118 ELEMENTARY CHEMICAL PROCESSES 



Elementary Chemical Processes Characteristic of Water 



In pure water the primary processes are usually written 



H2O ^ H2O+ + e (1) 



H2O+ + aq -> H3O+ + OH (2) 



H20 + aq + e -^ H + OH--aq (3) 



Processes involving excitation of water in the primary physical act are 

 usually neglected because it is the general opinion that the Franck- 

 Rabinowitch cage either prevents formation of free hydrogen atoms and 

 hydroxyl radicals or causes their immediate recombination without con- 

 sequent secondary chemical effects; that is, 



H2O ^ H2O* -^ H + OH ^ H2O (4) 



A feature peculiar to the set of reactions 4 is that, unlike the first three 

 listed, they occur without any local changes of hydrogen- or hydroxyl- 

 ion concentration. The reactions ensuant on reactions 1 to 3 depend on 

 the relative proximity to each other of the products of reactions 2 and 3. 

 In fast-particle irradiation (that is, high-energy electrons, x-rays, etc.) 

 the processes of reaction 1 may occur several hundred molecules apart, 

 whereas in slow-particle irradiation (that is, the usual alpha and neu- 

 tron processes) the distances between adjacent hydroxyl radicals, formed 

 by the successive reactions 1 and 2, may be less than 10 molecules. 

 Reaction 3 usually occurs a considerable distance, perhaps as much as 

 10 molecules, from the locale of origination of the electron involved. 

 Thus, we may expect a distribution of atoms and radicals somewhat 

 isotropic for a fairly homogeneous beam of fast-particle irradiation, 

 definitely anisotropic for slow-particle irradiation.* In the latter case 

 each ionization column consists essentially of a core of hydroxyl radicals 

 and oxonium ions surrounded by a sheath of free hydrogen atoms and 

 hydroxyl ions. 



* Note added in proof {August 29, 1951): In a rapidly developing field, new facts 

 are found and new ideas develop in the course of a year. This picture and its conse- 

 quences have now been greatly modified. In both fast-particle and slow-particle 

 irradiation, about three-quarters of the primary physical effects are in spurs which 

 contain approximately the same number of ions approximately similarly distributed 

 in both cases. The important parameter, which must account for the difference in 

 the effects of the two types of radiation, is, therefore, the distance between spurs, 

 which is relatively large for fast particles. In further development of the theory 

 this fact must be carefully considered (cf. forthcoming paper in Nucleonics by Burton 

 and Magee). 



