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IV. Resonance and Ionization Potentials for Electrons in 

 Metallic Vapours. By John T. Tate, Ph.D., and Paul 

 D. Foote, Ph.D.* 



IT has been shown by Franck and Hertz and others that 

 when electrons are accelerated through gases or vapours 

 there are, especially for gases having small electron affinity 

 such as the inert gases and metallic vapours, well-defined 

 potentials at which a large transfer of energy takes place 

 between electrons and gas atoms. The evidence for this 

 transfer of energy is in the emission at these definite 

 potentials of radiations having frequencies characteristic of 

 the gas. 



This absorption by the atom of the kinetic energy of the 

 electrons only at definite velocities of the electrons is to be 

 expected from a purely mechanical standpoint. A consi- 

 derable transfer of energy from the electron to the relatively 

 heavy atom can be explained only by assuming that there 

 are in the gas atom certain vibrational degrees of freedom of 

 which the characteristic period bears some simple relation 

 to the time of encounter between the electron and atom. 

 Evidence of the existence of these characteristic vibrations is 

 seen in the absorption spectra of gases, and it is therefore 

 to be expected that, when the potential accelerating the 

 electrons reaches a value such that the time of encounter 

 between electrons and atoms is simply related to the natural 

 period of vibration of an absorption line of the gas in 

 question, a loss in the kinetic energy of the electrons and 

 an emission of a radiation of the frequency of the absorption 

 line will be observed. The essential similarity between the 

 phenomena of the absorption by a gas of radiant energy and 

 of the absorption of the kinetic energy of motion of electrons 

 should be emphasized. 



The experimental results thus far obtained indicate that 

 there are in general two types of inelastic encounter between 

 electrons and atoms — first, those encounters which are accom- 

 panied by the emission of a radiation of a single frequency 

 without ionization of the gas, and second, those encounters 

 which ionize the gas and excite the radiation of a composite 

 spectrum of frequencies. The potential giving the first type 

 of encounter may be termed a resonance potential, that 

 giving the second type of encounter, an ionization potential. 



* Communicated by the Director, United States Bureau of Standards. 



