77 



of events is typical, and will appear again in discussions of energy migration 

 and triplet state excitation. 



Of course, it is clear that this radiationless transition may occur also to 

 repulsive states, which is just the familiar process of predissociation. 



We may now ask: What is the relative probability of heating by these vari- 

 ous processes as opposed to radical formation or splitting? In general one can 

 say that vibrational dissipation becomes more likely than splitting for a com- 

 plex molecule - that complexity of the system alone will favor a local internal 

 heating process rather than dissociation, for several reasons. In the first 

 place, if we have a complex molecule to deal with there is going to be a time 

 lag between the initial activation of the vibration and the time necessary for the 

 energy to flow, as it surges through the various modes of the molecule, to the 

 relevant bond that we are interested in and to rupture that bond. This time is 

 in competition with the vibrational deactivation time which is very fast. So in 

 general for a condensed system one might expect that if there are many modes 

 of vibration, the chances are that the energy will pass over into local heating. 



FANO: You say internal conversion can be very fast. How fast? 



LINSCHITZ: Probably the best evidence comes from studies on fluoresence 

 yields of complex molecules in solution, and it is found there that fluorescence 

 invariably comes from the lowest of the excited electron states. The radiative 

 lifetime for states reached by excitation in intense absorption bands, as in the 

 case of certain dye stuffs, may be as short as 10"° seconds. However, you 

 never see any emission from the upper states in a condensed phase which 

 would limit, let us say, the fluorescence yield to something perhaps as small 

 as 0.01%. Any fluorescence appreciably above this would have been seen in 

 experiments which have been done. That means that for a lifetime of the upper 

 state of 10 _ 9 seconds, the factor 10"^ (from the 0.01% efficiency) reduces this 

 to the order of 10"^ seconds. Within this time one can say that the energy is 

 already on its way down from the upper state, by internal conversion, to a low- 

 er state. 



I would say for states like this, excitation to an upper state may lead to ra- 

 diationless transition and internal conversion into a lower electronic state in a 

 time as short as 10-13 seconds. 



FANO: It sounds mighty fast. 



LINSCHITZ: These are what the facts indicate. 



PLATZMAN: Surely not quicker. You say "as short as". Well, let's not 

 go to shorter times. 



LINSCHITZ: All right. This, of course, is an extraordinarily short time 

 - of the order of a single vibration. I do not mean that the time is always so 

 short, but this is the limiting value and does seem to occur in many cases. 



BURTON: May I say to Dr. Fano that the thing which characterizes this 

 type of internal conversion is that it occurs in a condensed system. Thus in- 

 duced internal conversion may be involved and consequently the selection rules 

 are not so restrictive as they are, for example, for a gaseous molecule. Con- 

 sequently, the internal conversion process may occur in a very short time, 

 even as Dr. Linschitz indicates, in a single vibration period. 



