics in dense environments. In the following, the photoisomerization reactions of cycloheptatrienes and of diphenyl polyenes serve as examples for the various aspects and the possibilities of a quantification.