52 RADIATION BIOLOGY 



bond well separated to approximately the distance required in the acti- 

 vated complex for this particular dissociation reaction. Under these cir- 

 cumstances the lifetime in that state may be great enough to permit 

 passage over the reaction barrier to dissociated products. It is observed 

 that the ion usually dissociates at that bond which is weakest in the 

 ground state. Absolute-rate theory has been applied to this complicated 

 process on the well-founded assumption that the migration of the ion 

 among electronic-vibrationaF states is sufficiently rapid to produce the 

 state of the activated complexes with a probability equal to the statistical- 

 mechanical probability of occurrence of activated complexes in an equi- 

 Ubrium situation (Wallenstein et al, 1951 ; Rosenstock et at., 1952). The 

 processes have several novel features from a rate-theory point of view. 

 Single ions with fixed energy form the reactants, thus necessitating the 

 use of a microcanonical ensemble in calculating probabilities for various 

 states. For any given excited ion there exists a variety of processes with 

 very nearly the same energy requirement relative to the total excitation 

 energy. All the processes must be included at each step, and all steps of 

 the cascade of decompositions, as the parent ion becomes successively 

 degraded, must be considered. 



Quantitative application of the absolute-rate theory using potential- 

 energy surfaces in configuration space is complete if the transmission 

 coefficient is calculated quantum-mechanically. Even without such 

 refinements it is as complete as collision theory. It would appear profi-t- 

 able to apply the theory to a wide variety of collisional processes in the 

 same way that absolute-rate theory has been applied to chemical reac- 

 tions. Speaking conservatively, absolute quantitative application would 

 be an ambitious undertaking for the simple collisions here discussed. 

 Detailed numerical investigations of the theory for collisions involving 

 polyatomic molecules are out of the question. Nevertheless even the 

 more complicated energy-transfer processes of these molecules are 

 described in a useful qualitative manner if attention is fixed on the two 

 degrees of vibrational freedom exchanging energy. The presence of 

 other vibrational modes will generally increase the restrictions on suc- 

 cessful collisions for diabatic as well as adiabatic energy-transfer processes. 



5. CHEMILUMINESCENCE 



The principle of microscopic reversibility which applies at thermal and 

 radiation equilibrium requires that the processes of energy transfer dis- 

 cussed in preceding sections be reversible, thus producing radiation from 

 chemical, electrical, mechanical, or thermal energy. Under the non- 

 equilibrium conditions of rate processes, probabiUty considerations are 

 generally unfavorable to high yields of luminescence. We have already 

 briefly discussed thermally induced luminescence in Sect. 1-3 on Black- 



