Collapse of the4forbital for Xe-like ions

The effect of $4f$ orbital collapse on the spectra of Xe-like ions in the region of $4d\to nf,\epsilon f$ excitations is studied with the use of a term-dependent Hartree-Fock technique. We find that the collapse of the $4f$ orbital is strongly term dependent, and that the appearance of intense absorption lines in the observed ${\mathrm{Ba}}^{2+}$ spectrum is due to the partial collapse of the $4f$ orbital in the $4d\to f{}_{}{}^1{}_{}{}^{}P$ channel. We also show that after the $4f$ orbital is completely collapsed for high degrees of ionization along the isoelectronic sequence, the bulk of absorption oscillator strengths is concentrated at the $4d^{9}4f{}_{}{}^1{}_{}{}^{}P$ level so that the only strong line in the spectrum is again the $4d\to 4f{}_{}{}^1{}_{}{}^{}P$ transition.