CAPTURE AND LOSS OF ELECTRONS BY POSITIVE IONS 105 



varieties of radiation. For radiations of high specific ionization the rela- 

 tive yield will be greater, but it would be difficult to treat this situation 

 even semiquantitatively. 



That multiple ionization must be extremely effective chemically can 

 be concluded from the fact that even doubly ionized molecules tend to be 

 highly unstable; they are, for example, observed only rarely in mass 

 spectrometric studies. The basis for this instability is clear: even if the 

 doubly ionized molecule should be formed in a stable state, the potential 

 surface for this state will in general cross that of the repulsive state 

 formed from two singly ionized radicals. The latter state always lies 

 lower than the former at great nuclear separation because more energy 

 is required to doubly ionize one atom than to singly ionize two. This 

 instability can be pictured very crudely as resulting from the capture 

 by a doubly ionized atom of an electron from an adjacent neutral atom 

 in the molecule, the two singly ionized atoms then dissociating the mole- 

 cule by their repulsion. (Recall that two electronic charges separated 

 by a distance of 1 A, the C — H bond distance, repel each other with an 

 energy of 14 ev.) The problems posed by multiple ionization of com- 

 plex molecules are obviously much too elaborate to permit their dis- 

 cussion here. (For the lines along which an analysis should proceed, 

 cf . the contributions to this volume by Eyring et al. and by Livingston.) 



We note finally that multiple ionization is to be expected for all high- 

 energy radiation and for all media except hydrogen and helium as the 

 result of Auger processes follow^ing ionization in inner electron shells. 

 The yield relative to single ionization events, for biological media, should 

 be of the order of 10^^ to 10~*, and will usually exceed that of direct 

 multiple ionization. These effects and their consequences are discussed 

 in detail below. 



Capture and Loss of Electrons by Positive Ions 



Another mode of energy loss, possible only for a positively charged 

 particle, is the capture of an electron from a molecule of the medium 

 into a discrete orbit about the particle and its subsequent loss in a later 

 collision. This pair of events occurs a few thousand times, for example, 

 in the absorption of a single alpha particle. _ It is important, for light 

 ions, only when they are slow — and thus for an energetic particle only 

 near the end of its range. Indeed, it is a principal mode of energy loss 

 for an alpha particle of energy between (roughly) 1 and 500 kev. The 

 net effect of such a pair of events is simply the ionization of a single 

 molecule of the medium, the positive ion and electron being formed, how- 

 ever, at a distance from each other. This should be an unimportant dis- 



