Some Bands in the Extreme Ultraviolet Spectrum of Helium

The band at $\lambda 600$ in the extreme ultraviolet spectrum of helium, first observed by Lyman and first attributed to the helium molecule ${\mathrm{He}}_2$ by Sommer, has been investigated under various discharge conditions. Low voltage arc, Schüler hollow cathode and uncondensed capillary discharges have been used as sources with a one-meter vacuum spectrograph. The helium used was circulated from the spectrograph back to the discharge tube through the purifying system of a misch metal arc and a chabazite trap in liquid air. The band at $\lambda 600$ appeared under those conditions of purity which favored the appearance of the visible bands of helium and under such conditions that there can be little doubt that it is due to helium. The intensity of this $\lambda 600$ band varied directly with the first power of the current in the discharge, whereas the visible bands are known to vary as the square of the current. This suggests a difference of mechanism in the two cases, and in the light of this, various explanations of the origin of the $\lambda 600$ band are considered but none found satisfactory. A possible explanation of the diffuse structures appearing at $\lambda 647$ and $\lambda 662$ is advanced.