  We have succeeded in establishing the crystal-field ground state of
CeRu2Al10, an orthorhombic intermetallic compound recently identified as a
Kondo insulator. Using polarization dependent soft x-ray absorption
spectroscopy at the Ce M4,5 edges, together with input from inelastic neutron
and magnetic susceptibility experiments, we were able to determine
unambiguously the orbital occupation of the 4f shell and to explain
quantitatively both the measured magnetic moment along the easy a axis and the
small ordered moment along the c-axis. The results provide not only a platform
for a realistic modeling of the spin and charge gap of CeRu2Al10, but
demonstrate also the potential of soft x-ray absorption spectroscopy to obtain
information not easily accessible by neutron techniques for the study of Kondo
insulators in general.
