  Momentum-resolved resonant inelastic x-ray scattering (RIXS) spectroscopy has
been carried out at the Fe K edge, successfully for the first time. The RIXS
spectra of a FeBO3 single crystal reveal a wealth of information on 1-10 eV
electronic excitations. The IXS signal resonates both when the incident photon
energy approaches the pre-edge (1s-3d) and the main-edge (1s-4p) of the Fe K
edge absorption spectrum. The RIXS spectra measured at the pre-edge and the
main-edge show quantitatively different dependences on the incident photon
energy, momentum transfer, polarization, and temperature. Electronic
excitations observed in the pre-edge and main-edge RIXS spectra are interpreted
as Mott-Hubbard (MH) and charge-transfer (CT) excitations, respectively. The
charge-transfer gap = 3.81 (0.04) eV and the Mott-Hubbard gap = 3.96 (0.04) eV,
are determined, model independently from the experimental data. The CT and MH
excitations are assigned using molecular orbitals (MO) in the cluster model and
multiplet calculation in the many-electron multiband model, respectively.
