The $4d$ relative photoabsorption cross section of cesium has been observed to change dramatically in appearance along the isonuclear sequence Cs through ${\mathrm{Cs}}^{4+}.$ In each case, discrete structure is observed below threshold and for Cs through ${\mathrm{Cs}}^{2+},$ a giant dipole $4\overrightarrow{d}\varepsilon f$ resonance is also present above threshold. Between ${\mathrm{Cs}}^{2+}$ and ${\mathrm{Cs}}^{3+},$ there is a striking change, with much of the oscillator strength available in the $4\overrightarrow{d}f$ channel being abruptly transferred to the discrete spectrum. Despite the complexity of the transitions involved, the oscillator strength envelope for the discrete transitions yields relatively few features and remains essentially static in energy regardless of the parent terms. Hartree-Fock with configuration interaction and time-dependent local-density-approximation calculations successfully account for this behavior and permit identification of the discrete features.

• Received 21 June 2000

DOI:https://doi.org/10.1103/PhysRevA.63.022702