Determination of optical constants and thickness of passivating films on nickel from multiple-angle-of-incidence reflectivity measurements

A method is proposed which allows to determine without non-optical assumptions the complex refractive index and the thickness of thin surface films from measurements of the relative changes of reflectivity due to the formation or removal of the film at different angles of incidence with monochromatic light polarized perpendicular or parallel to the plane of incidence. The exact equations for the reflectivity change are used, since the linear approximation introduces systematic errors which are too large in view of the experimental accuracy. The method is applied to the passivating film formed on nickel in 1 M sulphuric acid after 1 h at U_HSS=0.85 V. A complex refractive index n₂=2.10 (±0.05)–0.46 (±0.02) i and a film thickness d=2.1 (±0.1) nm were found. In the spectrum of the optical constants in the range 1.4≤hv/e V≤4.8 a characteristic peak of the absorption coefficient was observed at 3.1 e V. Both the values at γ=500 nm and the spectra of the optical constants are explained by a partly hydrated nickel oxide film.