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LXVI. Ionization and Production of Radiation by Electron 



Impacts in Helium investigated by a New Method. By 

 K. T. CoMPTON, Ph.D., Professor of Physics in Princeton 

 University*. 



Introduction. 



THE recent studies of the ionization oE metallic vapours 

 and the excitation of their spectra by electron impacts^ 

 have established relations between the absorption spectra of 

 the unexcited vapours and their resonance and ionization 

 potentials which are in accord with a theory of radiation 

 and atomic structure such as that proposed by Bohr. Briefly 

 stated, the results are as follows : — The atoms gain no internal 

 energy as a result of electron impacts unless the kinetic 

 energy of the electrons exceeds the amount hv n where v r is 

 the frequency of the first (longest wave-length) member of 

 the absorption series of the vapour. If the energy exceeds 

 hv r the atom may absorb this amount of energy, which it 

 subsequently re-emits as radiant energy of this frequency. 

 If the energy of the impinging electron exceeds JiVi, where 

 vi is the convergence frequency of the absorption series of 

 which v r is the frequency of the first line, the atom may be 

 ionized. The potential differences Y r and V» through which 

 an electron must fall to acquire the energies hv r and 

 hv% respectively, are termed the Resonance and Ionization 

 Potentials, and are given by the quantum relation eV = hv. 

 There is evidence J that there may be two resonance po- 

 tentials, corresponding to two different absorption series 

 which have the same convergence frequency, but no one has 

 yet proved the existence of additional resonance potentials 

 corresponding to intermediate members of a given absorption 

 series. 



These phenomena, together with phenomena of dispersion 

 of excited and unexcited gases, seem to prove the existence 

 within the atom of series of related states or orbits in which 

 electrons may exist in more or less stable equilibrium. 

 Energy derived from absorbed radiation or from impact by 



* Communicated by the Author. 



t McLennan, Proc. Phys. Soc. Lond. xxxi. p. 1 (1918) ; Tate and 

 Foote, Phil. Mag. xxxvi. p. 64 (1918); Foote, Roo-nley, and Mohler. 

 Phys. Rev. xiii. p. 59 (1919) ; Foote and Mohler (in press); etc. 



% Davies and Goucher, Phys. Rev. x. p. 101 (1917) ; McLennan, 

 loc. cit. 



Phil. Mag. S. 6. Vol. 40. No. 239. Xov. 1920. 2 



