14 



BURTON: We have spoken about ionization as having a different effect from 

 excitation. I think that during the course of the discussion there may be quite 

 a question raised as to whether the ionization is actually persistent enough so 

 that you can say that any chemical process ensues from the ionized state itself. 

 Instead, we might be dealing with higher excited states. Thus we must note that 

 when we say ionization in contrast with excitation, all we are really concerned 

 with are high energy states compared with somewhat lower ones. 



POLLARD: There is the other purely empirical fact that the quantum yield 

 is low for most of the studies of cases of excitation, whereas the ionic yield is 

 close to 1. This is in the region, say, of 1900 a on out. The quantum yield for 

 most biological molecules is pretty low. In the order of 0.001 or something 

 like that, and this would look as though it would take 1000 excitations to produce 

 that effect. 



KASHA: For inactivation ? 



POLLARD: Yes. 



KASHA: That would seem reasonable from the spectroscopist's point of 

 view. 



POLLARD: Then one has to blame something other than excitation. 



BURTON: Dr. Pollard, the energies of the excited level could be different. 

 After all, the phenomena produced depend also on the amount of energy available 

 for an individual process. 



ALLEN: It is also a matter of geometry. 



POLLARD: We are going to check these by studies in the vacuum ultra- 

 violet„which should give us an answer to that. We are proposing to go down to 

 1100 A. There are^some effects which indicate that the molecules are more 

 sensitive in that region, but it is the whole region where physical studies are 

 necessary. 



KASHA: I should like to make a comment about a simple experiment by 

 Ageno, of the Institute of Health in Rome, which appeared in an Italian Journal 

 (28) and which has to do with an attempt to find out how far out the emission in 

 a liquid scintillation counter travels. Roughly, the experiment was to take a 

 nickel button, which was flat, and which he coated with polonium by electroplat- 

 ing, I believe. Of course, he knew exactly the length of the alpha- particle path. 

 He set up a camera to photograph a picture of the scintillation (in the dark) from 

 this button in a side view, and he found, roughly, that the emission occurred 

 over approximately ten times the known path of the alpha particles. He care- 

 fully tested to see if diffusion of radioactive material was occurring during the 

 experiment time. He found it was not. 



ALLEN: Ten times the length, or ten times the breadth? 



KASHA: Ten times the length. He concluded that it was possible in the 

 liquid scintillation counter and that an important mechanism was one of emis- 

 sion of some radiations which could travel through the solution as electro- 

 magnetic radiation which would then excite the emitting molecule. This inter- 

 ested me very much, and I should like to say something about it later. 



POLLARD: It must be high energy. 



