PHOTONS AND ELECTRONS 19 



spectrum, absorption of light by an atom entails detachment of an elec- 

 tron from the atom, a process known as "ionization." The convergence 

 frequency v^.^^ of a series in an absorption spectrum is the least frequency 

 for which ionization takes place,'' and consequently the energy hv^^ 

 of the corresponding photons is the energy which just suffices to detach 

 an electron from the atom. This is known as the "ionizing energy" or 

 "ionization potential" of the atom. When the atom has thus been 

 ionized by light of frequency Vn^, we may consider the ionized atom and 

 its divorced electron as a system in a state, the "state of the ionized 

 atom," having an energy equal to /iv,;^ when reckoned from our regular 

 zero. 



THEORY OF ABSORPTION LINES 



Consider an absorption spectrum consisting of a line series having 

 limit frequency Vy^^, and of the adjoining continuum. Photons of 

 frequencies greater than Vy^^ are absorbed by atoms and expel electrons. 

 Photons of certain discrete frequencies Vi smaller than Vy^,^ are absorbed 

 by atoms but do not expel electrons. It is natural to infer that while 

 a photon of an absorption line does not have energy enough to detach 

 an electron completely from the atom, it does succeed in shifting an 

 electron partway outward from its equilibrium position in the atom. 

 The various stationary states of energy hvi (reckoned from that of the normal 

 state as zero) would then involve different positions of an electron in the atom. 

 The normal state would involve the position of permanent equilibrium; 

 the excited states, positions of merely temporary equilibrium; the transi- 

 tion of an atom from one state to another would involve the transfer of 

 an electron from one position to another. 



This also is one of the fundamental ideas of Bohr. In his earliest 

 papers Bohr (and subsequently many other theorists) went much further 

 in defining the pictures or models of these stationary states. They 

 considered that each stationary state involves a definite and characteristic 

 orbit of the electron in the atom. Take, for example, the simplest case 

 of all, that of hydrogen, the atom of which consists of a single electron 

 of charge —e and a nucleus of charge +e. Let it be assumed that the 

 electron may revolve around the nucleus only in one or another of certain 

 circular^ orbits, to wit, those circular orbits in which the angular momen- 

 tum of the atom is some integer multiple of h/2Tr — is equal to nh/2ir, 

 where n stands for any integer greater than zero. (This is not a purely 

 ad hoc assumption, but one suggested by general quantum theory!) 



' Except when ionization occurs as the result of two or more processes acting 

 simultaneously . 



* Certain elliptical orbits are also permissible, but it would complicate this brief 

 account too much to take due note of them. 



