240 RADIATION BIOLOGY 



evolved is not emitted as radiation, but rather transferred to these 

 entities. Thus electron recombination usually proceeds through a stage 

 corresponding to high electronic excitation, and the discussion of con- 

 sequences of such excitation in Sect. 3 is therefore applicable. 



Electron capture by a positive molecular ion, with dissociation of the 

 product molecule in the same step, is another possible process. An 

 example, which is under consideration for its possible contribution to 

 recombination in the upper atmosphere, is the recombination of 0^ and 

 an electron to form two oxygen atoms in (different) excited states. 



If there are present in the system atoms or molecules having appreciable 

 electron affinity (which comparatively few do; notable cases are the hal- 

 ogen atoms, O and O2), electrons may be captured to form negative ions. 

 (This process occurs only in three-body collisions; radiative capture is an 

 order of magnitude less probable than it is for capture by positive ions; 

 and capture to a highly excited state is not possible, at least in the case of 

 atomic ions, because negative atomic ions have no excited states of appre- 

 ciable binding energy.) An important elementary process in such sys- 

 tems is the recombination of a negative and a positive ion. There are 

 three important possible products of such a reaction: (1) the positive and 

 negative ion may simply neutralize one another (charge exchange) ; (2) 

 they may form two neutral entities (atoms, molecules, radicals) differing 

 in atomic composition from the two original ions; and (3) they may com- 

 bine, in a three-body collision, in an association reaction. It is important 

 to observe that, because ionization potentials are in general much greater 

 than electron affinities, such recombinations are usually strongly exo- 

 thermal. In two-body collisions, highly excited products are at all events 

 very likely. The three-body recombination of negative and positive 

 ions is the predominant process in the recombination in ionized air (or 

 atmosphere) at pressures greater than about 1 mm of mercury. For fur- 

 ther details on the recombination of gaseous ions the review by Massey 

 (1952) may be consulted. 



4-3d. Electron Attachment to Neutral Molecules. If a molecule contains 

 an atom which can form a negative ion (i.e., which has appreciable elec- 

 tron affinity), it can capture an electron in the important elementary 

 process 



AB -\- e ^ A -\- B- 



Such processes have been demonstrated with particular clarity for the 

 case that B is atomic: examples are capture by CU, Br2, I2, and O2. If the 

 electron affinity of B is smaller than the dissociation energy of AB, the 

 energy deficit must be supplied in the form of kinetic energy of the 

 electron; the process is somewhat less probable but still possible, and 

 there will be a definite appearance potential for the ion B~ to be so 

 formed (if accurately measured, this appearance potential can be a basis 



