  We report layer-resolved measurements of the \textit{unoccupied} electronic
structure of ultrathin MgO films grown on Ag(001). The metal-induced gap states
at the metal/oxide interface, the oxide band gap as well as a surface core
exciton involving an image-potential state of the vacuum are revealed through
resonant Auger spectroscopy of the Mg $KL_{23}L_{23}$ Auger transition. Our
results demonstrate how to obtain new insights on \textit{empty} states at
interfaces of metal-supported ultrathin oxide films.
