Ionization energies of doubly and triply ionized rare earths

Values for the ionization energies of the doubly and triply ionized rare earth atoms have been derived from interpolated spectroscopic properties of the 4f^Nns series, and from interpolated energy intervals relating the first series member, 4f^N6s, to the ground state. The results in eV are $$\begin{array}{cccccc}\mathrm{La\; III}& \text{19.1774(6)}& & \mathrm{Tb\; III}& \text{21.91(10)}& \text{IV 39.79(20)}\\ \mathrm{Ce}& \text{20.198(3)}& \text{IV 36.758(5)}& \mathrm{Dy}& \text{22.79(30)}& \text{41.47(20)}\\ \mathrm{Pr}& \text{21.624(3)}& \text{38.98(2)}& \mathrm{Ho}& \text{22.84(10)}& \text{42.48(32)}\\ \mathrm{Nd}& \text{22.14(30)}& \text{40.41(20)}& \mathrm{Er}& \text{22.74(10)}& \text{42.65(21)}\\ \mathrm{Pm}& \text{22.32(36)}& \text{41.09(32)}& \mathrm{Tm}& \text{23.68(10)}& \text{42.69(20)}\\ \mathrm{Sm}& \text{23.43(30)}& \text{41.37(38)}& \mathrm{Yb}& \text{25.03(2)}& \text{43.74(20)}\\ \mathrm{Eu}& \text{24.70(32)}& \text{42.65(32)}& \mathrm{Lu}& \text{20.9596(10)}& \text{45.19(2)}\\ \mathrm{Gd}& \text{20.63(10)}& \text{44.01(35)}& \mathrm{Hf}& & \text{33.33(2)}\end{array}$$ The values for the doubly ionized atoms agree to within ∼ 1% with those deduced from thermodynamic measurements on lanthanide oxides. A value for the ionization energy of Gd II of 12.09(8) eV was determined by using new spectroscopic data for Gd II and Gd III.