The Third Spectrum of Cerium (Ce III)

Thirty-three triplet and singlet terms of Ce III have been recognized, and account for 294 lines, including almost all of wave-length exceeding 2000A. The electron configurations $4f5d$, $4f6p$, ${\mathrm{}\left(5d\right)\mathrm{}}^2$, $4f6s$, $4f6d$, and $5d6s$, have been almost completely identified. The last three show evidence of $\mathrm{JJ}$ coupling. Zeeman data by de Bruin and King confirm the classification. The lowest known level is $4f5d{}_{}{}^1{}_{}{}^{}G_4^{}{}_{}{}^{}$ and the ionization potential from this to $4f{}_{}{}^2{}_{}{}^{}F_{2\frac{1}{2}}^{}{}_{}{}^{}$ of Ce IV is 19.5 volts. The ${\mathrm{}\left(4f\right)\mathrm{}}^2$ configuration may be lower, but has not been discovered, as its combinations lie in the infrared and the far ultraviolet. The ultimate lines must be in the infrared, as is probably the case for the third spectrum of most of the rare earths.