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 (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 5p⁵²P of Cs III. Five of the eight resonance lines found in the ultraviolet region fitted into Sommer's scheme. One new level with $J=0$ has been found in this investigation, thus completing the group of the six 5p⁵6p levels built upon the ²P11/2 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 5p⁶¹S₀ ground level to new 5p⁵(s, d) levels built upon the ²P1/2 limit. It was possible to separate the ³P₂ and ³P₁ levels due to 5p⁵(²P11/2) ($6s$ and $7s$) from the 5p⁵(²P11/2) ($5d$ and $6d$) levels. The ionization potential of Cs II was computed to be approximately 23.4 volts.