Authors: F. Manghi, L. Pardini, V. Bellini

The excitation associated to X-ray absorption is a complex phenomenon where many body effects may play a major role. The independent particle picture fails in describing features of X-ray absorption spectra in all materials where e-e interactions are non negligible. To account for these failures various theoretical methods have been developed that differ on how the e-e Coulomb interaction is taken into account. They range from atomistic approaches where a parameterized many body Hamiltonian is solved via configuration interaction methods to solid state first-principles schemes that include properly the structure of valence electrons but loose some of the atomic many-body effects that can be relevant to the physics of the process. In this paper we present a new approach that treats on the same footing the localized and itinerant character of the electron states and of the interactions exploiting on one side the first principle calculations of the band structure and on the other side the localized picture to treat many body effects associated to electron-electron on-site interactions.

We will show that the on-site interaction between core and valence electrons responsible of satellite structures in core level photoemission spectra is also responsible of excitonic effects and of modified line shapes in XAS and XMCD. Application to the prototypical case of absorption from the L_2,3 edge in 3d transition metals is presented.