In situ x-ray specular reflectivity and glancing-incidence-angle x-ray-diffraction measurements have been performed at the Au(001) surface in a 0.01M ${\mathrm{HClO}}_4$ solution under potential control in an electrochemical cell. At -0.4 V versus an Ag/AgCl electrode, the gold surface exhibits a hexagonal reconstructed layer with a mass density 21% greater than the underlying bulk layers. The reconstruction disappears above 0.5 V, and the excess atoms form a new atomic layer with a density corresponding to 22% of a bulk layer. The reconstruction fully recovers below -0.3 V.

• Received 31 May 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1466