Diabatic and Adiabatic Processes in Photochemistry

Photochemical reactions may be classified as adiabatic or as diabatic whether the chemical change occurs on the same potential energy surface or not. In the adiabatic cases deexcitation occurs either in the reactant (I) or in the product (II) while in the diabatic case it occurs in between them (III). The potential energy surfaces of polyatomic systems are discussed in relation to photochemistry and the possible conditions for the different behaviour of photo-reactions are formulated. Experimental evidence for the appearance of excited products in photoreactions in solution is given. I. CLASSIHCATION OF PHOTOCHEMICAL REACTIONS Thermal reactions with rare exceptions occur adiabatically on the lowest potential energy surface. An equivalent statement for photochemical reactions is less easy. They start in one of the electronically excited states of a reactant but end with the products in their ground states. This corresponds to a transition from a higher potential surface of the system to the lowest one. So, in principle, all photoreactions include diabatic processes. If one regards, however, electronic excitation and deexcitation as physical rather than chemical processes one may classify a reaction as adiabatic or as diabatic whether the chemical change takes place on the same potential surface or not. This leads us to a distinction between three different classes of photoreactions in which deexcitation occurs (I) in the reactant(s), or (II) in the product(s), or (III) anywhere in between on the reaction path. Schematically, these classes may be characterized as follows: