Formation of d-holes in the initial stages of the oxidation of Ag(001)

We demonstrate that double ionisation of the Ag atoms, leading to the formation of d -holes, takes place in the initial oxidation stage of Ag(001). The complex formed by the Ag 2+ ion, two oxygen adatoms and one oxygen atom in the octahedral interstitial gives rise to a dipole active electron energy loss at 130 meV. Signature of the electronic origin of this loss is the inverse isotope shift and the peculiar temperature dependence of the energy loss, which can be reproduced by a theoretical model.