30 



entirely satisfactory values for AH} andAH-,. This is to be ascribed to the fact 

 that one does not have adequate information concerning the OH" ion -- e.g. , its 

 heat of solution in water and the electron affinity of OH. The reactions were 

 considered carefully by Dr. Magee and Dr. Burton (8), who for the first time, 

 to my knowledge, pointed out (reference (8), page 53Z) that reaction (B) is endo- 

 thermal, and that (A) therefore proceeds only by utilizing the hydration energy 

 of the OH" ion. This important conclusion apparently did not make the impres- 

 sion that it warrented. 



I should like at this point simply to mention a new way of attaining a value 

 for A Hi. On the basis of a recent theory (9) of the absorption spectra of ions 

 in electrolytic solution, it is possible to deduce AHj from the position of the 

 maximum of the characteristic absorption spectrum of the OH" ion in water; 

 this deduction is independent of any assumptions regarding the electron affinity 

 of the OH radical or the hydration energy of the OH" ion, and is contingent only 

 on the applicability of the aforementioned theory. The result is: AHj= -1.6 + 

 0.2 ev. For the reason already mentioned, it is not possible satisfactorily to 

 translate this to a value for AH2, but a crude estimate places the latter at, very 

 roughly, + Z ev. These calculations entirely confirm the previously mentioned 

 assertions of Dr. Magee and Dr. Burton. 



I now come to a further point which is, I believe, entirely new: reaction 

 (A), just because (B) is endothermal, runs into difficulty with the Franck-Con- 

 don principle, and is probably impossible. This is immediately apparent from 

 an examination of the nature of reaction (B), which is an elementary process of 

 the type called "dissociative attachment". For a simple molecule, a process of 

 this type is essentially completed in (i.e. , cannot be reversed after) a time of 

 the order of magnitude of 10" 1 3 second. But if the energy of hydration is to be 

 utilized to satisfy the energy requirement, a time of 10-11 second, the dielec- 

 tric relaxation time, is needed. In simple terms, this disparity in time means 

 that instead of being captured by and dissociating the water molecule, the elec- 

 tron simply moves on. I therefore suggest that we have been mistaken in the 

 past in presuming that reaction (A) is important in the radiation chemistry of 

 water and aqueous systems, if the electron is "free" -- or, more accurately, is 

 only weakly interacting with the medium, i. e. , interacts only by the electronic 

 polarization. Rather, the thermal electrons have a relatively long lifetime of 

 at least the relaxation time (10"H second), diffuse a comparatively great dis- 

 tance (of the order of microns), and are ultimately "captured" or "trapped" by 

 the medium as a result of the dipolar relaxation. It seems likely that only after 

 all of this has occurred can the dissociative attachment (A) take place. Per- 

 haps the most important consequence of such behavior would be the existence of 

 track effects for the OH radicals but not for the H atoms. 



I should say that this presentation has been necessarily oversimplified in a 

 number of respects. For example, it is conceivable that a free electron is at- 

 tached in a process, to be thought of as one involving a very large polyatomic 

 molecule, in which the energy defect is provided by a slight orientation of the 

 water molecules about the OH" ion. Such slight twisting is possible in the case 

 of water; it can occur much more quickly than the true dipolar relaxation, and 

 is responsible for the high value of the infrared dielectric constant. It seems 

 likely, however, that such a process, even though energetically possible, has 

 a very low probability. 



FANO: The electron is not solvated? 



PLATZMAN: The electron must be solvated first. The reaction will then 

 proceed, but more time will have elapsed. 



