Covalency in bis((8)annulene)uranium from photoelectron spectroscopy with variable photon energy

Relative partial photoionization cross sections and photoelectron branching ratios of the valence bands (6-22-eV binding energy) of bis((8)annulene)uranium(IV) are reported over the photon energy range 24-125 eV. The f{sup {minus}1} band shows cross-section features attributable to 5f resonant photoemission in the vicinity of the 5d {yields} 5f giant resonant absorption (hv = 101 and 110 eV). The e{sub 2g}{sup {minus}1} and e{sub 2u}{sup {minus}1} bands also show small cross-section maxima at these energies, that for the e{sub 2u}{sup {minus}1} ionization being the more intense. The f{sup {minus}1} band shows a large maximum at 39 eV, which is assigned to a delayed maximum due to the centrifugal barrier. The three highest ionization energy bands (from ligand-based orbitals) show essentially monotonic cross section decrease with photon energy. The mapping of the intensity changes of the f{sup {minus}1} band by the e{sub 2u}{sup {minus}1} band provides strong evidence for f orbital covalency in this molecule.