25 



different words, the electron can absorb as well as emit vibrational quanta; 

 however if its energy drops below 0.2 ev, only absorption is possible. 



KASHA: Are you referring to a single vibrational frequency? There are 

 several. 



PLATZMAN: There are three normal vibrations, but their frequencies are 

 roughly the same. Actually one is at about 0. 2 ev and the other two are at 

 about 0. 5 ev. When I say lower than this energy, I mean lower than the lowest 

 of the three, i. e. 0. 2 ev. 



Let us now summarize what has been said. The rate at which a moving 

 electron loses energy in exciting molecular vibrations is a function of the ve- 

 locity, and is of order of magnitude 10* ^ ev/sec in the energy range between 

 several ev and about 0.2 ev. Below that value (dw/dt) is actually positive, but 

 is small enough to be unimportant. 



ALLEN: How does this compare with what you get for the homopolar case? 



PLATZMAN: It is considerably greater. The excitation of molecular vi- 

 bration in the homopolar case is quite small. I might mention that, as far as 

 experimental information on this very important question is concerned, there 

 is very little. There were good experiments done by some students of Franck 

 many years ago on the homopolar case. For example, the probability that an 

 electron, passing in the vicinity of H^, excites its vibration was found to be 

 about 1 per cent. This shows that one must be careful about associating this 

 process with the existence of a permanent electric dipole moment. No ex- 

 perimental work of this type has been done which can be taken as an indication 

 of the energy loss to such a dipolar molecule as water. 



MAGEE: Why do you discount the work of Bailey and Duncanson (5) with 

 H 2 0? 



PLATZMAN: I haven't discounted it yet. I am going to discount it in a few 

 moments. 



Controlled impact experiments on the loss of energy of very slow electrons 

 to important dipolar molecules, then, have not yet been undertaken The only 

 experimental work on dipolar gases that is available is that from drift experi- 

 ments -- for example, measurement of the mobility of electrons as an electron 

 swarm having some kind of thermal or pseudo- thermal velocity distribution 

 passes through the gas. This is the case, for example, with the work of Bailey 

 and Duncanson on H2O that Dr. Magee mentioned, and that he and I have de- 

 bated. I have lately tried again to find out how one can interpret the electron- 

 swarm experiments and arrive at any information of significance to the problem 

 we are discussing. My conclusion is that one cannot, at least not easily. 

 These experiments, in my opinion, are not so interpretable at the present 

 time --at least not by me. In any case, they refer to the vapor and not to the 

 condensed phase. And this makes a great deal of difference, as we shall now 

 see. 



Now I shall proceed to the second type of interaction, the interaction of the 

 moving charge with the dipole moments, considering the molecules as rigid 

 dipoles. This is an interesting problem which, incidentally, has never been 

 treated before. I was very fortunate to have the collaboration of Professor H. 

 Frohlich, who was recently a visiting professor at Purdue, for this work. To- 

 gether we worked out a simple theory for the rate at which a moving electron 



