The experiments are performed in an ultrahigh vacuum (UHV) chamber (pressure < 5´10 -10 mbar) at room temperature (RT). XPS core level spectra were obtained using the Mg Ka (1253.6 eV) and Al Ka (1486.6 eV) lines and a Phoibos 150 electron analyzer with the axis coincident with the surface normal. The core level binding energies are calibrated using the binding energies of C 1s and Ag 3d in contact with the sample as references.