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 $4f5d$, $4f6p$, (5d)², $4f6s$, $4f6d$, and $5d6s$, have been almost completely identified. The last three show evidence of JJ coupling. Zeeman data by de Bruin and King confirm the classification. The lowest known level is 4f5d¹G₄ and the ionization potential from this to 4f²F21/2 of Ce IV is 19.5 volts. The (4f)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.