In-Situ Atomic Structure of Underpotentially Deposited Monolayers of Pb and Tl on Au(111) and Ag(111): A Surface X-ray Scattering Study

In-situ surface X-ray scattering measurements are described of the electrochemical underpotential deposition (UPD) of Tl and Pb monolayers on Au(111) and Ag(111) with the aim of understanding the factors that influence the atomic structure and compressibility of these systems. It is found that the UPD deposits form incommensurate, hexagonal monolayers that are compressed compared to the bulk metals and rotated from the substrate direction by several degrees. Our results show that for these incommensurate UPD systems, (i) the adatom-adatom interaction is the primary force determining the monolayer structure, (ii) although the adatom-substrate interaction is strong and results in the formation of the monolayer, it only influences the monolayer structure slightly, and (iii) the presence of the large concentration of water molecules or adsorbing anions in contact with the monolayer does not affect its structure. 70 refs., 9 figs., 1 tab.