THE STAGES OF OXYGEN-ADSORPTION ON POLYCRYSTALLINE IRON STUDIED THROUGH FACTOR-ANALYSIS APPLIED TO O KLL AND FEM23VV D(NE)/DE AUGER-SPECTRA

Previous studies of oxygen adsorption on polycrystalline iron under UHV conditions have distinguished at least three stages at room temperature: dissociative chemisorption, a "linear" initial oxidation mechanism with oxygen incorporation, and growth of bulk oxide. Identification of the nature of the adsorbate and the surface chemistry in this case and for crystalline surfaces have depended on studies and assignments of appropriate signatures in UPS, AES and EELS spectra. In the present work, factor analysis of AES lineshapes, under conditions of gradually increasing oxygen exposures, is shown to permit the decomposition of the spectra in components relating to chemisorption and two chemically different oxides building up. We find that the initial chemisorption (0-1.5 L) affects the iron surface atoms, that the final oxide phase, γ-Fe2O3, starts to develop only after 6 L, and that the first oxide component, FeO, peaks at 10 L and continues to appear after very large exposures during formation of the bulk oxide. The "linear" oxidation regime, 5-10 L, thus contains mainly the initial oxide, as two-dimensional islands. © 1990.