PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 241 



for the determination of the electron affinity of B ; the dissociation energy 

 of AB must then of course be known). 



4-4. ELEMENTARY PROCESSES FOLLOWING IONIZATION 

 IN CONDENSED SYSTEMS'^ 



The presence of a liquid or solid environment profoundly affects the 

 course of a chemical reaction caused by penetration of high-energy radia- 

 tion. The influence is felt in all three stages of progress of the reaction: 

 in the initial energy transfers to molecules of the medium, in the imme- 

 diate consequences of this energy transfer, and in the subsequent elemen- 

 tary reactions (and kinetics thereof) which the primary products undergo. 

 In the following brief discussion attention mil be restricted to litiuid 

 water, a medium of great complexity and, of course, of unique importance 

 in both chemistry and biology. However, many of the remarks will apply 

 at least qualitatively to other polar solvents as well. 



Energy transfers from a swiftly moving charged particle to molecules 

 of a medium include excitation acts, and ionization acts covering a wide 

 spectrum from small energy transfers of magnitude close to the ionization 

 potential, which produce an ion and a very slow electron, to very great 

 energy transfers that produce the same positive ion but a very energetic 

 electron. The latter group of ionization events will not be influenced by 

 the fact that a molecule is immersed in a liquid; indeed, even the binding 

 of the atomic electrons in the atoms and molecule has only a minor effect 

 on great energy transfers. The small energy transfers, however, will be 

 very sensitive to finer details of the electronic binding, and the relative 

 numbers of different possibilities for small energy transfer will depend 

 intimately on the state of aggregation. The same argument applies to 

 ionization by secondary, etc., electrons. One consequence of this must 

 be a small but appreciable difference between gaseous and liquid water in 

 the stopping power for swiftly moving charged particles. Another, and 

 much more marked one, will be a difference in the mean energy recjuired 

 to form an ion pair: the value for the Uquid would be anticipated to be 

 about 10 to 20 ev instead of the often-used value of 30 to 35 ev, taken 

 without justification from experiments on gases. Unfortunately, it has 

 not yet been possible to determine this important quantity directly from 

 experiment. 



The very concept of what constitutes an ionization act requires new 

 attention when a liquid like water is considered, and the ionization poten- 

 tial, which must in any case be smaller than it is in the gaseous phase 

 (because of the influence of the dielectric constant in decreasing the energy 

 required to separate ion and electron) is difficult to define. It is possible, 

 for example, that an electron be transferred through a distance of several 

 molecular diameters, within a particular group of closely coupled water 



18 Cf. Platzman (1952b). 



