A SEARCH FOR SURFACE ALLOY FORMATION IN FACETING INDUCED BY MONOLAYERMETAL-FILMS - PD W(111) AND NI W(111)

A monolayer of Pd on a W(111) surface will cause faceting of the surfa ce upon annealing to T> 700 K, with the formation of (211) oriented mi crofacets. In order to learn whether surface alloy formation or interm ixing of surface layers occurs during faceting we have employed low en ergy ion scattering (LEIS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) to investigate the Pd/W(111) s ystem. We find that 3.0 Angstrom Pd deposited at room temperature almo st completely covers the substrate W. In the temperature range from ro om temperature to similar to 1125 K (the onset of Pd desorption) there is no intermixing of Pd atoms with W atoms: Pd atoms stay on top of t he planar surface as well as the faceted surface. In comparison with t he Pd/W(111) system, we have also studied the Ni/W(111) system which d oes not exhibit faceting behavior. Within experimental uncertainty we find that there is similar to 10% intermixing of Ni atoms with W atoms in the topmost surface layer, after annealing at 1175 K.