Abstract
The excitation of the 4 3S, 4 1S, 3 3P, 3 1P, 3 3D, 4 3D and 4 1D levels of helium by electron impact has been studied by observing the light emission following the excitation of the levels concerned. The electron-energy region considered extends from 55 to 62 eV. The measurements show that in this energy range a number of resonances occur in the optical excitation curves. In the 3 3D and 4 3D excitation a resonance is found which is very large compared to the resonances observed in the other channels. The observed resonances can be explained in terms of intermediate negative-ion states associated with doubly excited states of the neutral helium atom.
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