S2 Dr. P. D. Foote and Dr. W. F. Meggers on Atomic 



an electron from the l m os ring to infinity, in excellent agree- 

 ment with the experimentally determined value of 3*9 volt?. 

 Thus the ionization of the normal caesium atom is produced 

 by ejecting an electron from the ] '5s ring. If the outermost 

 stable orbit were the 2p rings the ionization potential would 

 be 2*4 volts, unless Ave make the improbable (see later) 

 assumption that electrons colliding with an atom could not 

 displace an electron from the 2p ring. 



Thus it is safe to conclude that any displacement of an 

 electron from its orbit in the stable condition of the atom 

 takes place from the l'5s ring. The question arises as to 

 what type of displacements may occur in electronic-atomic 

 impact. Inelastic conditions t;ike place when the impacting 

 electron has the energy e V where J" is the ionization poten- 

 tial, this kinetic energy being just sufficient to account for 

 the increase in total energy of the ionized atom. Another 

 type of inelastic impact has been observed in which the 

 colliding electron has the energy eV where V is the re- 

 sonance potential*, this kinetic energy being just sufficient 

 to account for the increase in total energy of the atom arising 

 from a displacement of an electron from the l"5s ring to the 

 2p ring. The displaced electron being then in an unstable 

 orbit falls to the 1*55 ring, giving up the quantum of energy 

 eV received from the collision as a quantum of energy liv 

 of radiation of frequency v, as is shown by the experiments 

 described later. Thus inelastic impacts are known when the 

 colliding electron has sufficient energy to eject an electron 

 from the 1*5* ring to infinity or to the 2]> rings. In the 

 former case the electron returning to the atom causes the 

 emission of various lines, and with many eiectrons in different 

 ionized atoms returning by various paths we obtain the com- 

 plicated line-spectra in which the intensities of the lines 

 represent to some degree the probabilities of each particular 

 type of transition from orbit to orbit. 



Suppose the colliding electron possessed just sufficient 

 energy to eject an electron to some ring intermediate to 

 go and 2p. Could the normal atom absorb the entire kinetic 

 energy of the impacting electron ? The only known series 

 converging at 1*55 is the principal series l'Ss—mjn. Hence 

 it is extremely probable that, if a displaced electron falls into 

 the 1*55 orbit, it comes from one of the p rings. Accordingly 

 it is reasonable to assume that in the original displacement 

 from the 1*55 ring the electron is ejected to some p ring. 

 Thus inelastic impacts might occur when the colliding elec- 

 tron had fallen through any of the following potential 

 * Foote ; Rognley, and Mohler, he. cit. 



