HETEROGENEITY IN THE SURFACE-STRUCTURE OF A CU(100)-C(2X2)-PD SURFACE ALLOY

The structure and compositional profile of a Cu{100}-c(2x2)-Pd surface alloy formed by deposition of 0.55+/-0.1 ML of Pd on Cu{100} at 300 K has been examined by a range of surface sensitive techniques and is shown to be highly heterogeneous. Using CO as a probe molecule, thermal desorption spectroscopy demonstrates that a significant fraction (approximate to 40%) of the outermost layer is pure Cu. X-ray photo-electron polar intensity plots along the [010] azimuth reveal a zeroth-order diffraction peak at a polar emission angle of 40 degrees-45 degrees indicating significant penetration of Pd beyond the outermost Cu layer. Absence of a strong 0 degrees forward scattering peak limits the compositional profile to one in which the majority of sub-surface Pd is located within the second layer. Model single scattering cluster calculations confirm this and indicate a local expansion of the outermost interlayer spacing of 0.4+/-0.2 Angstrom in regions of high local Pd concentration.