Local effects in electron capture processes of fluorine atoms interacting with an oxidised Mg surface

Changes in electron capture rates during the oxidation of metal surfaces are investigated on the example of F- formation in fluorine ion/atom scattering on a Mg surface exposed to O-2 and also for MgO(100). In the low-coverage, chemisorption range, F- formation decreases in spite of a decrease in the surface work function. This is assigned to a local effect due to specifics of the electronic structure at the adsorbate site, which is akin to surface poisoning effects. At high exposures corresponding to oxide formation electron capture is very efficient and can be understood in terms of a quasi-resonant localised charge exchange mechanism between a F atom and a MgO lattice O anion.