Collapse of 4f electrons in the isoelectronic series of xenon

The Hartree-Fock method with a frozen core is used to calculate the excited $^1P(3d^9nf)$ states (n = 4,5,6) in the isoelectronic series of Xe with continuous variation in nuclear charge in the range $53.9 \le Z \le 55.1$. The singularities in the behavior of the single-particle energies and mean radii of the excited nf levels in the collapse of 4f electrons are investigated, the parameters of the transition region are determined, and the resonance of the levels is discussed in detail. It is shown that, within the framework of the method employed, the 4f level in the $Cs^+$ ion collapses, which corresponds to experimental data on photoabsorption.