General Features of Photochemical and Photophysical Processes

In order to avoid such ambiguities, the definition of chemical species will depend on the simple concept of stability. In the absence of chemical reactions, a chemical species will last indefinitely. Thus an ion is a distinct chemical species, and an electron transfer reaction must be seen as a chemical change. However, an electronic excited state of an atom or molecule must inevitably decay back to the ground state, so the processes of excitation, emission and non-radiative deactivation are photophysical processes. 

In the Arrhenius-Eyring model of a chemical reaction which takes place without the intervention of light, the reactant(s) R go over to the product(s) P through a transition state (X) which determines the activation barrier Ea in the rate constant equation 

The non-radiative decay of the excited state (M ) to the transition state (X) of the ground state reaction is so unlikely that it can be altogether forgotten. Even if there was a single example of such a process, it could not be described as catalysis by light. The expressions photoinduced or photo-activated reactions are accurate they do not imply that light acts as a catalyst, but rather as a reactant which is consumed in the chemical process. 

3 The Range of Photochemical Reactions Vibrational Photochemistry and Radiation Chemistry 

The definition of a photochemical reaction depends on the definition of light . Indeed, a photochemical reaction is a chemical reaction induced by light, a reaction in which the energy of light is used to promote molecules 

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