The First Spark Spectrum of Caesium (Cs II)

The resonance lines of Cs II have been photographed in the ultraviolet region with the vacuum spectrograph. The mode of excitation was the hollow cathode discharge in helium. The values of the wave-lengths of the ultraviolet lines for Cs II were obtained from second order measurements. With the separations of the resonance lines as guides, it was possible to assign electron configurations to the energy levels given by Sommer and to extend his scheme somewhat. The coupling is that of approximate ($\mathrm{jj}$) type. The characteristic feature of the Cs II spectrum was the decomposition of the levels arising from the various configurations into two distinct groups due to the large separation of the $5p^5{}_{}{}^2{}_{}{}^{}P$ of Cs III. Five of the eight resonance lines found in the ultraviolet region fitted into Sommer's scheme. One new level with $J=0\mathrm{}$ has been found in this investigation, thus completing the group of the six $5p^{5}6p$ levels built upon the ${}_{}{}^2{}_{}{}^{}P_{\frac{11}{2}}^{}{}_{}{}^{}$ limit. The classification, assignment of inner quantum numbers, and the identification of levels agreed with the theoretical considerations as well as with the predictions from the Xe I classification. The remaining three resonance lines appeared to be due to transitions from the $5p^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ ground level to new $5p^5(s, d)$ levels built upon the ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ limit. It was possible to separate the ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ levels due to $5p^5\left({}_{}{}^2{}_{}{}^{}P_{\frac{11}{2}}^{}{}_{}{}^{}\right)$ ($6s$ and $7s$) from the $5p^5\left({}_{}{}^2{}_{}{}^{}P_{\frac{11}{2}}^{}{}_{}{}^{}\right)$ ($5d$ and $6d$) levels. The ionization potential of Cs II was computed to be approximately 23.4 volts.