  We have successfully determined the hitherto unknown sign of the B44 Stevens
crystal-field parameter of the tetragonal heavy-fermion compound CeCu2Si2 using
vector q dependent non-resonant inelastic x-ray scattering (NIXS) experiments
at the cerium N4,5 edge. The observed difference between the two different
directions q||[100] and q||[110] is due to the anisotropy of the crystal-field
ground state in the (001) plane and is observable only because of the
utilization of higher than dipole transitions possible in NIXS. This approach
allows us to go beyond the specific limitations of dc magnetic susceptibility,
inelastic neutron scattering, and soft x-ray spectroscopy, and provides us with
a reliable information about the orbital state of the 4f electrons relevant for
the quantitative modeling of the quasi-particles and their interactions in
heavy-fermion systems.
