242 RADIATION BIOLOGY 



molecules, and that such a transfer would effect chemical change (the 

 products escaping the "cage") ecjuivalent to that effected by a truly freed 

 electron. 



Following the production of ions, the role of the medium is felt even 

 more strongly: the ions become "hydrated." This process, which 

 involves more or less orderly alignment of water molecules in the vicinity 

 of the ion, takes place within a period after the ionization act approxi- 

 mately equal to the dielectric relaxation time of water, namely, 10~'^ 

 second. Thus it is very fast, almost as fast as direct dissociation and 

 about as fast as internal conversion. If a primary ion undergoes internal 

 conversion or dissociation, these processes will occur in conjunction with, 

 and may be intimately coupled to, the alignments of hydration. Evi- 

 dently much of the energy transferred to the medium in an ionization act 

 (not including the kinetic energy of the ejected electron) will be dissipated 

 almost at once as heat; a portion may be retained as chemical energy, but 

 only a negligible ([uantity will reside in the ions per se, for recombination 

 of hydrated ions like H+ and 0H~ evolves only 0.6 ev in the liquid state, 

 as compared to an estimated 15 ev for the free ions in the gaseous state. 

 Thus, in the particular case of water, the events following an initial ioniza- 

 tion act are very complex: the adjacent water molecules orient about the 

 ion, any additional excitation energy of the H2O+ is transformed by 

 internal conversion to oscillations of H2O+ and thence to oscillations of 

 the neighborhood, the H2O+ dissociates by transferring a proton to a 

 neighboring water molecule to form H3O+ (it will be that proton which 

 happens to be caught in the more favorable position), and the heat gen- 

 erated by both H3O+ formation and hydration excite the oscillations 

 about the ion to a degree far in excess of that corresponding to the ambient 

 temperature. 



In the capture of an electron by H2O to form 0H~ -|- H, the hydration 

 energy again enters in intimate fashion, so that the process is closely akin 

 to one proceeding in a large polyatomic molecule. It is complicated, 

 however, by the mobility of the electron and the complex and rather 

 subtle nature of the energy balance, and is not understood at the pres- 

 ent time. 



Elementary reactions of radicals, ions, etc., formed by the primary 

 processes will proceed as they do in corresponding thermal processes, the 

 medium again exercising an influence which may be small or great. Thus 

 the association reaction OH + OH -^ H2O2 is probably affected in a 

 relatively minor way, chiefly by polarization and small orientation effects 

 of the electric charge distribution in water, and by a modification of the 

 details of diffusion. On the other hand, an electron exchange reaction 

 such as OH + Br~ — ^ OH" + Br is in actuality an entirely different 

 reaction from the corresponding one in a gas: the negative ions act like 

 large polyatomic molecules, the energy of the reaction depends not only 



