28 



LIGHT AND LIFE 



r — 



r — 



r — 



interaction None 



Energy mnsfer ^^^^ 



Energ^yjronsfer ^^^^ 



Weak 

 Strong 



Weak 



Strong 

 Weak 



Weak 



Fig. 3. Zero, weak, and strong interactions between electronic states of a diatomic 

 molecule showing how strong interactions inhibit energy transfer while weak inter- 

 actions, because the states remain close together, may favor rapid energy transfer 

 through closely spaced environmental levels. Similar considerations should apply 

 for more complex molecules. ,\ fluid environment introduces a fluctuating (time- 

 dependent) perturbation. This can cause states 2 and 1 to spend a small, but 

 significant, part of the time in a region of mutual overlap, whereby the non-radia- 

 tive transfer of energy from 2 -* 1 is greatly enhanced. 



benzene and similar molecules, one component of the spin orbit 

 operator has the correct symmetry to mix excited singlet and triplet 

 states (21a) . 



Effect of environment upon energy transfer. It is known that the 

 rate of non-radiative processes between excited states and the ground 

 state is very .sensitive to the environment. In rigid media, this process 

 is usually very slow (lifetime » 10 sec for the benzene triplet to 

 ground singlet state) , while in liquids or gases the process is very 

 rapid (lifetime of the order of 10-"* to 10-^ sec or less) . In certain 

 cases the non-radiative process can be catalyzed by a special environ- 

 ment. This is illustrated by the known quenching effect of paramag- 

 netic molecules on the lifetime of the triplet state (28). In the absence 

 of such catalyzing agents it is the fluctuations in fluid media which 

 quench the radiative processes. A possible mechanism can be con- 



