RECONSTRUCTION KINETICS OF A STEPPED METALLIC SURFACE - STEP DOUBLINGAND SINGLING OF NI(977) INDUCED BY LOW-OXYGEN COVERAGES

Helium atom diffraction has been used to study the reconstruction kine tics of a stepped metallic surface, Ni(977), which sequentially underg oes step-doubling and -singling upon dosing with low coverages of oxyg en. Over the temperature range 390-470 K it was found that less than 2 % of a monolayer of oxygen was sufficient to transform the initially p repared single-stepped surface to a new steady state having a double-s tepped structure. The thermal range over which the doubled phase exist s extends to higher temperatures when more oxygen is present. At low o xygen exposures this doubled interface reverts to the single-stepped s urface above 470 K. Singling can also be driven by more extensive leve ls of oxygen adsorption. The kinetics of the step-doubling transformat ion which occurs below 470 K was determined to be second order with re spect to single-step density while, for temperatures above 470 K, step -singling followed first-order kinetics with respect to the double-ste p density. Oxygen atoms adsorbed at step edges play a crucial role in these transformations. Arrhenius analysis is used to extract energetic s for the step-doubling and -singling reconstructions. These results d elineate the sequence of mechanistic stages which occur during the ini tial stages of oxidation of a stepped metallic interface which precede the onset of bulk oxidation, findings which are important for develop ing an improved understanding of metallic oxidation and corrosion.