On the Excitation of the Helium Spectrum by Electronic Bombardment

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Abstract
Variation in relative intensity of the ortho- and parhelium spectra with the energy of the exciting electrons, 25 to 100 volts.—In the tubes used, electrons were accelerated from a Wehnelt cathode through a platinum grid 1 mm away into a metal box, and the spectrum of the light produced inside the box was photographed. A pressure of about 1mm was used. At the higher voltages, the two spectra were of about equal intensity, but at low voltages the orthohelium spectrum was relatively much the stronger (see Plate I). This is explained by supposing that the orthohelium spectrum is to a large extent due to bombardment of metastable atoms by electrons with energies corresponding to less than 5 volts. A special experiment, however, in which the helium was bombarded by both 5 volt and 21 volt electrons, gave a negative result, no additional light being excited by the slower electrons; but this is not considered as conclusive.

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