132 \V. ARNOLD 



the time in the dark. A Chlorella suspension that has been in the dark 

 for 20 to 30 minutes and then exposed to one single flash of light 

 (from a photographic flash bulb) gives over the region of 10 seconds 

 to 10 minutes, a simple second-order decay curve (Fig. 2). However, 

 if the single flash is replaced by continuous light for a few minutes, 

 the results are quite different (Fig. 3). 



The intensity of the delayed light, as measured with the phos- 

 phoroscope (Fig. 4), increases with increase in the number of flashes 

 per second up to 500 to 1000, and then becomes independent of the 

 frequency of flashing. It may be that this break in the curve can be 

 associated with the new fast reaction in photosynthesis shown by 

 Kok (this volume) in his kinetic studies. 



Throughout the entire decay curve for the delayed light, the in- 

 tensity of the exciting light needed to give maximum signal is in- 

 creased as the time in the dark is decreased until, as the two curves 

 in Fig. 4 show, at the shortest dark times used (^'gooo second), there is 

 no evidence of saturation. 



Discussion 



Weigl: The second-order decay after flash illumination suggests the electron- 

 hole recombination kinetics commonly observed in crystal phosphors and photocon- 

 ductors. Debye and Edwards (J. Chem. Phys., 20, 236 (1952); Science, 116, 143 

 (1952)) and Yastrebow {Doklady Akad. Nauk. SSSR, 90, 1015 (1953)) have 

 reported similar afterglow in a variety of organic compounds, including proteins. 

 Recombination appears to be limited by the rate at which electrons return to 

 their radicals, and emission takes place by way of the lowest triplet state of the 

 parent molecules. In a chlorophyll-protein complex, recombination at the protein 

 could conceivably sensitize luminescence of the dye — even though the observed 

 emission comes from the lowest singlet level of the chlorophyll. 



Arnold : And an electron takes a very long time finding the radical. 



Weigl: Electron diffusion can give rise to lifetimes of many minutes or even 

 hours, but only at about — 196°C. Deep electron traps or inefficient recombination 

 would have to be demonstrated to make this mechanism seem plausible at room 

 temperature. 



Rosenberg : The use of a single flash seems to have simplified the kinetics of 

 recombination. 



Arnold : For the longer times. 



Rosenberg : In chloroplasts the situation may be easier to investigate where we 

 don't have induction phenomenon and we don't have pools of intermediates that 

 have to be brought up. Is the kinetics simpler for the chloroplast situation? 



Arnold: I checked the question of whether or not the saturation depended on 

 the time afterwards that you measured, and it is much the same for chloroplasts. 



