Spectral and Impact Phenomena in the Faraday Dark Space

Observations on the spectra of the negative glow and Faraday dark space in pure helium and in helium containing a trace of nitrogen lead to the conclusion that in the negative glow the phenomena are due mainly to collisions of fast (primary) electrons with normal atoms while in the Faraday dark space impacts of the second kind between normal and metastable atoms are of primary importance. The metastable atoms present in the latter region are probably produced by the absorption of resonance radiation from the negative glow and are thus in the $2^{1}S$ state. The negative glow proper emits the He I and He II lines strongly and a band spectrum of ${\mathrm{He}}_2$. The band spectrum is, relatively, much stronger in the region of transition between the negative glow and the Faraday dark space and this last region emits only the He I line $\lambda 5016 (2^{1}S-3^{1}P)$ in sufficient intensity to be observed. When a trace of nitrogen is added to the helium, the N I spectrum replaces the ${\mathrm{He}}_2$ bands in the transition region. A consideration of the phenomena observed leads to the conclusion that ${\mathrm{He}}_2{2}^{1, 3}\Sigma$ molecules are formed by the union of a $1^{1}S$ atom and a $2^{1, 3}S$ atom in a three body collision and that the possibility of observing the visible ${\mathrm{He}}_2$ bands at all depends on the metastable nature of the $2^3\Sigma$ level of the ${\mathrm{He}}_2$ molecule. These metastable molecules are excited to higher levels by slow electron impacts in pure helium, whereas when nitrogen is present, they are destroyed by impacts of the second kind with nitrogen molecules before they can be excited. ${\mathrm{N}}_2$ molecules may be dissociated in these collisions, the dissociation occurring as a secondary consequence of the molecule's being raised to a higher electronic level from which dissociation occurs through predissociation or because the upper level is repulsive or simply because the molecule is excited to a degree above its dissociation asymptote. The effects from admixtures of oxygen and of carbon monoxide to helium and similar experiments on neon and argon support the conclusions drawn from the phenomena observed with helium and helium-nitrogen mixtures.