54 



RADIATION BIOLOGY 



According to Evans et al. (1938), two general types of potential-energy 

 diagrams are responsible for chemiluminescence. They proposed a dia- 

 gram of the type of Fig. 1-18 for the chemiluminescent decomposition 

 of sodium azide (Audubert, 1937). Luminescence occurs in transition 2. 

 Reaction on the upper curve will be important only if the energy required 

 for excitation to that surface Ei is small and the activation energy for the 

 process on the upper surface is small with respect to that on the lower 

 surface, i.e., Eo,e > £'o,m- Though reaction via the upper surface may be 

 a relatively poor process, a considerable intensity of luminescence may 

 be caused by the small amount of reaction which does take place on that 



DISTANCE ALONG REACTION COORDINATE 



Fig. 1-18. Typical potential-energy curves for a chemiluminescent reaction occurring 

 via black-body or collision excitation. 



surface. Excitation to the upper surface can occur through absorption 

 of black-body radiation or through collisions, but we have seen that the 

 latter processes will be more efficient by a factor of about 10^ (Sect. 1-3). 

 A more probable mechanism whereby chemical energy is converted 

 into light can occur in the second kind of potential-energy diagram, typi- 

 fied by Fig. 1-19. Activation to the upper surface via path 1 is highly 

 improbable, but the crossing conditions at the top of the potential barrier 

 I on the lower surface may, under a large variety of conditions, favor 

 migration of the configuration point to the upper surface during chemical 

 reaction (Sect. 2-1). The intensity of luminescence via path 2 will, of 

 course, depend on the transmission coefficient for crossing at I and the 

 height of the potential barrier II on the upper surface. Undoubtedly 

 the majority of Audubert's observations are explained by the existence 

 of potential-energy surfaces, such as shown in Fig. 1-19 for the reactions 

 he studied. 



