True and apparent unimolecular reactions

In unimolecular reactions only one molecule is involved in the reaction. Since, however, the molecules prior to the reaction are stable, it is necessary to activate them in order to initiate the reaction. Based on the different methods of activation, we distinguish between the following two types of unimolecular reaction  

In a true unimolecular reaction, the activation is done by exposing the molecules to electromagnetic radiation, whereas activation is accomplished by inelastic collisions with other molecules in an apparent unimolecular reaction. The condition for the latter process to be unimolecular is that the time scales of the activation process and the chemical reaction are very different, so that the chemical reaction is much slower than the activation process. 

A true unimolecular reaction is induced by electromagnetic radiation. That is, only one molecule takes part in the reaction and the energy is provided by the electromagnetic field. In fact, chemical reactions induced by electromagnetic radiation form such an important subfield of chemistry that it has its own designation photochemistry. 

There are also many interesting photochemical rearrangements of organic molecules. One example is the cis-trans isomerization of retinal induced by visible light, a basic 

The electromagnetic radiation will normally induce a transition from the electronic ground state to an excited electronic state, where the reaction takes place. 

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