Initial stages of Al(111) oxidation by oxygen: Temperature and surface morphology effectsThe interaction of oxygen with the ordered Ni 3 Al ͑111͒ surface has been investigated in the temperature range from 300 to 1000 K using high-resolution electron-energy-loss spectroscopy ͑HREELS͒ and low-energy electron diffraction ͑LEED͒. The ''2ϫ2'' LEED pattern of the clean Ni 3 Al ͑111͒ surface indicates a bulklike termination. After oxygen adsorption at 300 K the LEED pattern is diffuse suggesting the formation of an amorphous overlayer. The HREELS spectra show evidence for oxygen interaction with both aluminum and nickel atoms. At 600 K adsorption temperature the fcc surface order is restored, however, the observed (1ϫ1) LEED pattern indicates the loss of chemical order. Again HREELS spectra suggest interaction of oxygen with both aluminum and nickel. For an adsorption temperature of 800 K LEED shows an unrotated oxygen induced superstructure with a lattice spacing of 2.93 Å in addition to the (1ϫ1) substrate spots. The HREELS spectra exhibit an intense loss at 81.9 meV, which is also known from oxygen in threefold hollow sites on Al ͑111͒. Since such sites are not available on the Ni 3 Al ͑111͒ surface, we conclude the buildup of an oxygen covered aluminum overlayer. Finally, during oxygen exposure at 1000 K we observe the growth of a ␥Ј-Al 2 O 3 structure on the reordered Ni 3 Al ͑111͒ substrate surface. This structure has been identified by means of the characteristic vibrational losses in HREELS at 54.6, 80.3, and 112.8 meV together with the emergence of overlayer spots in LEED exhibiting the lattice spacing of ␥Ј-Al 2 O 3 ͑3.02 Å͒. For oxygen exposures at 800 and 1000 K an island growth of the overlayer has been found.

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