THE VISUAL PIGMENTS 



ultra-violet light, some dianthracene is formed. The quantum 

 efficiency depends on the concentration of anthracene but is not 

 affected by the temperature. Writing A for an anthracene molecule, 

 A2 for dianthracene and ^4* for an activated anthracene molecule, the 

 primary process is, 



A -{- hv -> A"^ 



while the secondary process can be either 



A* + A^A^ 



which yields a molecule of dianthracene, or, 



^* -> ^ + hv' 



in which the activation energy is lost by fluorescence. 



Which of these two alternative secondary processes occurs 

 depends on the chance of the activated anthracene molecule meeting 

 another anthracene molecule. The chance is less in a dilute solution 

 than in a concentrated one. Consequently dilute solutions fluoresce 

 strongly, and in these the quantum efficiency is low. In stronger 

 solutions the fluorescence is less — and the yield of dianthracene 

 more — up to the point (0-1 molar in anthracene) where the loss of 

 energy by fluorescence is negligible. Further increase in concentra- 

 tion is then without effect on the quantum yield. Deactivation of 

 excited molecules by collision with solvent molecules is not important 

 in this reaction, as is shown by its indifference to temperature or the 

 nature of the solvent. 



Sometimes, following absorption of a quantum, an energy or 

 atomic chain of reactions may be initiated. Thus in the photo- 

 chemical reaction between hydrogen gas and chlorine gas, thousands 

 of HCl molecules are formed for each quantum absorbed. The 

 primary process is the absorption of a quantum by a chlorine 

 molecule. In this instance the energy acquired is sufficient to raise 

 the chlorine molecule to an unstable electronic level with the result 

 that the molecule dissociates into atoms, 



CI2 + hv -> CI + CI 



If no other kinds of molecules are present these dissociated atoms will 

 recombine to form the molecule again. If, however, hydrogen is 

 present then the following reaction can occur, 



CI + Hg -> HCl + H (a) 



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