Electron stimulated oxidation of the Ni(111) surface: Dependence on substrate temperature and incident electron energy

Abstract

The effect of surface temperature on the rate of oxidation of the Ni(111) surface with and without electron irradiation has been determined for temperatures between 120 and 340 K. The oxidation rate in the presence of an electron beam demonstrates an inverse dependence on the substrate temperature, while without an electron beam we observe a decrease in oxidation rate with decreasing substrate temperature, decreasing almost to zero at 120 K. Similar rates are observed near room temperature for the two cases. We have found that oxidation of this surface can be well described by either of two rate expressions: one that relates the oxide growth rate to the rate of lateral growth of two dimensional oxide islands, and another that is first order in oxide and oxygen coverages at the surface. The physical implications of each model are discussed in terms of the nucleation sites created by the electron beam, and the rate constants for oxidation at these nucleation sites. We present evidence that the nucleation sites created by the electron beam are metastable, with an unusually long half-life of about 600±150 s. We have also investigated the dependence of the cross section for nucleation center creation as a function of incident electron energy at constant electron flux and constant oxygen exposure. The energy dependencies of the cross section for nucleation center creation and the yield of secondary electrons produced by irradiation from the incident electron beam are compared, leading to consideration of the role that such secondary electrons may have in the creation of nucleation centers. The results presented herein delineate the correct low temperature oxidation kinetics for Ni(111) in the absence of perturbing electrons. They also provide a cautionary note for experiments which use electron-based probes, or optical probes which generate intense swarms of electrons, for studying the oxidation kinetics of metals, and perhaps other classes of interfacial reactions.