Relativistic component of chemical shift of Uranium X-ray emission lines

Twelve most intensive hard X-ray emission lines belonging to two multiplets (2p-3d and 2p-4d) and three doublets (2s-3p, 2s-4p, and 2p-3s) of the uranium L-series were studied by using a precise crystal-diffraction X-ray spectrometer. Chemical shift (CS) values were obtained for the first time for uranium (VI) oxide and uranium fluoride with respect to uranium (IV) oxide. The first attempt was made to separate CS of hard X-ray lines of an actinide atom into two components: the non-relativistic CS part characterizing the charge distribution of valence electrons between the atom and its partners in chemical bonding and the relativistic part reflecting the effects which are not related to electron removal from the atom. A method for calculating the relativistic CS component is proposed. It is concluded that it is the relativistic component that provides information about the changes in the valence state of the actinide atom in a compound. On the basis of model Dirac–Hartree–Fock calculations, it is shown that in the case of the uranium atom the relativistic component of CS is a direct consequence of intra-atomic electrons redistribution between 5f7/2- and 5f5/2-split levels of the uranium atom fine structure without changes in its charge state.