Substrate surface relaxation for Cl and S on Cu(001)

Using a combination of x-ray standing waves and surface extended x-ray-absorption fine structure, we have determined the interlayer relaxation of a metal surface in the presence of an adsorbate. The Cu(001) surface was studied with Cl c(2×{}2) and S p(2×{}2) overlayers using a back-reflection diffraction geometry from (111) planes at 2.9 keV. For the Cl-covered surface we find a 0.07±{}0.04 A{} outward relaxation while for the S-covered surface the relaxation is more difficult to determine because of possible substract reconstruction.