Contemporary Advances in Physics, XXI 



Interception and Scattering of Electrons and Ions 



By KARL K. DARROW 



This article deals with a couple of aspects of one of the amplest questions 

 of modern experimental physics: the question of what happens when an elec- 

 tron (more generally, an electron or a proton or a charged atom of any kind) 

 collides with an atom or a molecule. It is well known, of course, that if the 

 electron has energy enough, it may excite or ionize the atom. There are 

 many different modes of excitation, and often several of ionization; the 

 variety of possibilities is wide. If any of them occurs at an encounter, the 

 electron loses energy and speed, and may suffer a change in the direction 

 of its motion — a "scattering," as this is called. Even if it loses no measur- 

 able amount of energy at a collision, it may be "scattered," that is to say, 

 deflected. The scattering and the energy-losses of the electrons are 

 studied both on their own account, and because of the light they shed on 

 what is happening to the atoms. ^ 



IMAGINE a stream of electrons projected, all with known and uni- 

 form velocity and along the same direction, into a rarefied gas. 

 Perhaps it ionizes the gas; if so, positive ions appear, and one may de- 

 tect them and identify them and count them in any of various ways, 

 without concerning oneself about the destiny of the ionizing electrons. 

 Or perhaps it excites the gas without producing ions; if so, the atoms 

 (or molecules) send forth light, and one may detect the excitation and 

 identify the manner and measure the likelihood thereof, without paying 

 any attention to the corpuscles responsible. Nevertheless, these 

 corpuscles also must have been affected; they must have given up 

 some at least of their kinetic energy, and if they still retain some 

 motion, it is probably no longer in the same direction as at first. 



If there is ionization or excitation of the gas, there should be electrons 

 wandering off obliquely from the stream, and moving more slowly than 

 when they entered the gas; in technical language, there should occur 

 "scattering with loss of energy." Even if the incoming corpuscles are 

 moving too slowly to ionize or excite, there might be — and there are — 

 electrons wandering off obliquely with practically undiminished speed; 

 they have suffered "elastic impacts" with atoms or molecules, they 

 have been deflected merel}-, or "scattered without loss of energy." 

 And even if the incoming corpuscles are moving fast enough to ionize 

 or excite, some may be scattered with undiminished speed while others 

 are spending some of their energy in these operations. Also, some of 



1 This article, in somewhat altered form, is to appear as a chapter in a forthcoming 

 book on ionization and conduction in gases. 



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