[ 33 ] 



III. Ionization and Resonance Potentials for Electrons in Va- 

 pours of Magnesium and Thallium. By Paul D. Foote, 

 Ph.D., and Fred. L. Mohleb, Ph.D.* 



TWO types of inelastic collision between electrons and 

 atoms o£ metallic vapours are known to exist. One 

 type of collision results in the expulsion of an electron from 

 the atom, and the other type of collision produces an 

 agitation, or transfer from one orbit to another, of one or more 

 electrons bound in the atom. Collision resulting in ioni- 

 zation is accompanied by the emission of the complicated 

 series spectra of the metals, presumably due to recom- 

 bination following ionization. The second type of collision 

 is generally supposed to result in the emission of a single 

 line spectrum f. 



The potential difference through which an electron must 

 fall in order to acquire sufficient energy to produce ionization 

 upon collision with an atom is known as the ionization po- 

 tential, and the corresponding potential difference for the 

 second type of collision is known as the resonance potential 

 for the particular vapour in question. Dr. Tate and one of 

 the writers^ have determined the ionization and resonance 



* Communicated by Dr. S. W. Stratton, Director Bureau of Standards. 



f Dr. F. A. Saunders suggests there is reason for believing that the 

 term single line spectrum is a misnomer. It is possible that the entire 

 series of which the so-called single line is the first member appears 

 simultaneously with this line. In most cases where single line spectra 

 have been studied, experimental difficulties are such that other members 

 of the series could not be detected. This suggestion raises a very im- 

 portant question which might be decided experimentally by a spectro- 

 scopic study of low voltage discharge through sodium or potassium 

 vapour. It may be pointed out, however, that conclusive evidence in 

 this regard will be difficult to obtain ; for if the other lines of the series 

 should be found to be present, they would appear in such low intensity 

 that their emission might be accounted for by the presence of a few 

 electrons of velocity greater than that corresponding to the ionizing 

 potential. These high velocity electrons are always present on account 

 of the Maxwell distribution of velocities of the electrons emitted by the 

 hot cathode. A second difficulty, even if a univelocity stream of electrons 

 were employed, would be to assure that the collision of the electron was 

 with the normal atom, and not with an atom having an electron already 

 displaced from its normal orbit by a previous collision. This latter 

 question has been considered by Van der Bijl, Phys. R. x. p. 546 

 (1917). 



1 Tate and Foote, J. Wash. Acad. Sci. vii. p. 519 (1917) ; Bur. Stds. 

 Sci. Paper No. 317 ; Phil. Mag. xxxvi. p. 64 (July 1918). 



Phil. Mag. S. 6. Vol. 37. No. 217. Jan. 1919. D 



