470 BELL SYSTEM TECHNICAL JOURNAL 



tential energy. The ionizing-potential of the atom is then, by defini- 

 tion, equal to the potential energ}^ of that state of ionization which 

 differs least in energy from the normal state. 



Another way of describing the ionizing-potential is to say that it is 

 the energy required to detach the loosest electron from the atom. This in- 

 volves a picture of an atom as a system of separately-identifiable elec- 

 trons, "bound" with various degrees of looseness or tightness. Such a 

 picture is so nearly indispensable, that there need be little hesitation 

 about introducing it here. Frequently the term valence-electron is used 

 instead of "loosest electron"; there is little in its favour beyond the 

 general inability of physicists to think of a better one.^ 



Detachment of the Loosest Electron by Other Agencies than 



Electron-Impacts 



Other agencies than the blows of electrons are capable of detaching 

 the loosest electron from an atom; but it is very much more difficult 

 to obtain simple and intelligible information about their immediate 

 effects than about those of electron-impacts. 



The study of ionization by radiation involves a host of new problems. 

 Theoretically the conditions seem simple enough. Radiation of any 

 frequency v behaves in some respects as though it consisted of streams 

 of particles each having energy hv and momentum hv/c. Since it be- 

 haves in this manner in so far as absorption in gases and ejection of 

 electrons from solids are concerned, we should expect it to do likewise 

 in effecting ionization of atoms. If so, radiation should ionize atoms 

 if and only if its frequency v equals or exceeds a critical or threshold 

 value vo, expressed in terms of the ionizing-potentials Vo of the atoms 

 (measured in equivalent volts) by 



hpo = eVo/300. (6) 



Projecting light from a spectrum upon a gas, and passing steadily from 

 low to high values of v, we should expect ionization to commence 

 abruptly at vq. 



Experimentally, the task of testing this inference has baftled every- 

 one, at least until very recently. In the first place, the values of thresh- 

 old-frequency Vo for various atoms correspond to values of threshold- 



^ The terms "optical electrons" and "series electrons" are sometimes seen; they 

 are derived from theoretical pictures which are in danger of mutation (some people 

 now ascribe most series-spectra to displacements of electrons in groups). The Ger- 

 man term "Leuchtelektron" probably sounds better in German than its equivalent 

 "shining electron" would sound in English. It may be remembered that difficulty in 

 choosing a good name for a concept sometimes signifies that the concept is essentially 

 vague and not rooted in Nature. 



