IN-SITU STM INVESTIGATION OF TL AND PB UNDERPOTENTIAL DEPOSITION ON CHEMICALLY POLISHED AG(111) ELECTRODES

In-situ STM was used to investigate the adlayer formation and structur e of Tl adsorbates at chemically polished Ag(111) electrodes in perchl orate electrolyte, and to study the effect of extended I polarization of incomplete Tl and Pb adlayers at high initial adsorbate coverage. I n-the Tl+/Ag(111) system, the full first monolayer adsorbate has a hex agonally close packed structure with interatomic distances that are co mpressed in comparison with the bulk Tl phase. The subsequently formed second monolayer has also a hexagonally close packed (hcp) structure, but shows more pronounced disordered domains. Incomplete first Tl mono layers of high coverage have the same hcp structure as a complete mono layer, but do not extend to the very boundary of the substrate terrace . Extended polarization of such monolayers leads to a conversion of th e original hcp layer into a coverage with [root 3 x root 3]R30 degrees -symmetry which is interpreted as a mixed coverage, where every third Ag atom is replaced by a Tl atom. Similar phenomena are observed durin g long-time polarization of an incompletely formed, Pb-coverage. The r esults are discussed with regard to the role of the atomic steps and r eveal the importance of surface singularities for the understanding of the adsorption behaviour at real electrodes.