Metals on Metal Oxides: Study of Adsorption Mechanisms With the Metastable Impact Electron Spectroscopy (MIES)

摘要

The Metastable Impact Electron Spectroscopy (MIES) spectra obtained from oxide surfaces (TiO_2, MgO), exposed to metal atoms (Mg; Cs), are discussed. MgO-films (2nm thick) on transition metals (Mo;W) and TiO_2-single crystals were chosne as substrates. Guided by ab initio electronic structure calculations, in particular for K/TiO_2, Mg/MgO and Ag/MgO, several mechanisms important during the initial stage of the nucleation and the growth of metal clusters/films could be identified: (1) Chemisorption caused by charge transfer from the metal adsorbate into unoccupied states of the oxide (type I). For the case of alkali/TiO_2 this leads to the reduction of the Ti-cation. (2) Hybridization of the O2p-orbitals of the oxide anions with the s-valence orbitals of the adsorbate (type II), as verified for Mg/MgO. For Mg/TiO_2 both mechanisms, I and II, play a role. (3) For Cs/TiO_2 mechanisms I loses much of its importance for longer exposures: instead, adsorption is due to the electrostatic interactio nbetween the enhanced electron density, concentrated between adjacent metal adatoms, and surface ions of the underlying substrate (type III).