1868 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



method used in section 3 for crystals. With a natural life time of 1.2 X 

 10~^ sec. (section 1), and a fluorescence yield of 1%, the time available 

 for energy migration is 1.2 X 10~^° sec. Since the shape of the band 

 indicates the maintenance of the coupling of electronic excitation with 

 molecular vibrations having periods of the order of 10~^^ sec, the possible 

 number n of transfers during the actual life time is n < 1.2 X 10~^VlO~^' = 

 1200. It is interesting to note that conditions for energy migration are 

 more favorable in vivo than in crystal layers in vitro, the unfavorable 

 effect of lesser density being more than offset by the favorable effect of a 

 less efficient quenching through energy conversion into lattice vibrations. 



(b) Sensitized Fluorescence in vivo 



We now turn to sensitized fluorescence in vivo, and the evidence of 

 energy transfer, derived from it. 



\.o 



0.8 



V) 



> 0.6 



o 



UJ 



- 0.4 

 < 



02 



X Chlorophyll a 

 O Phycoerythrin 

 D Phycocyanin 



450 500 



WAVE LENGTH, mfi 



Fig. 37C.41. Relative intensity of fluorescence of three pigments in Porphyridium 

 cruentum. Monochromatic excitation (after French and Young 1952). 



In chapter 24 we gave a brief summary of the important work of Duy- 

 sens, and of French and Koski, on the fluorescence of red algae. The two 



