44 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



The main quantitative theory on energy transfer, due to Forster, 

 predicts that efficiency of energy transfer is proportional to intensity 

 of absorption of the acceptor molecule. This would argue against our 

 suggestion. However, it is doubtful whether Forster would insist that 

 his mechanism would apply to all types of energy transfer. 



We should like to point out that Dr. Duysens' statement (that the 

 similarity of the "action spectrum of fluorescence" of chlorophyll a 

 and the "action spectrum of photosynthesis" requires that the "fluo- 

 rescent" level be involved in photosynthesis ) is incorrect. In the normal 

 excitation of phosphorescence the "action spectrum" of fluorescence 

 and phosphorescence are necessarily identical, since both first involve 

 lowest singlet-singlet absorption. 



Dr. Mason: Would you describe the operations which characterize 

 fluorescence and phosphorescence? Have phenomena such as delayed 

 fluorescence or premature phosphorescence been observed? What op- 

 erations must be performed in order to decide in which category the 

 emission falls? 



Dr. Kasha and Mr. Becker: Dr. Mason's first question is answered 

 in the published literature (Kasha, 1947; Lewis and Kasha, 1944). 



In answer to the second question, there is no sharp distinction be- 

 tween fluorescence and phosphorescence of molecules on the basis 

 of lifetime. Thus, fluorescence is usuaUij very short lived, in the range 

 10~^ to 10" ^'^ second (mean life) because the emission corresponds 

 to intense absorption bands (cf. Section III, 1, our paper). However, 

 singlet-singlet luminescence, or fluorescence may be long lived, as 

 long as 10"'^ second ("delayed" has unhappy connotations) if the 

 singlet-singlet absorption is weak. On the other hand, phosphores- 

 cence lifetimes are usiioUy long, in the range 10 to 10 "^ second. 

 Nevertheless, under the action of certain perturbations, such as sub- 

 stitutions in the molecule involving atoms of high atomic number 

 (e.g., Br I), the inherent phosphorescent lifetime may be shortened 

 as much as by a factor of 1000. Thus, a 10 '-second phosphorescence 

 could be observed. Obviously, there is overlap between lifetimes of 

 fluorescence and phosphorescence. 



Distinctions which are qualitative may be made, however, between 

 fluorescence and phosphorescence as follows : ( 1 ) The excited state 

 giving rise to phosphorescence is paramagnetic. This has been proved 

 by direct susceptibility measurements in one case of phosphorescence. 

 It is a difficult measurement and cannot be applied easily enough to 



