2 



RADIATION BIOLOGY 



ous other common examples of processes depending on specific effects of 

 visible light have provoked research interest for many years. These 

 phenomena are gradually becoming understood, largely in terms of the 

 simpler photochemical systems, which can be duplicated in the laboratory. 

 The general principles of the energy-exchange processes that occur in 

 chemical and physical systems were made understandable during the 

 period 1925-1942. Photoreactions provide a special case only in so far as 

 the magnitude of the energies involved is greater than ordinary thermal 

 energies and in so far as special transfer of electronic excitation energy 

 may be important. There are many important gaps in our knowledge 



INTRAMOLECULAR PROCESSES 

 EXCITED STATE 



INTERMOLECULAR PROCESSES 



Fig. 1-1. Processes that follow the absorption of radiant energy. Vertical lines 

 denote radiation processes. Diagonal lines indicate internal conversion or the 

 exchange of energy among degrees of freedom other than electronic. The number 

 following each term is that of the section or subsection in which that phenomenon is 

 primarily discussed. 



of energy-transfer processes. Nevertheless there exists a voluminous 

 literature on the subject, so that we can hope to abstract only a few of 

 the major areas of the field here. General discussions of energy-transfer 

 processes have been given by Bethe and Teller (1940), Oldenberg and 

 Frost (1937), Zener (1935), and Franck and Livingston (1949). 



In Fig. 1-1 the various possible sequences of steps following light 

 absorption or preceding luminescence emission are diagramed. Diago- 

 nal steps represent processes that oc(!ur without absorption or emission 

 of light. The number that follows each term refers to the section of this 

 chapter which deals with the phenomenon. No claim is made for the 

 completeness of coverage of any topic. However, the years have shown 

 that the most convenient and lucid visualization of kinetic systems is in 

 terms of potential-energy surfaces on which each point represents the 

 potential energy for a distinct configuration of the atoms of one or several 



