Some photochemical terminology and principles

An important principle of photochemistry is that each photon absorbed by a molecule activates that molecule in the initial (or absorption) step of a photochemical process. For example, the absorption of a photon by a general molecule, indicated by XY, can be represented by 

If the energy of dissociation for this reaction is 304 kJ mole , what is the minimum wave number of the EM radiation that can cause this dissociation  

Solution. If we consider the dissociation of 1 mole of NO2, we will need to have 1 einstein of photons, which will need to have an energy of at least 304 kJ to produce the dissociation. Therefore, from Eq. (7.4) 

Since wave number = 1/A, the minimum required wave number of the radiation is 2.6 x 10 m or 2.6 /tm . (Dissociation of some NO2 may occur at somewhat smaller wave numbers than this due to internal energy already present in the NO2 molecules.) 

Reaction (7.6) provides an example of one pathway that an excited molecule may follow, that is, to dissociate chemically. This pathway can be indicated in general by 

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