THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



The particular locality in a molecule where absorption takes place 

 is known as the chromophore. When a beam of hght passes through 

 a substance, a part is absorbed and the rest transmitted. On the 

 quantum theory, the energy in the beam is more Hke the energy of a 

 hailstorm than that of a waterfall in that it consists of a vast number 

 of discrete quanta. If, after passing through a substance the intensity 

 of a light-beam is found to have been weakened to 50 per cent (say) 

 of its original value, this means that one half of the quanta were 

 wholly absorbed by the chromophores of the substance and that the 

 other half emerged completely unscathed. 



Photochemical reactions can be regarded as taking place in two 

 stages; the primary process and the secondary process. According 

 to the Stark-Einstein equivalence law, the primary process is the 

 absorption of a single quantum by an individual chromophore. As a 

 result the molecule is raised to a higher electronic level which may be 

 either stable, or unstable and cause dissociation of the molecule. 

 From the standpoint of the primary process every interaction between 

 radiant energy and matter has a quantum efficiency of unity. 



(^ . Number of chromophores affected \ 

 Quantum efficiency = — — — ; --— — I 

 Number of quanta absorbed / 



The overall quantum efficiency, however (as measured by some 

 permanent change), depends on the secondary process, namely, the 

 chemical consequences of the original activation. For example an 

 excited molecule may lose all or some of its extra energy by fluores- 

 cing or by suffering deactivating collisions with other molecules. In 

 the former case at least some of the absorbed energy is re-radiated, 

 usually as light of longer wavelength (i.e. in smaller quanta) ; in the 

 latter the energy is dispersed throughout the system as heat energy. 

 In either of these events there is no permanent change, and the 

 quantum efficiency is zero. 



In some instances there may be the possibility of permanent change 

 to an activated molecule, provided that in the brief period of excita- 

 tion — before it must fluoresce or lose its energy by an unlucky 

 encounter — it meets another molecule of a kind with which it can 

 react. A proportion of activated molecules may thus undergo an 

 irreversible change. This proportion may be affected by the tem- 

 perature of the system and the concentrations of the various molecu- 

 lar species present. 



For example, when a solution of anthracene is irradiated with 



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