27 



PLATZMAN: It must affect it very much. 



ONSAGER: Could you give us an idea of what the order of magnitude is? 



PLATZMAN: That is just what I was going to do. So far I have talked about 

 the individual mechanisms, but now I am going to try to consider them together 

 and arrive at some estimates for the electron path. It is very easy to calculate 

 how long it takes an electron to go from 5 or 10 ev to thermal energy, and this 

 turns out to be very roughly 10" 13 second. It is very important to note that this 

 time is considerably greater than the reciprocal of the infrared absorption fre- 

 quencies. 



KAMEN: This is water vapor? 



PLATZMAN: This is for liquid water, and that time, of course, involves 

 the density. 



ALLEN: When you go to 50 electron volts do you have any idea of how much 

 of the stopping power is contributed by this? 



PLATZMAN: About 0.1%. 



ALLEN: That is, at 50 electron volts it is about as important or unimpor- 

 tant as at one million? 



PLATZMAN: About. Within a factor of 10. 



To return to the argument: The average time required for an electron to 

 be moderated from the lowest electronic excitation potential down to thermal 

 energy is about 10" ^ second. This time is much longer than the reciprocal of 

 the infrared, frequencies -- i.e. , the vibration periods of the molecule. Hence, 

 in the period during which the electron is slowing down, there is ample time 

 for the infrared polarization of the dielectric to develop, which means that 

 once it is thermalized, the electron is surrounded by polarized medium to the 

 extent given by a dielectric constant of five. 1 That is important. 



POLLARD: What is the positive ion doing all this time? 



PLATZMAN: I am glad you mentioned that. I have ignored the influence 

 of the field of the positive ion on the electron. It is, of course, pulling the 

 electron back. However, I have not considered this effect in detail yet. I do 

 not believe, at the present time, that it will be of paramount importance. 

 This is where Dr. Magee and I depart very much from each other in our inter- 

 pretations. He will have ample opportunity today and tomorrow to bring this 

 out. 



POLLARD: Doesn't Coulomb's law hold? 



PLATZMAN: Coulomb's law certainly holds, but I do not believe that the 



1 . At the time of the meeting, I had not taken into account the effects of the 

 "resonance absorption" part of the dielectric absorption. Further work has 

 indicated that this is highly important in the case of liquid water. It seems 

 likely, however, that it will affect the conclusions I have drawn only quantita- 

 tively, and not qualitatively. R. L. P. 



i IBB Ah y 



