28 RADIATION BIOLOGY 



reaction may be either the optical dissociation of the oxygen molecule or 

 the photoexcitation or dissociation of a molecule of the reducing agent. 

 Since oxygen absorbs chiefly at wave lengths less than 1800 A, reactions 

 of the first type are limited to relatively transparent substrates such as 

 hydrogen or carbon monoxide (Noyes and Leighton, 1941, pp. 246-254). 

 In those cases where the reducing agent absorbs the light, the initially 

 reactive species may be an excited singlet (i.e., fluorescent) state, a long- 

 lived (triplet) excited state, or a pair of radicals, produced by some type of 

 photodissociation. 



Solutions of aryl hydrocarbons in hexane or similar solvents are fluores- 

 cent. In the presence of o.xygen, their fluorescence is quenched and 

 peroxides are formed (Bowen and Williams, 1939). With few exceptions 

 the sum of the fluorescent yield and the peroxide quantum yield is dis- 

 tinctly less than unity, in some cases being as small as 0.1. None of the 

 data for the 14 hydrocarbons investigated by Bowen and Williams are 

 consistent with the view that the only effect of o.xygen is to quench the 

 fluorescence by reacting with the excited molecule (in its singlet, fluores- 

 cent state) to form a peroxide. Apparently oxygen can quench the 

 fluorescence of these molecules without forming any detectable product. 

 In five cases (benzene, w-xylene, fluorene, acenaphthene, and triphenyl- 

 methane) the evidence is compatible with the postulate that only the 

 singlet, fluorescent state is involved in the peroxide formation. For the 

 others (especially hexamethyl benzene, anthracene, naphthacene, toluene, 

 and p-.xylene) the experimental results strongly indicate that some or all 

 of the peroxide is formed by a reaction between an oxygen molecule and 

 an energy-rich nonfluorescent (triplet?) state of the hydrocarbon. This 

 is particularly obvious for hexamethylbenzene, where a quantum yield 

 of peroxide formation almost fivefold greater than the maximum fluores- 

 cent yield was observed. The preceding conclusions are based on the 

 assumption that only the 10 following reaction steps occur. In these 

 ecjuations, A* stands for the singlet, excited state and A' for the triplet 

 state of the hydrocarbon molecule, A. 



Oxidations which are initiated by the photochemical dissociation of the 

 reductant frequently exhibit the characteristics of chain reactions. Their 

 quantum yields are functions of the temperature, of the concentrations of 

 reactants and products, and sometimes of the intensity of the absorbed 

 light. Often the products are complex, and the relative amounts of the 

 several compounds formed vary with the conditions. The detailed 



