OXYGEN INTERACTION WITH NI-FE SURFACES (1) LEED AND XPS STUDIES OF NI 76-PERCENT-FE 24-PERCENT (100)

The interaction of oxygen with a Ni 76%/Fe 24% (100) surface has been studied at 293 K. XPS measurements were made as a function of emission angle to obtain depth distributions. Slight Fe segregation is observed for a clean annealed surface. It is enhanced by oxygen adsorption. By 0. 7 monolayers oxidized Fe species are detected, but the Ni remains metallic until well above one monolayer. The Fe concentration in the outer 4 A has then increased twofold. At about similar 2. 7 monolayers (passivation) the surface Fe concentration is nearly three times the original and entirely oxidized. Heating causes further Fe segregation which reduces the Ni**2** plus to yield Ni**0. LEED shows a c (2 multiplied by 2) pattern which persists above one monolayer. Correlations of the XPS and LEED data and comparison with XPS/LEED studies of oxygen on Fe(100) and Ni(100) show that between 0. 7-2 monolayers, the surface must be heavily patched with Fe oxide and metallic Ni aggregations, the Ni being covered with a c (2 multiplied by 2) oxygen superstructure.