IONIZATION 65 



well as ionization accompanied by dissociation of the molecule, also 

 occurs in a variety of ways. Some ions are metastable and dissociate 

 spontaneously after a short lapse of time. 



Negative ions (22) can be formed by the capture of an electron by a 

 neutral molecule or atom. Whereas most atoms and radicals have 

 positive electron affinities and can therefore form stable negative ions, 

 many simple molecules (especially those which have a S ground state) 

 cannot form stable negative ions. Other molecules, particularly those 

 which are strongly polarizable or have permanent dipole moments, form 

 stable negative ions but usually have small electron affinities. In 

 addition to the simple capture of an electron, negative ions may be 

 formed by capture accompanied by dissociation. For example, a 

 hydrogen bromide molecule can interact with a slow electron to yield a 

 hydrogen atom and a bromine negative ion. 



Ionization may migrate through a system either by simple exchange 

 of charge between molecules or because of the conductivity of the 

 medium. Measurements of the effect of traces of impurities upon the 

 mobility of positive ions in gases (23) (such as helium) demonstrate 

 that the exchange of charge between unlike molecules occurs effectively 

 at ordinary pressure. Lind's demonstration (6, 24) that radiochemical 

 reactions can be sensitized by inert gases is additional independent 

 evidence that the exchange of ionization takes place readily, at least 

 under the conditions of the experiments. A similar exchange of charge 

 between a negative ion and a neutral molecule is to be expected. Failure 

 to take the exchange of ionization into account is likely to invalidate 

 any analysis (25) of the kinetics of radiochemical reactions. In solution, 

 particularly aqueous, the solvation of the ions may influence profoundly 

 the rate of exchange. Fortunately the rates of such solution reactions 

 involving electrolytic ions are subject to direct study. 



Migration of either positive or negative charges in a crystalline solid 

 (26) may occur readily by electron exchange or by electron migration 

 in the conductivity bands of the crystal. Although such conductivity 

 is observed most readily in ionic crystals, it should also occur in atomic 

 lattices (like diamonds) and in molecular lattices. However, in crystals 

 made up from organic molecules there will be httle electron migration 

 on account of the frequent disturbances by thermal vibrations of the 

 lattice and the molecules. Electron conductivity is also to be expected 

 in liquids such as water, but here it should be limited to quite short dis- 

 tances corresponding to the short range order of liquids. 



Reactions between ions and molecules are of importance in radiation 

 chemistry. Except for the existence of the charge on one of the reactants, 

 these reactions are in every way similar to the kinetic reaction steps of 



