﻿810 Mr. A. J. Saxton on Impact Ionization by 



(d) More than one o£ the above sources may be concerned 

 in the ionization of an atom. Thus an atom may be partially 

 ionized by absorption of radiation, and then the process may 

 be completed by electron impact*. 



2. Ionization by Positive Ion Impacts. 



The method of studying the ionization produced by one of 

 these four methods alone must be by some form of discharge 

 through the gas at low pressures. The conditions of the 

 experiment must approximate to the following ideal condi- 

 tions. Only the source of ionization, e. g. positive ion impact 

 which is being studied, must contribute to the ionization of 

 the gas, other sources being eliminated as far as possible by 

 suitably designing the apparatus. We must also be able to 

 distinguish between ionization produced by a single process 

 and that produced by the cumulative effect of several colli- 

 sions. 



Assuming the foregoing principles concerning the energy 

 exchanges in ionization, and given the ideal conditions stated 

 above, we may expect the following result : — That for a posi- 

 tive ion accelerated through an electric field and striking 

 an atom of the gas, in order just to ionize the atom its 

 kinetic energy must be equal to the ionization energy of the 

 atom. Thus if it has fallen unimpeded through an acceler- 

 ating field of V, a singly- charged ion will have a kinetic 

 energy of 



Ye = ^mv 2 . 



This accelerating P.D. of V reduced to volts is the ioniza- 

 tion potential for positive ions. In a p irticular gas it should 

 have the same value as the ionization potential for electron 

 impact. 



This ionization potential for positive ions should be inde- 

 pendent of the nature of the ions, the effect depending only 

 on their kinetic energy. These principles will apply only to 

 the case where an atom is ionized by the impact of a single 

 positive ion (possessing one positive charge) which has been 

 accelerated unimpeded by a P.D. of Y volts. 



The case of positive ion impact may differ, however, from 

 the case of electron impact in the following manner : — 

 Though the kinetic energy of the accelerated positive ion is 

 the same as that of an electron accelerated through a P.D. 

 of the same value, the energy exchanges with the " struck " 



* H. D. Smyth and K. T. Compton, Phys. Rev. xvi. p. 501 (1920). 

 (Iodine vapour.) 



