  Triple photoionization of Xe3+, Xe4+ and Xe5+ ions has been studied in the
energy range 670-750 eV, including the 3d ionization threshold. The photon-ion
merged-beam technique was used at a synchrotron light source to measure the
absolute photoionization cross sections. These cross sections exhibit a
progressively larger number of sharp resonances as the ion charge state is
increased. This clearly visualizes the re-ordering of the $\epsilon$f continuum
into a regular series of (bound) Rydberg orbitals as the ionic core becomes
more attractive. The energies and strengths of the resonances are extracted
from the experimental data and are further analyzed by relativistic
atomic-structure calculations.
