Local structural and compositional determination via electron scattering: Heterogeneous Cu(001)-Pd surface alloy

We have measured the structure and chemical composition of ultrathin Pd films on Cu 001 using lowenergy electron microscopy. We determine their local stoichiometry and structure, with 8.5 nm lateral spatial resolution, by quantitatively analyzing the scattered electron intensity and comparing it to dynamical scattering calculations, as in a conventional low-energy electron diffraction LEED -IV analysis. The average t-matrix approximation is used to calculate the total atomic scattering matrices for this random substitutional alloy. As in the traditional LEED analysis, the structural and compositional parameters are determined by comparing the computed diffraction intensity of a trial structure to that measured in experiment. Monte Carlo simulations show how the spatial and compositional inhomogeneity can be used to understand the energetics of Cu-Pd bonding.