  We have measured the intricate temperature dependence of the Co L2,3 x-ray
absorption spectra (2p-3d excitations) of CoO. To allow for accurate total
electron yield measurements, the material has been grown in thin film form on a
metallic substrate in order to avoid charging problems usually encountered
during electron spectroscopic studies on bulk CoO samples. The changes in
spectra due to temperature are in good agreement with detailed ligand-field
calculations indicating that these changes are mostly due to thermal population
of closely lying excited states, originating from degenerate t2g levels lifted
by the spin-orbit coupling. Magnetic coupling in the ordered phase, modeled as
a mean-field exchange field, mixes in excited states inducing a tetragonal
charge density. The spin-orbit coupling induced splitting of the low energy
states results in a non-trivial temperature dependence for the magnetic
susceptibility.
