58 REACTIONS INDUCED BY IONIZING RADIATION 



data, and the interpretation of the results on the oxidation of carbon 

 monoxide, which was pubHshed by Hirschfelder and Taylor (8) in 1938, 

 appear entirely satisfactory and prove, at least for these cases, that it 

 is unnecessary to assume that there is any special or unusual character- 

 istic of the kinetics of radiation chemistry. 



To sum up, information which was not available when the cluster 

 hypothesis was first introduced by Lind is sufficient to enable us to reject 

 the assumption that the formation of clusters plays any important role 

 in the kinetics of radiochemical reactions. We are not justified in 

 believing, however, that the part played by ions is necessarily a minor 

 one. Although the historical reason for reporting the results of radio- 

 chemical reactions in terms of ion-pair yields is certainly untenable, the 

 use of the ion-pair yield, at least for gas reactions such as the oxidation 

 of CO, is a convenient and reasonable one. It is, of course, for just 

 these systems that the ion-pair yield and the energy yield (for example, 

 the yield per 100 ev) are closely proportional to one another. It must 

 be remembered that, when the ion-pair yield is used for reactions 

 occurring in condensed systems, the number of ion pairs is not directly 

 determined but is extrapolated from gas-phase measurements. The 

 similarity between the ion-pair yield and the quantum yield of photo- 

 chemistry should not be too heavily stressed, since it is likely to bias 

 one's thinking about such reactions. 



The earlier classical work on radiation chemistry was largely a study 

 of reactions involving simple molecules in the gas phase. Recently, 

 because of the obvious biological implications of complex molecules, for 

 practical reasons, and on account of the greater simplicity, in at least 

 one sense, in interpreting the results, the tendency amongst radiation 

 chemists has been to study chiefly reactions involving complex molecules 

 and, frequently, condensed systems. Although undoubtedly these 

 studies are of vital importance, it is somewhat to be regretted that the 

 investigation of gaseous systems of simple molecules has been allowed 

 to lie dormant. The newer viewpoints and the additional information 

 which has been gained, partly from mass spectrographic studies, about 

 the nature of the reactions involving simple ions should permit much 

 more rapid and sound progress to be made in the interpretation of these 

 simple processes. For example, the published data (6) of the water- 

 formation reaction still exist as a challenge to any theoretical student of 

 radiation chemistry. As Franck has stated several times in the last few 

 years, it should also be interesting to study the products of radiochemical 

 reactions involving simple gaseous compounds of carbon, hydrogen, and 

 nitrogen. The mystery of the origin of complex organic material upon 

 the earth might be solved by such experiments, since it is reasonable to 



