SURFACE-POTENTIAL AROUND POTASSIUM PROMOTER ATOMS ON RH(111) MEASURED WITH PHOTOEMISSION OF ADSORBED XE, KR, AND AR

The effect of potassium promoter atoms on Rh(111) on the surface potential has been investigated with photoemission of adsorbed Xe, Kr, and Ar. By using probe atoms of different sizes, we have measured the variation of the surface potential in the vicinity of a potassium atom. The surface potentials have been probed on Rh(111) surfaces, covered with 2.7, 4.1, and 5.0 at. % potassium. At all these coverages, we find a decrease in potential close to the potassium atoms (<4 angstrom) on the order of 1-2 eV, which depends on the lateral distance from the potassium atom. Further away from the potassium atoms we find a nearly constant potential that is 0.4-1.0 V lower than on unpromoted Rh(111). This implies that all available adsorption sites on the promoted surface are affected by the presence of potassium. The measured potentials depend on the potassium coverage, indicating that the potentials result from a cumulative effect of all potassium atoms on the surface. The observed changes in surface potential agree well with the potential in a hexagonal network of dipoles, indicating that they are of electrostatic origin and do not require long-range changes in electronic structure.