The Fifty-Three Electron Spectra of Caesium and Barium: Cs III and Ba IV

The separations of the lowest levels $5s^{2}5p^5{}_{}{}^2{}_{}{}^{}P$ of the Cs III and Ba IV spectra have been predicted and observed. In the case of Cs III, the prediction was made from the $s^2{p}^{5}p$ levels of Cs II by use of Shortley's equations. The separation as found is 13,870 ${\mathrm{cm}}^{-1\mathrm{}}$ and several combinations of the terms with higher levels were located. The ${}_{}{}^2{}_{}{}^{}P$ separation for Ba IV was estimated by the irregular doublet law and located from the data as 17,830 ${\mathrm{cm}}^{-1\mathrm{}}$. New data were obtained for caesium by use of the vacuum spark, for barium by the vacuum spark and the condensed discharge in helium. The more complete data for barium enabled the location of a group of terms which combine both with the $\left(s^2{p}^5\right){}_{}{}^2{}_{}{}^{}P$ terms and also with a higher group of terms allowing the classification of about fifty lines in this spectrum.