79 



BURTON: I think there is evidence to the contrary. For example, in the 

 aldehydes these internal conversion processes occur, and they occur slowly. 



KASHA: How does one know that? You see, all the absorption bands in the 

 excited states of these molecules are diffuse. That is a novel feature compared 

 with diatomic molecules. 



BURTON: Perhaps we are going too far afield. We can save this point for 

 private discussion. 



LINSCHITZ: This is certainly a lower limit for the time that we could get 

 in general. Possibly if you want to say that the fluorescence measurements 

 would pick up even as much as 0. 1 per cent, then 10 _ 12 seconds, which would 

 allow several vibrations to occur, would be the time required for internal con- 

 version. These times, though, are by no means well defined and of course are 

 sensitive to the nature of the vibrational modes that are initially excited. 



We were discussing the factors that would channel the original electronic 

 excitation energy either into vibrational dissipation or bond rupture. In partic- 

 ular, the role of molecular complexity favors the former because of the long 

 time required for bond rupture. For complex molecules there is also a better 

 chance for matching the vibrational levels of one electronic state to those of the 

 next lowest state. So that this would also be a factor that would make it easier 

 for a complex molecule to lose energy by this process than for a simple one in 

 condensed phase. 



Finally I want to point out that for complex molecules, especially those with 

 large aromatic systems, the lower electronic states are more likely to be stable 

 than are those of simple molecules, so that for this type of case you can expect 

 to get possibly a couple of electron volts of energy set free when the molecule 

 ultimately ends in the lowest excited electronic state even if fluorescence can 

 occur. For a complex molecule the very fact that there are so many excited 

 states which may be stable would also tend to concentrate heating effects at those 

 sites, since the tendency may be to release vibrational energy in relatively 

 small amounts. 



KAMEN: What you are saying is that any biological molecule you can think 

 of is already so complex that the chance of getting -- 



LINSCHITZ: I would not go so far as to say that categorically. You have 

 these two competing processes and in general you are going to get less free 

 atom formation with the more complex molecules, but it can be shown that if 

 you take molecules even as complex as aniline, excitation in the ring will lead 

 in the gas phase to splits of the C-N bond which can be prevented by the admix- 

 ture of simple gases into the system. 



KAMEN: How do you account for the localization of energy in one bond in 

 this picture? 



LINSCHITZ: This is a function entirely of the shape of the energy surface, 

 of the mode of vibration which happens to be excited in transition and of the time 

 required for collisional deactivation. Again one has no knowledge at all for this 

 complex system of how the energy surface lies, and I would hesitate to try to 

 make any systematic rules. However, if the chromophoric group is adjacent to 

 a vulnerable bond, internal conversion may lead to rupture in that nearby bond 

 before the energy has had a chance to get diffused through the various modes. 

 This is the case in aniline and other complex molecules. 



