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 x 2) and S p(2 x 2) overlayers using a back-reflection diffraction geometry from (111) planes at 2.9 keV. For the Cl-covered surface we find a 0.07angstroms +- 0.04 outward relaxation while for the S-covered surface the relaxation is much smaller.