Surface segregation of gold for Au/Pd(1 1 1) alloys measured by low-energy electron diffraction and low-energy ion scattering

The surface composition of a Au/Pd(1 1 1) alloy formed by depositing five monolayers of gold onto clean Pd(1 1 1) at 300 K in ultrahigh vacuum and heating to various temperatures is measured from an analysis of the low-energy electron diffraction (LEED) intensity versus energy curves and by low-energy ion scattering (LEIS). The LEIS and LEED data yield values of the outer-layer gold coverage that are in good agreement and LEED shows that the second-layer gold concentration is intermediate between that of the surface and bulk. Plotting the gold coverage versus the bulk gold mole fraction estimated using Auger spectroscopy, clearly indicates that gold preferentially segregates to the surface and can be modeled by a simple Langmuir-McClean equation. A fit to the experimental data yields a value of the equilibrium constant for segregation of 3.2 ± 0.4, in good agreement with the value of 5.4 ± 0.2 obtained for Au/Pd(1 1 1) alloys grown on Mo(1 1 0) substrates.