Ag-adsorption on MgO: investigations with MIES and UPS

MgO films (2 nm thick) were grown on W(1 1 0) while metastable impact electron (MIES) and photoelectron (UPS(HeII)) spectra were collected in situ; apart from the valence band emission no additional spectral features could be detected. The oxide surface was exposed to Ag (substrate temperature at 300 K). The additional Ag-induced emission found in the MIES spectra is attributed to the formation of Ag islands. In the very early stage of island growth a characteristic initial decrease of the surface work function is observed; it is therefore proposed that in the initial phase of the Ag exposure, surface defects (Vs-center-type and extended defects) play an important role as nucleation sites for the island formation. Initially, the islands show 2D growth; above a critical coverage of 0.08 monolayers this is followed by preferential 3D growth.