5- 



General Statements about Chemical Reactions 

 Induced by Ionizing Radiation 



ROBERT LIVINGSTON * 



Chemistry Department 

 University of Minnesota 

 Minneapolis, Minnesota 



Introduction 



Molecules, in order to react, must be activated. If a certain system 

 reacts only very slowly, its rate of reaction will be greatly increased by 

 a moderate increase in the temperature of the system. In such a system 

 of thermal reactions, the energy of activation is obtained from the 

 ordinary thermal energies of the molecules. Only that small fraction of 

 all the molecules, which (by the Boltzmann distribution) have energies 

 greatly in excess of the average, will be able to react. When such a 

 slowly reacting system is illuminated with light of wave lengths which 

 are absorbed by one or more of its components, there is a chance that 

 the rate of the reaction will be accelerated. The energy of activation of 

 such photochemical reactions is provided by the energy of electronic 

 excitation of the molecules which have absorbed photons. In radiation 

 chemistry there are two paths of activation. Some of the mole- 

 cules are electronically excited whereas others are ionized. Both the 

 excited molecules and the ions are capable of undergoing further 

 reaction. 



As a consequence of electronic excitation, the molecules may dis- 

 sociate directly into radicals or may undergo a process of internal con- 

 version (1), that is, transfer of the excitation energy of the electronic 

 system into oscillational energy of the atomic constituents of the mole- 

 cule. Thus the molecule will oscillate like a "hot" molecule and will 

 undergo chemical reactions in some respects like a molecule at high 

 temperatures. These reactions may involve other molecules, or the 

 original excited molecule may break up into radicals or stable molecules. 



* The author is greatly indebted to Professor James Franck for his encouragement 

 and advice. The general outline of the paper and any new concepts which it may 

 contain are due entirely to Professor Franck. 



56 



