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Dynamical ellipsometric investigationsof the initial oxidation of the Ni(ll0) surface have been performed.Ellipsometry appears to be well suited to distinguish between chemisorbed oxygen and nickel oxide on the surface. Annealing at 740 K causes the nucleation of nickel oxide to proceed faster than at 570 K. After equal exposures however, the nucleation is slower with an oxygen pressure of 2X10W7 Torr than with a pressure of 1.3X IO-' Torr. Spectroscopic elhpsometric measurements (400-800 nm) have been performed on clean and oxygen-exposed (at saturation) Ni(ll0)surfaces. The complex dielectric function of the clean surface has been determined. To explain the results of the oxygen exposed surface a model is discussed -a surface layer with a thickness of 6 A and a mixture of 90% NiO and 10% Ni -in which changes with substrate optical properties are taken into account.

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