66 PHOTOCHEMICAL PRINCIPLES 



duce the transition from the ground state band of an aggregate of 

 chlorophyll molecules to the first excited singlet state band (process 

 1 ) . From the first excited singlet, the energy is split between conver- 

 sion to an exciton in the first excited triplet band (process 2) and 

 fluorescence (process 2a). All these processes are well known in 

 ordinary molecular systems, and all will have time constants of the 

 order of 10"^ sec or faster. 



According to this picture, the triplet state excitons will undergo 

 ionization resulting in electrons and holes in the conduction band 

 (process 3). At the instant the exciting light is turned off, then, a cer- 

 tain fraction of these electrons and holes will be in the traps (processes 

 4 and 4'). The number of these traps in the chloroplast is probably 

 very small, perhaps of the order of one per several thousand chloro- 

 phyll molecules. Thus, this scheme leads directly to the idea of a 

 "photosynthetic unit." (Gaffron and Wohl, 1936.) A small proportion 

 of the remaining electrons and holes will be near enough to each other 

 to recombine (process 3a) and return to the ground state via process 

 3b. We will identify this recombination process with the rate-limiting 

 step of the 0.15-sec emission. The decay constant of such a process 

 should be relatively temperature independent and the experimental 

 results are in accord with this. The fact that the intensity of this emis- 

 sion decreases with the temperature suggests the existence of a process 

 the rate of which increases with decreasing temperature and which is 

 competitive with the recombination. Whether this is the actual trapping 

 of the electron or hole or is some side process is not known. 



Arthur and Strehler (1957) observed a temperature-independent 

 emission in chloroplasts with a half-life of about 0.01 sec. It is possible 

 that this emission represents the direct decay of the exciton via process 

 3b. 



The electrons and holes that are trapped will give rise to a spin 

 resonance signal. The traps will be thermally depopulated and the 

 resultant electrons and holes in the conduction band will recombine 

 and a temperature-dependent luminescence will result. The 2- and 

 15 -sec lifetime emissions may then be identified with the depopulation 

 of traps of different depths. At low temperatures, the thermal energy 

 will be insufficient to excite the electrons and holes out of the traps. 

 This will result in the disappearance of the luminescence and the 



