Mg clusters on MgO surfaces:: characterization by MIES and electronic structure ab initio calculations

MgO films (2 nm thick) were grown on Mo and W substrates while metastable impact electron (MIES) and photoelectron spectra (UPS(HeI)) were collected in situ. Apart from the valence band (VB) emission no additional spectral features have been detected with electron spectroscopies. After exposing the oxide surface to Mg (substrate temperature between 100 K and RT) an additional peak, not seen with UPS, located within the band gap, shows up in MIES. it is located at about 2 eV above the top of the VE with a FWHM of about I eV at the lowest exposures. Electronic structure calculations based on the density functional theory (DFT) are presented for the adsorption of up to Four Mg atoms to the defective MgO surface. Based on these calculations we attribute the additional emission to the formation of small Mg clusters. The calculations suggest that the nucleation starts at extended defects, steps in particular. (C) 1999 Elsevier Science B.V, All rights reserved.