An Attempt to Observe the Spectrum of Doubly Excited Helium

Abstract

In this paper is described an electron-impact discharge tube which was designed to give excitation conditions favorable to the production of doubly excited helium. The tube furnished a concentrated 1000 ma beam of 300 to 500 volt electrons which passed through purified helium gas at a pressure of approximately 0.01 mm Hg. The comparatively feeble glow produced was photographed with a high speed two-prism glass spectrograph. Long exposures yielded only very much over-exposed ${\mathrm{He}}_{\mathrm{I}}$ lines and rather strong ${\mathrm{He}}_{\mathrm{II}}$ lines. This result is consistent with certain approximate theoretical calculations of Kreisler which show that the lifetimes of the doubly excited states of helium are limited by autoionization to approximately ${10}^{-14\mathrm{}}$ sec., so that any spectral "lines" would be far too broad to be observable. This result tends to weaken the argument that the far ultraviolet helium lines, $\lambda \lambda 320.4 \mathrm{and} 357.5$ A, found by Kruger, and by Compton and Boyce, are due to transitions from doubly excited states to singly excited ones; and that the 60-volt energy losses of electrons passing through helium, observed by Whiddington, are due to a transition from the ground state to a doubly excited state.