Site dependence of core-electron binding energies of adsorbates: I/Pt(111)

Core-electron binding energies of iodine adsorbed on Pt(111) surfaces have been measured as a function of coverage in both the ($\sqrt{3}\times \sqrt{3}$) and ($\sqrt{7}\times \sqrt{7}$) regimes. In the former, in which the iodines occupy the threefold hollows, the binding energy decreases only slightly up to the saturation coverage of ⅓ monolayer. In the latter regime two distinct binding energies are obtained, one close to the value in the ($\sqrt{3}\times \sqrt{3}$) regime, the other smaller by 0.95 eV. The smaller binding energy corresponds to atoms in the atop sites of the ($\sqrt{7}\times \sqrt{7}$) structure. The initial- and final-state contributions to this binding-energy shift are examined.