228 RADIATION BIOLOGY 



photochemical cis-trans isomerization about a carbon double bond appear 

 to proceed in this manner. Another possibility is the decomposition of a 

 molecule through an internal conversion process into two fragments 

 involving substantial reorganization of its component parts. This could 

 not be accomplished by a simple predissociation, which merely splits a 

 molecule into two (swiftly moving) fragments. An example of this 

 appears to be the monomolecular component of the photolysis of aliphatic 

 aldehydes, a hydrocarbon and CO2, or a ketone and H2 being produced. 



Because predissociation and internal conversion are such very probable 

 consequences of electronic excitation of large molecules, they must play 

 an important role in biological reactions caused by visible light, or by 

 ultraviolet or high-energy radiation. The shortness of the time usually 

 required for these processes is borne out by the fact that relatively few 

 polyatomic molecules exhibit fluorescence. 



3-3e. Collisions of the Second Kind. Collisions of the second kind 

 following excitation of polyatomic molecules have not often been 

 observed. The simpler molecules of this class have been studied chiefly 

 in the gaseous state, and usually have a higher probability of losing their 

 excitation energy by dissociation or by internal conversion than by any 

 kind of impact of the second kind. For instance, no case of sensitized 

 fluorescence (type III) in the gaseous phase seems to have been con- 

 clusively established, although some have been postulated; for example, 

 the reaction 



C0| + O2 -> CO2 + O.* 



has been considered in attempts to explain the radiation by excited 

 oxygen in a CO flame. For the same reactants, the occurrence of sensi- 

 tized dissociation (type VI), 



CO2* + O2 -> CO2 + + 



has also been suggested. However, since sensitized fluorescence is easily 

 observable with suitable dyestuff molecules in solution, doubtless the 

 phenomenon could be found in this group of complex molecules in the 

 vapor phase too. Certain dyes have vapor pressures high enough, at 

 temperatures at which thermal disintegration is still absent, for such ex- 

 periments to be feasible. High probability in favorable cases should be 

 shown by exchange reactions, and especially by association reactions, for 

 in the latter a reaction of the type 



A* -\- B-^ AB 



which cannot occur without triple collisions if both .4 and B are atomic, 

 may have high probability if one or both is polyatomic: the recombina- 

 tion energy may then be quickly dissipated to the many oscillational 

 degrees of freedom and so rendered unable to dissociate the newly formed 



