Lanthanide Ionization Energies and the Sub-Shell Break. Part 1. The Second Ionization Energies

By interpolating a 4fq6s to 4fq7s transition within the sequence f1 to f14 rather than between f0 and f14, revised second ionization energies of the lanthanides have been obtained. In the early part of the series, between cerium and samarium, the revisions are significantly higher than currently accepted values. The revised values have been used to calculate the energy variation for the conversion of Ln(g, 4fq6s2) to Ln2+(g, 4fq) across the series Ba to Yb. The variation is smooth between lanthanum and ytterbium but the barium point at q=0 is displaced downwards by more than 0.3 eV, thereby representing a significant sub-shell break. A case is also made for a substantial change to the second ionization energy of lutetium, the revised value being 14.13 ± 0.10 eV