  The complex electronic structure of metallic ferromagnets is determined by a
balance between exchange interaction, electron hopping leading to band
formation, and local Coulomb repulsion. The interplay between the respective
terms of the Hamiltonian is of fundamental interest, since it produces most, if
not all, of the exotic phenomena observed in the solid state. By combining high
energy and temporal resolution in femtosecond time-resolved X-ray absorption
spectroscopy with ab initio time-dependent density functional theory we analyze
the electronic structure in fcc Ni on the time scale of these interactions in a
pump-probe experiment. We distinguish transient broadening and energy shifts in
the absorption spectra, which we demonstrate to be caused by electron
repopulation and correlation-induced modifications of the electronic structure,
respectively. Importantly, the theoretical description of this experimental
result hence requires to take the local Coulomb interaction into account,
revealing a temporal interplay between band formation, exchange interaction,
and Coulomb repulsion.
