The Absorption Spectra of Salts of Ytterbium in Crystals and Solutions Compared with Those of Cerium

The absorption spectra of salts of ytterbium [the chloride YbCl3–6H2O, the acetate Yb(C2H3O2)3–4H2O and the ethylsulfate Yb(C2H5SO4)3–9H2O] were found to resemble those of cerium very closely, especially in their diffuseness even at low temperatures. This is in marked contrast with the sharp spectra of the other rare earths. The resemblance was anticipated on the supposition that the thirteen electrons in the 4f shell of Yb+++, one short of the maximum the shell can hold, would give rise to but one term 2F, the same term as the one electron of Ce+++ gives rise to. That is, activations aside from fine structure, are impossible when the thirteen electrons remain in the 4f shell. This prohibition is derived from the Pauli exclusion principle as it has been applied in gases. The lattice, however, may possibly modify the application of this principle. It was thought likely that the activated states of the ions in crystals would respond to any change in the application of the principle the lattice might impose. The spectra show that the principle holds for these ions in crystals in exactly the same way as it does in gases. Strong confirmation is obtained that the sharp spectra of the other rare earths consist of ``forbidden transitions,'' arising from a change in coupling among the electrons of the 4f shell.