Kinetic energy effects on the oxidation of Ni(111) using O-2 molecular beams

The oxidation kinetics of the Ni(111) surface have been quantitatively exam ined 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 t o 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 s imple ambient gas dosing. In contrast to this, exposure to a relatively hig h-energy 600 meV O-2 beam yielded a fundamentally different oxidation behav ior: 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(l ll) could not be oxidized using either of these incident-beam conditions, i ndicating that the energetic constraints needed to move from oxygen chemiso rption to actual metallic oxidation cannot be simply overcome using inciden t O-2 kinetic energy. (C) 1998 Published by Elsevier Science B.V. All right s reserved.