ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 



53 



been Birks (1953). There is some disagreement as to the importance 

 of this mechanism (Bowen and Lawley, 1949; Bowen, 1951), and it 

 has been shown to be of little significance in a number of instances 

 (Ferguson, 1956a, b). 



2. Resonance transfer analogous to unordered systems (Franck and 

 Livingston, 1949; Livingston, 1957). According to this theory, the 

 energy is transferred by an overlap of the electronic systems of the 

 excited donor and the unexcited acceptor molecules. In this case, the 

 interaction between molecules is small enough to permit them to be 

 considered as individual electronic systems. The probability of transfer 

 will increase with the magnitude of the interaction. 



3. The migration of excitons throughout the crystal (Curran, 1953; 

 Davydov, 1948; French and Teller, 1938; Frenkel, 1931;Kittel, 1956; 

 Leverenz, 1950; Peirls, 1922). The main features of this theory may 



s' 



s' 



S' 



S' 



S' 



G 



I 



-H- 



-^ 



^ 



H- 



H 



^ 



^ 



H 



^ 



Fig. 2. Schematic representation of the formation of energy bands in 

 crystals through the interaction of n molecules. The pairs of arrows 

 represent electrons with antiparallel spins. G, ground state; 5', lowest 

 excited singlet state. 



be understood by a consideration of the diagram in Fig. 2. The inter- 

 actions between the pi-orbitals of the n molecules in the crystal are 

 so large as to lead to a splitting of the energy levels, resulting in the 

 formation of /; closely spaced levels. In general, the lower band of 



