Abstract
The surface ionization comparison technique was used to measure the first ionization potentials of the lanthanide elements. Values obtained were La, 5.55 ± 0.05 eV; Ce, 5.54 ± 0.06; Pr, 5.40 ± 0.05; Nd, 5.49 ± 0.05; Sm, 5.61 ± 0.05; Eu, 5.64 ± 0.05; Gd, 6.16 ± 0.05; Tb, 5.89 ± 0.04; Dy, 5.82 ± 0.03; Ho, 5.89 ± 0.03; Er, 5.95 ± 0.03; Tm, 6.03 ± 0.04; Yb, 6.04 ± 0.04; and Lu, 5.32 ± 0.05. The IP of Pm is estimated to be 5.55 eV. The reference element used was silver. A tabulation of all known reported values is included, and comparisons are made. Agreement is generally good.

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