Collapse ofdandforbitals in the isoelectronic sequence of singly ionized ytterbium

Abstract

The phenomenon of ``collapse'' of $5d$ and $5f$ orbitals in the ${\mathrm{Yb}}^{+}$ isoelectronic sequence occurring when the increasing Coulomb and exchange attractions overcome centrifugal repulsion is studied in this paper using both the relativistic model-potential and single-configuration Dirac-Fock techniques. As before for xenon and cesium sequences, valence-core electron exchange is found to play an essential role in the collapse phenomenon. Furthermore, the collapse greatly enhances the sensitivity of the properties of the orbitals that undergo this phenomenon to small changes in the effective potential, so that even much more subtle effects such as nonlocal exchange effects or core-valence electron correlation (core polarization) became more than usually important.