29 



o 



MAGEE: I believe that even so the coulombic energy at 50 A is more than 

 0.04 ev at that point, isn't it? 



PLATZMAN: Yes. But I believe that one must be wary of drawing any con- 

 clusions from that simple fact. 



The preceding discussion has all applied to liquid water at room temperature. 

 With ice, for instance,^" is very much greater and the rate of energy loss by 

 dipolar relaxation becomes so small as to be entirely negligible. Once they get 

 below 0.2 ev, therefore, the electrons just move around for a long time; per- 

 haps here is where the charge of the positive ions would dominate the picture. 



KAMEN: May I bring up a point? What happens when you substitute am- 

 monia instead of water? 



PLATZMAN: The figures should be of about the same order of magnitude, 

 except for one qualification: I don't believe that anybody knows the value of € ^ r 

 for ammonia. 



KAMEN: I was looking for some explanation of why ammonia is so different 

 from water as far as the length of time the electron exists in the free stage is 

 concerned. 



PLATZMAN: That I will come to later. I believe that this is easy to under- 

 stand, but it does not concern what we have thus far discussed. 



This ends the consideration of electron moderation. We now have electrons 

 of thermal energy, and there arises the question of what happens to them. For 

 years we have been saying that the reaction that takes place is 



e"+ H2O > H + OH'. 



This is the basis for the interpretation of radiation effects in water given by 

 Dr. Burton and Dr. Allen and by so many others. 



BURTON: The ideas in reference are properly to be attributed to Franck 

 in the first place. Dr. Allen and I were both involved in the development of the 

 theory when it was still entirely secret; the most interesting later features 

 were entirely Dr. Allen's contribution. 



PLATZMAN: I should now like to examine the physical mechanism of this 

 reaction in some detail. First let us consider the energetics. Here we might 

 be interested in either of two distinct reactions: 



e" + H 2 OUq y H aq + OH^q (A) 



and 



e" + H 2 O g > H g + OH g , (B) 



which refer to the liquid and gaseous phases, respectively. In both cases the 

 electron (e~) is to be though of, insofar as the energetics are concerned, as 

 one which is not interacting with the medium. Then the heats absorbed in the 

 two reactions, A Hi and AH 2 respectively, differ chiefly by the "heat of solu- 

 tion", or "hydration energy, " of the OH" ion. 



Although it would be difficult to think of simpler reactions in radiation 

 chemistry than (A) and (B), it has not heretofore been possible to arrive at 



