PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 239 



Such dissociations of ions are quite evidently very important processes 

 for the mechanisms of radiation-chemical and radiobiological reactions. 

 One of the most powerful tools for their study is the mass spectrometer, 

 and a great deal of experimental information is available from this source 

 (Massey and Burhop, 1952), both for simple and for complex molecules. 

 It is commonly possible, in these experiments, to determine both the 

 nature of the various ionic species and their yields for various values of 

 the kinetic energy of the electrons which are used to produce the initial 

 ionization. From such data there can also be determined the appearance 

 'potential of a particular ion, which is the minimum electron energy at 

 which the ion can be detected. A given molecule has a number of differ- 

 ent appearance potentials, each corresponding to production of a different 

 excited state of the resultant ion — and each such state has a characteristic 

 fate, for instance, a particular mode of dissociation. The appearance 

 potential corresponding to the lowest electronic state of the complete 

 molecular ion is often equal to the ionization potential of the molecule; 

 indeed, measurement of this appearance potential is usually a con- 

 venient, and often the only practicable way to determine the ionization 

 potential. 



4-3 c. Recombination. The charge on a primary positive ion or on an 

 ion which is one of its dissociation products must ultimately be neutral- 

 ized. This may take place by recombination with an electron, or with a 

 negative ion. 



Capture of a slow electron by a positive atomic ion to form the ground 

 state of the atom (and therefore with radiation of a photon of energy equal 

 to the ionization potential) is a process of such low intrinsic probability 

 that it may be entirely neglected in ordinary chemistry (it may play a 

 role in such special circumstances as electric discharges or under certain 

 extraterrestrial conditions). Capture of a swiftly moving electron is 

 even less probable. Even capture to the ground state in a three-body 

 colhsion is relatively unlikely, unless the third bod}^ is an electron. The 

 most probable process in most conditions encountered in gaseous sys- 

 tems is capture to a very highly excited state. ^^ Unless the pressure is 

 extremely low, this type of capture can indeed occur, by "long-range" 

 interaction with other entities, in such a way that the very small energy 



" To recombine with a positive ion, a free electron has to lose energy by radiation; 

 it does so because it is accelerated in the positive field of the ion. Small energy losses 

 connected with relatively small accelerations can be sufficient for a transition of the 

 free electron into the closed but large orbits of the highly excited states. Great 

 accelerations and great radiation losses are needed for a recombination into the small 

 orbits of the ground state. They occur only by nearly central encounters between 

 electron and ion, and they are, of course, rare. For accurate treatment of the recom- 

 bination process quantum mechanical calculations are needed. They reveal that 

 another factor favoring recombination into highly excited states is the high statistical 

 weight of certain of those states. 



