SYNTHESIS OF ORGANIC COMPOUNDS WY I()M/,1\(; RADIATION 



measure ov calculalc the total iniiiil)cr of ions formed in the system and tlic 

 experimental results were expressed in ion-pair yields either lor the destruc- 

 tion or for the formation of molecules. It was recognized later, particularly 

 in connection with the enormous amoimt of work on water and aqueous 

 systems, that excited molecules and free radicals were perhaps pred(jminant 

 factors. Experimental results are therefore now expressed in terms of the 

 amounts of material destroyed or formed in the absorption from the radiation 

 field of a fixed, measurable amount of energy. These are 'G'-values'. G^^q 

 is the number of molecules of \vater destroyed for each 100 e\' of energy 

 absorbed, and Gn_ would be the comparable yield of hydrogen in the same 

 system. However, it is now believed that the formation of charged ions is 

 responsible, at least in the primary events, for much of the final state of an 

 irradiated system^- '''^. 



It is clear that the types and interactions of species formed in the primary 

 radiation interactions will determine the overall l)ehaviour of the system. 

 The problems involved are divisible into three sections: (a) the nature and 

 energies of primary fragments must be established, {h) the kinetics and 

 energetics of reactions of the primary fragments with other constituents of 

 the system must be known and (c) the exact nature and amounts of final 

 products of the irradiation must be determined. In general we have been 

 concerned with reactions in the gas and liquid phases and seldom with the 

 solid phase, although the latter aspect will increase in importance. 



Information on the problems of section (a) is available from electron 

 impact data** using mass spectrometers to determine the types and energies 

 of primary charged fragments. There are two deficiencies in the data avail- 

 able with this technique. In general, positively charged species only have 

 been measured and there is an appreciable time, up to 10^'^ sec, in which 

 reactions of primary fragments can occur in the source before fragments 

 pass to the analyser. There is an increasing amount of work now being done 

 on negative ion formation, metastable transitions and rearrangements and 

 secondary reactions, which will soon be extremely valuable in radiation 

 chemistry. Another approach, which should provide similar data in the 

 gas phase, is the use of scavengers for reactive species. Several scavengers 

 have been proposed but the most useful so far is radio-active iodine, because 

 the organic iodides produced with reactive radicals can now be identified 

 and measured with some confidence^"'^. In addition, in simple systems at 

 least, an attempt is being made to study the theoretical basis of the disrup- 

 tion of molecules and reactions between fragments and molecules^^ 



An equally complex problem is found in the nature of the secondary 

 reactions which occur (section (b) above). In this case, it is essential to dis- 

 tinguish clearly between the reactions which can occur in different phases. 

 The nature of the environment and the average times required for reactive 

 species to form and react are the basic considerations which determine the 

 ultimate behaviour of the system. A good picture of the time scale is given 

 in the review by Magee^^^ and a modified chronological table is shown in 

 Table 1. The physical picture of the primary processes is usually as follows : 

 along the paths of the radiation through matter secondary ionization and 

 excitation occur with great intensity in isolated 'spurs'; primary fragments 

 are formed within the spurs and it may be taken that any reactions of the 



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