XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111) |
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Authors: | S.B. DiCenzo G.K. Wertheim D.N.E. Buchanan |
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Affiliation: | Bell Laboratories, Murray Hill, New Jersey 07974, USA |
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Abstract: | We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ~0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The core electron binding energy in both cases decreases by ~0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas. |
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