PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 235 



excitation processes. The ionization potentials of atoms vary from 

 3.9 ev for Cs to 24.6 ev for He. 



Ionization of a molecule results from absorption of a photon or from 

 an impact with a charged particle if the energy transferred is sufficiently 

 great. If this minimum necessary energy be again defined as the ioniza- 

 tion potential, note should be taken that it will usually be greater than the 

 difference in energy between the ground electronic states of molecule and 

 molecular ion. The excess energy is just that which must be transferred 

 (either in light absorption or in impact) to potential energy of the atomic 

 constituents of the molecule and therefore to oscillational energy of the 

 final state, because the ionization occurs so quickly that the interatomic 

 distance(s) remains the same (Franck-Condon principle; cf. Fig. 3-4 and 

 the text pertaining thereto, which explains the entirely analogous case of 

 electronic excitation). This oscillational energy may indeed be so great 

 that dissociation of the molecule will ensue (Sect. 4-3b). The ionization 

 potentials of most stable molecules lie between about 10 and 16 ev. 



Ionization of an atom or molecule may also result from a collision of 

 the second kind between an excited atom or molecule and a neutral one 

 (type V; cf. p. 207). Such reactions are often very probable, because 

 exact resonance is fulfilled if the process is energetically possible (cf. 

 Sect. 3-lc) ; they play a role, for example, in certain electrical discharges, 

 and possibly also in some radiation-chemical reactions in gases, especially 

 those in which metastable atoms or molecules are formed. 



An atom or molecule may, under special circumstances, absorb a quan- 

 tity of energy in excess of the minimum ionization potential without being 

 immediately ionized, for the absorbed energy may excite a tightly bound 

 electron instead of ionizing a more weakly bound one. If this occurs, and 

 the system does not lose energy by radiation or in impact, ionization 

 (with ejection of an electron) must inevitably follow; such a process is 

 called preionization. For an atom, preionization occurs very quickly, 

 ordinarily in roughly 10~^^ second after excitation. For molecules, par- 

 ticularly polyatomic molecules, it may take a longer time — and, indeed, 

 may not occur at all if internal conversion transforms the excitation 

 energy to oscillational energy and the latter is removed in much later 

 impacts. Preionization in a very large polyatomic molecule often 

 requires a comparatively long time; in such cases it may be possible to 

 consider the excitation energy as migrating through the molecule until 

 it is "localized" at a weakly bound electron. 



Photochemical reactions follomng optical ionization have received 

 virtually no attention. This is because most stable atoms and molecules 

 have ionization potentials corresponding to wave lengths in the far ultra- 

 violet region (below 1000 A), so that experimental work would be 

 extremely difficult. In the vast majority of photochemical reactions 

 that have been studied, ionized species do not occur. This situation is 



