Role of Spin-Orbital Splitting of 5 f -Orbitals of Uranium Atom in the Formation of Its Chemical State
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Role of Spin-Orbital Splitting of 5f-Orbitals of Uranium Atom in the Formation of Its Chemical State Yuri F. Batrakov, Andrey G. Krivitsky, and Elena V. Puchkova Department of Radiochemistry, Faculty of Chemistry, St. Petersburg State University 7/9 Universitetskaja emb., St. Petersburg, 199034, Russia Email: [email protected] ABSTRACT Chemical shifts (ChSh) of nine emission lines of the uranium L-series in uranium oxides UO2+x (x=0-1) with respect to UO2 were studied by using a precise crystal-diffraction X-ray spectrometer and the changes in energy of spin-orbital splitting (SOS) - ∆δnl± of inner nl-orbitals of the uranium atom were calculated from the data of ChSh of spin-doublet lines. For UO2+x oxides, a linear decrease in ∆δnl± values with increasing degree of uranium oxidation was found. On the basis of the comparison of experimental ∆δnl± values with Dirac-Hartree-Fock atomic calculations, it was concluded that the observed variations in ∆δnl± values are due to the redistribution of electron and spin density between the 5f7/2- and 5f5/2-levels of the fine structure of the uranium atom without changes in atomic charge state. INTRODUCTION The chemical behaviour of actinide atoms (in particular, that of uranium) is determined by valent nl-electrons of three types: 7s, 6d, and 5f. Although the bond energies of these electrons are almost equal, their wave-functions differ greatly in distribution in the radial direction. It can be said that the 5f-electrons have an only core arrangement in the atom. Therefore, when actinides chemical bonding is studied several questions should be raised: 1) the possibility and form of 5f-electrons participation in chemical bonding; 2) the necessity for taking into account the splitting of valent levels of the atom into two sublevels nl+ and nl- with total angular momentum j = l ± ½ because of the relativistic effect (RE) of spin-orbital splitting (SOS) [1-3]; 3) the energetic stabilization of the specific chemical state of the heavy atom due to fine effects of electron density redistribution on valent orbitals; 4) the possibility of independent participation of split subshells in chemical bond formation. In X-ray photoelectron spectra (XPS), a regularly decreasing intensity of the narrow line of 5f-electrons (with an energy Eb = 2.0±0.1 eV) is observed in a series of UO2+x oxides with increasing x [4-6]. The explanations of this phenomenon are very contradictory [4-7]. This is probably due to insufficient XPS sensitivity to fine energetic changes of the atomic electron structure (state). It is known that the error in the determination of the binding energy of electrons and of the line width in the XPS method is about 100 meV and that in the determination of relative line intensity is 10% [4]. One of the methods of modern precise spectroscopy capable of providing a correct description of chemical bonging process is the chemical shift (ChSh) method of X-ray emission lines, i.e., the change in their energy when the chemical state of the emitting atom is changed [8,9]. Th
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