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^-1 band shows cross-section features attributable to 5f resonant photoemission in the vicinity of the 5d →5f giant resonant absorption (hv = 101 and 110 eV). The e_2g¹ and e_2u¹ bands also show small cross-section maxima at these energies, that for the e_2u¹ ionization being the more intense. The f^-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^-1 band by the e_2u^-1band provides strong evidence for f orbital covalency in this molecule.