24 J- FRANCK 



what he is asking? I think he is saying that, if you pnpulatf^ (lie ground triplet 

 state to a greater extent, which you presumably do by stronger irradiation, do you 

 increase the drainage from the excited singlet. 



Duysens: In which case you would lower the fluorescence yield. 

 Rosenberg: But I don't think that that happens to any ajjpreciable extent. 

 The steady-state population of the first fiuores(!eiit state and of the lowest triplet 

 state would both be proportional to the light intensity in close approximation. 



Duysens : I do not think that Dr. Rosenberg's remark is correct, because the 

 drain is proportional not only to the number of excited singlets but al.so to that 

 of non-excited triplets. 



Rosenberg : But also, at the same time, you are increasing the total number of 

 quanta absorbed per second so that the fractional yield of the molecules which are 

 fluorescing may remain the same. It is the fractional yield of fluorescence that is 

 constant experimentally. 



Lumry: If all the processes out of the first excited state are of the first order 

 in the concentration of the molecule in the excited state, then the only way you 

 can change the amount of fluorescence is by changing the crossing point. Unless 

 you are postulating a back reaction from the triplet into the singlet, the fraction 

 of the fluorescence yield isn't going to change, no matter what you do to how 

 many triplet molecules. 



Duysens : If you have no triplet, there is no loss of fluorescence. Assume the 

 fluorescent yield is, say, 50%. If you have a very efficient transfer, then the fluo- 

 rescent yield is zero. 



Lumry: This just does not have anything to say about the lifetime of the trip- 

 let or the population. It is a question of whether it can get out of the singlet into 

 the triplet. 



Duysens : But my contention is that the efficiency of transfer to the triplet 

 state is proportional to its concentration. If it is two million, then you have twice 

 as great a transfer as if you had one million. Then the fluorescence yield of chloro- 

 phyll changes. If the process of transfer is fairly efficient, it may change by, say, 

 50%. 



Rosenberg: I think there is a self-balancing mechanism in Franck's picture 

 which answers Duysen's question. In the Franck picture, the metastable state 

 would be formed at a rate proportional to the concentration of the fluorescent 

 state material. It would also di-sappear at a rate proportional to the concentra- 

 tion of the fluorescent state material. Therefore, the steadj'-state concentration 

 should be independent of the light intensity and it should be simply a ratio of rate 

 constants, i.e., the rate constant of formation divided by the rate constant of loss. 

 So we don't have to worry about the variation of jjopulation of the lowest triplet 

 state with the light intensity. 



Arnold : This is true until the rate is so slow you have to worry about the life- 

 time of this triplet. 



Rosenberg : Yes, I am forgetting the natural decay of the triplet. 



Duysens : But this triplet state is supposed to react chemically. 



Rosenberg : No, only in its excited state. 



Duysens : So the concentration remains constant? 



Rosenberg: Yes, as long as you don't worry about the natural decay of the 

 lowest triplet state. 



