MULTIPLE IONIZATION 103 



Multiple Ionization 



A primary process not often considered in radiation chemistry and 

 biology is the formation by the incident particle, in a single collision, of 

 a doubly (or, in general, multiply) excited or ionized atom or molecule. 

 Such a process, it will be shown, is probably not of importance in most 

 instances. Nevertheless, it certainly ought not to be completely ignored, 

 for although undoubtedly infrequent it could in some special cases con- 

 ceivably have much greater permanent effect than a number of isolated 

 single excitations or ionizations. (The latter are usually chemically 

 effective for simple molecules, but, as mentioned above, are perhaps less 

 commonly so for complex molecules.) 



Multiple processes produced by consecutive impacts of two different 

 particles of the radiation are, of course, utterly negligible in all cases. 



Multiple excitation or ionization in a single event finds perhaps its 

 most conspicuous physical manifestation in the excitation of non-dia- 

 gram, or satellite, x-ray emission lines, some of which owe their origin 

 to multiple ionization of inner shells of the target atoms by fast electrons. 



Unfortunately, contemporary theory does not offer much possibility 

 of accurately calculating the yield for this type of process — for example, 

 the relative probability that a charged particle will in a single collision 

 produce a doubly or singly charged ion — except for the simplest of 

 atomic systems. This is because the approximation that one is ordi- 

 narily obliged, for reasons of tractability, to use for the possible station- 

 ary states of the affected atom (the so-called single configuration, cen- 

 tral-field approximation) is such that multiple excitation and ionization 

 events automatically have zero probability, that is, are inherently neg- 

 lected. The use of a more realistic atomic model imposes the greatest 

 calculational difficulties, even for single processes; and, moreover, su- 

 perior models are not generally available for substances of chemical or 

 biological interest. The only theoretical treatment of a multiple col- 

 lision process thus far accomplished is one for the excitation of some of 

 the x-ray satellite lines mentioned above. 



It should be permissible, however, to disregard multiple excitation, 

 since this process seems less likely to play a significant role than does 

 multiple ionization. (By multiple excitation is normally meant the 

 simultaneous excitation, by a single passing charged particle, of several 

 electrons in a single atom or in atoms closely coupled together in a mole- 

 cule. Excitation by a single particle of two or more widely separated 

 atoms in a molecule will more often be important. It can be treated by 

 theory as a special case in the consideration of the spatial distribution 

 of the energy loss. Since the collision time for a fast particle and a not 



