Chlorine Adsorption on the Polycrystalline Fe, Ni and Fe-Ni Alloy Surfaces: A Photoemission Study

Chlorine adsorption on the polycrystalline Fe, Ni and Fe-Ni alloys at 300 K were studied by XPS and UPS. Chlorine is dissociatively adsorbed on all samples. The uptake curves show a two-stage process: an initial rapid chemisorption followed by a slow uptake to the saturation. The substrate dependence of the initial sticking coefficient and the saturation coverage was observed: Fe＞ Fe₆₄Ni₃₆ ＞ Fe₁₈Ni₈₂ ＞Ni.
With the increase of Cl₂-exposure the UPS spectra show that the d-band broadens and the shoulder peak of metal ion is developed at 2-3 eV, which indicates the surface oxidation.The Cl-adsorption leads to present the Fe⁺²(Fe⁺³) emission in Fe(2P_3/2) band of Fe and Fe-Ni alloys, But the Ni（2P_3/2） band of the same alloys does not change. It indicates that interaction of Cl with Fe is more stronger than with Ni.
The stronger Fe-Cl interaction induces the preferred adsorption of chlorine on Fe-sites and the Fe enrichment on the alloy surfaces.At elevated temperatures this segregation of Fe is more pronounced.