PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 243 



on the two electron affinities but also on the two hydration energies, and 

 the transfer of the electron takes place as the atom and rather large ionic 

 system interpenetrate during a collision; this electron exchange cannot 

 possibly be thought of as a simple "electron jump" because of the essen- 

 tial contributions of the hydration shells to the energy of the transition. 



4-5. MULTIPLE IONIZATION 



Multiple ionization is known to occur with ciuite appreciable yield in 

 electron impact experiments with atoms (Massey and Burhop, 1952). In 

 the case of molecules, little information is available because multiply 

 charged molecules usually dissociate quickly as a result of the Coulomb 

 repulsion. (Relatively few doubly charged molecules have been observed 

 in mass-spectrometric experiments, and apparently none of charge greater 

 than two has been detected. It is possible, in fact, that some or all of the 

 doubly charged molecules are not stable but metastable.) 



Multiply charged ions will always result (with atoms of small mass, 

 such as are found in biological systems) from single ionization acts in an 

 inner shell of an atom. With high-energy radiations such multiple 

 ionization is by no means a rare effect (Platzman, 1952a). 



Multiple ionization would appear to hold promise of explaining the pri- 

 mary physical process for chemical changes in particularly stable mole- 

 cules which are not transformed by single ionization or excitation acts, 

 for it would always have a potent influence on a polyatomic molecule of 

 not too great size, even in condensed phases. Although this possibility 

 has provoked some speculation, there seems to be no definite empirical 

 indication of such an effect at the present time. 



4-6. THE SIGNIFICANCE OF DENSITY OF IONIZATIONS^ 



The possible effects of "density of ionization" (and, of course, also of 

 "density of excitation") on radiation-induced reactions are ordinarily 

 considered to arise from the influence which the initial spatial distribution 

 of energy transfers has on the kinetics of elementary reactions having 

 reaction rates of order two or greater (cf. Sect. 1-3). This effect need 

 not, however, be the only one. If the reactant is a large polyatomic 

 molecule, it is necessary to consider how the molecule as a whole is 

 affected by a single large deposition of energy as compared to a number of 

 successively delayed smaller ones. If a particular effect on the molecule 

 were to have a threshold energy for transformation, for instance, different 

 types of radiation would have different effects and the differences would 

 not be interpretable on the basis of simple reaction kinetics: they would 

 stem instead from a complicated interplay of the electronic and vibra- 

 tional energies of various "parts" of the single molecule. An example of 



•9Cf. Chap. 1, Sect. 5-5b. 



