Kinetic energy effects on the oxidation of Ni(111) using O2 molecular beams

The oxidation kinetics of the Ni(111) surface have been quantitatively examined utilizing kinetic energy selected supersonic beams of molecular oxygen. Using in-situ high-resolution electron-energy-loss spectroscopy, we have observed notable differences in the oxidation mechanism for this interface asa function of incident-beam kinetic energy. Exposure of a 300-K surface to a relatively low-energy 60 meV O-2 beam leads to oxidation kinetics that follow an island growth model, qualitatively similar to what is seen with simple ambient gas dosing. In contrast to this, exposure to a relatively high-energy 600 meV O-2 beam yielded a fundamentally different oxidation behavior: the kinetics of oxidation no longer follow an island growth model but rather behave with a Langmuir-like sticking model, implying key differences in the nucleation stage for interface oxidation. Cryogenically cooled Ni(lll) could not be oxidized using either of these incident-beam conditions, indicating that the energetic constraints needed to move from oxygen chemisorption to actual metallic oxidation cannot be simply overcome using incident O-2 kinetic energy. (C) 1998 Published by Elsevier Science B.V. All rights reserved.