ELECTRON-CAPTURE ON SURFACES WITH ELECTRONEGATIVE ADSORBATES AND SURFACE POISONING

We present the results of a study of the modification in electron capt ure rates on metal surfaces with an electronegative impurity: oxygen. Electron capture is investigated on the example of O- and F- formation in oxygen and fluorine ion/atom scattering. O- and F- ion fraction me asurements were made For Mg and Al surfaces exposed to O-2. Changes in electron capture rates were continuously followed from the limit corr esponding to a metal with submonolayer chemisorbed oxygen to those on an oxide film. A comparative measurement was made for an MgO(100) surf ace. It is shown that in the low coverage, chemisorption range local e ffects due to specifics of the electronic structure al the adsorbate s ite can strongly attenuate electron capture. This effect, akin to surf ace poisoning, may be attributed to changes in the local density of st ates and modification in the positions and widths of the anion level n ear the adsorbate. At high exposures corresponding to oxide formation electron capture is very efficient and can be understood in terms of a gas-phase-like, localized non-resonant charge exchange mechanism betw een the incident atom and, for example, an MgO lattice O anion. At int ermediate coverages, the behaviour of the ion fractions is affected by appearance of oxide islands. (C) 1998 Elsevier Science B.V.