The minimum electron energies associated with the excitation of the spectra of helium

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In addition to the orthohelium and parhelium serial systems, the spectrum of helium includes the lines of the enhanced system and the many-lined, or band, spectrum. The lines of the orthohelium and parhelium series are attributed to transitions of electrons within uncharged helium atoms, while the enhanced lines are attributed to the transitions of the remaining electrons in already ionised helium atoms. It has been suggested that the band spectrum of helium arises from disturbances of helium molecules (He 2 ), for the formation of which electrical excitation is postulated as a preliminary condition. On the view that each of the lines of the orthohelium and parhelium series corresponds to a definite transition of one of the electrons in the normal helium atom, it might be expected that it would be possible to excite certain of the series lines without exciting others by suitably adjusting the dis­turbing force, so as to make the occurrence of certain transitions impossible. The investigations of Rau, of Horton and Bailey, and of Compton and Lilly, have led to the conclusion that it is not possible to excite certain of the orthohelium and parhelium series lines without exciting all of them. This conclusion is in complete agreement with the work of McLennan and Ireton, and of Foote and Meggers, on the spectra of metallic vapours. These investigators found that the only lines which could be excited without all the lines appearing were the first lines of the series whose limiting frequencies corresponded to the respective ionisation potential differences of the metallic vapours concerned, a result which was supported by the current-potential difference curves of Tate and Foote, and of Mohler, Foote and Meggers, etc. These observers found that the only types of inelastic impacts, which occurred between atoms of the metallic vapours and electrons, were those corresponding to the ionisation of the atom, and to the voltages which were connected by the quantum relation with the frequencies of the first lines of the series whose limits were connected with the ionisation potential differences. The lines of the helium spectrum which, by analogy with the cases of the metallic vapours, might be expected to appear before the other helium lines, are in the extreme ultra-violet region of the spectrum, and could only be detected with a vacuum grating spectroscope: this was not used by either of the investigators to whose work in helium reference has been made.