Early stages of surface alloy formation:: a diffuse LEED I(V) study

The technique of diffuse low energy electron diffraction (LEER) I(V) analysis is shown to be a useful quantitative probe of the structure of surface alloys in the early stages of,growth before long-range order develops. The Cu(100)/Pd system has been studied in the Pd coverage range 0.10-0.55 ML. We demonstrate the Pd adsorbs at all coverages primarily by substitutional replacement of top layer copper atoms forming a two-dimensional CuxPd1-x surface alloy. At low coverages a quasi-random substitutionally disordered two-dimensional alloy is formed with local order detected by LEED developing at Pd coverages above 0.25 ML. The surface geometry is not strongly coverage dependent with top layer Cu and Pd atoms almost coplanar and the first interlayer spacing slightly expanded (3%) with respect to the bulk value. (C) 2001 Elsevier Science B.V. All rights reserved.