The position of segregated Al atoms in the ordered (root 3x root 3)R30 degrees superlattice phase on the (111) surface of a Cu-9 at. % Al alloy was precisely determined by tensor low energy electron diffraction (LEED) analysis. Work function values of the superlattice phase, the alloy without Al segregation, and Cu (111) were measured and compared. Both the position of segregated Al atoms and the work function values were computed using first-principles calculations. The conclusion from the tensor LEED analysis that the segregated Al atoms, occupying substitutional positions in bulklike lattice sites within the top layer, were located outward by 6% with respect to the plane of the top layer Cu agreed well with the calculations. The results that the work function of the alloy surface with substitutional Al segregation was very close to that without Al segregation were obtained from both the experiments and the calculations. The effect of segregation of atoms and the position of segregants on the work function value was also discussed. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3273533]