76 



the upper electronic state, falls to its lowest vibrational level. From this 

 level fluorescence may occur after a suitable time, in general to a high vibra- 

 tional level of the lower state, followed by final dissipation of the remaining 

 vibrational energy. 



Excitation may also occur into unstable states, with immediate splitting or 

 into states which lie above the dissociation limits. If bond rupture results, the 

 fragments A and B may themselves be electronically excited and thus carry 

 away electronic energy, in addition to the high potential energy of the resulting 

 free radicals. 



Innumerable condensed systems are known in which excitation leads neither 

 to photochemistry nor fluorescence, but merely to dissipation of the absorbed 

 energy as heat. This simple observation makes it necessary to assume that 

 there is a pathway by which the molecule can get from the upper to the lower 

 state with neither radiation nor chemical change, and this is the well-known 

 process of internal conversion, described by several people, Franck, Livings- 

 ton, Teller, etc. (1) (2). The process is illustrated in Fig. 3. 



Fig. 3. Schematic Molecular Potential Curves. 



Excitation raises the molecule to point I on the upper surface, from which, in 

 its vibrations, it can reach the point II, a nuclear configuration at which one 

 may describe the system by saying there is resonance between the upper and 

 lower states. As the molecule swings through this configuration the electronic 

 state may switch over from that corresponding to the upper to that for the lower 

 curve. The molecule is thus transferred to a very high vibrational level of the 

 lower state. In a condensed system, as we have seen, this energy is lost rap- 

 idly, and the molecule is therefore trapped in the lower state. This sequence 



