The atomic structure of ordered Cu adsorbate layers on Au(111) and Au(100) electrode surfaces and of the clean substrates was resolved in scanning tunneling microscopy images taken in situ. For submonolayer coverages deposited from sulfuric acid solutions under potential control, various ordered structures were observed. The quasihexagonal arrangement of Cu atoms in these structures reflects increasingly repulsive interactions between closely spaced Cu adatoms. These structures differ from the pseudomorphic Cu adlayer formed under vacuum conditions, which demonstrates the structure-determining role of the coadsorbed anions.

• Received 6 February 1990

DOI:https://doi.org/10.1103/PhysRevLett.64.2929