  By combining x-ray excited Auger electron diffraction experiments and
multiple scattering calculations we reveal a layer-resolved shift for the Mg
KL23L23 Auger transition in MgO ultrathin films (4-6 \AA) on Ag(001). This
resolution is exploited to demonstrate the possibility to control Mg atoms
incorporation at the MgO/Ag(001) interface by exposing the MgO films to a Mg
flux. A substantial reduction of the MgO/Ag(001) work function is observed
during the exposition phase and reflects, both band-offset variations at the
interface and band bending effects in the oxide film.
