Double-electron excitation in absorption spectra of actinides

The photoexcitation of a inner-shell photoelectron is is usually accompanied by the excitation of outher-shell electrons. Early systematic studies were performed with noble gases because their absorption signal is not affected by photoelectron backscattering from neighbored atoms. Up to now, most of the elements until Bi were investigated for multieletron excitations even in the presence of photoelectron backscattering effects from extended X-ray absortion fine structure. All elemets with Z > Bi comprise only radioactive isotopes and their mutielectron features are more difficult accesible. Recently we found evidence for multielectron excitations even in the L-absorption edges of actinide hydrates [1,2]. The actinides investigated up to now are Th, Pa, U, Np, Pu and Am. The strongest resonances result from [2p4f] double electron excitations, but there is also evidence for [2p5d] excitations. The [2p4f] resonance energies follow systematically the trend observed by Di Cicco and Filipponi for Hg, Pb and Bi [3]. The actinide elements show numerous oxidation states in particular for Np and Pu, where they range from III to VII. The electronic configuration involves besides 6d states partly filled 5f shells. The 5f states are less strictly located than the 4f states of the lanthanides, and can therefore participate in the chemical bond. This electronic configuration influences the physical and chemical properties and posses a large variation in the coordination geometry including transdioxo cations, AnO2n+ (n = 1 and 2). There is a clear correlation of the oxidation state, coordination geometry and the spectral feature of the multielectron resonance, resulting from final state density. The change in the resonance intensity of the single electron 2p6d transition as function of the electron configuration correspond well with the double-electron [2p4f] resonance intensity.