Nitrogen Molecular Spectra in the Vacuum Ultraviolet

Nitrogen molecular spectra in the region from 900A to 2300A have been photographed with a 10-foot vacuum spectrograph giving a dispersion of 5.5A/mm. Partial rotational quantum analyses have been made for several bands of the $a{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to {{}_{}{}^1{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$ system yielding the constants ${B_0}^{\prime }=1.632\mathrm{}\pm 0.002$, ${B_0}^{\prime \prime }=1.998\mathrm{}\pm 0.002$, ${\alpha }^{\prime }=0.021\mathrm{}$, ${\alpha }^{\prime \prime }=0.018\mathrm{}$. Many new bands in a nitrogen band system with an origin at 1847A discovered by Hopfield in a condensed discharge in a helium-nitrogen mixture are reported. This system is due to ${\mathrm{N}}_2^{+}$, having the same lower state as that of the first negative system. The rôle of the helium is discussed. Birge and Hopfield's $b^{\prime }$ progression is found to consist of $Q$ branches only with $B^{\prime }=1.147\mathrm{}$. Several new ${\mathrm{N}}_2$ ground state progressions among the bands in the 900A-1300A interval are presented.