34 RADIATION BIOLOGY 



of this energy. Thus the transmission coefficient will vary with the state 

 of the activated complex, and not all activated complexes will be alike. 

 This behavior is in contrast to that observed in other types of reactions. 

 The problem is further complicated by the fact that at any instant most 

 of the activated complexes in any given energy state will be those just 

 returning from an unsuccessful attempt to pass the barrier. This situ- 

 ation destroys equilibrium distribution of complexes moving toward the 

 barrier and makes necessary further corrections depending on the trans- 

 mission coefficient for the state involved. It is just these transmission 

 coefficients that depend on internal coupling and the full potential-energy 

 diagram for the molecules. 



Activation energies are not usually observed in photoreactions because 

 the excitation energy exceeds the thermal requirement for the reactions 

 that occur. The thermal values of the activation energy provide a lower 

 limit for the photoenergy required if thermal and photoreaction mecha- 

 nisms are identical. Thermal energies can occasionally make up a deficit 

 in the energy for a photoreaction (Franck and Herzfeld, 1937). An inter- 

 esting example is the discovery by St. George and Wald (1949) that 

 mammalian vision has a temperature coefficient at the red end of the 

 spectrum in contrast to the absence of a thermal dependence at shorter 

 wave lengths. The observation can be explained readily as a deficiency 

 of long-wave-length radiation for the primary process of vision. The 

 thermal increment may serve as activation energy for crossing to a lower 

 potential surface or as activation energy for the subsequent process on 

 the lower surface. In the photolysis of I2 (Rabinowitch and Wood, 

 1936b) and S2 (Durand, 1940), collisional transfers of energy provide the 

 necessary extra energy for these molecules, when photoexcited, to reach 

 their crossing points. 



Emphasis must be placed on the activation rather than the thermo- 

 dynamic energy requirements of a given photoreaction. In diatomic 

 molecules the only allowed unimolecular process is bond rupture, in which 

 the heat of reaction and the activation energy are identical. In larger 

 molecules there are usually alternative chemical processes that can occur 

 with lower activation energies. Consider the case of ethane photolysis 

 in the gas phase studied at 1470 and 1295 A by Faltings (1939). The 

 quantum yields <S> are $h2 = 0.96, $CjHj = 0.20, ^c^n, = 0.56, ^ch^ = 0.05, 

 and $CaH6+c4Hio = 0.04, indicating preference for the primary reaction 



C2H6 + hv -> C2H4 + H2, (1-26) 



with the low activation energy Eo of about 20 kcal/mole. Some methyl 

 radicals are also produced by the reaction 



C2H6 + fip - 2CH3, (1-27) 



with Eo =: 78 kcal/mole. The difference in yields is explained by the 



