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

The effect of potassium promoter atoms on Rh(111) on the surface poten tial has been investigated with photoemission of adsorbed Xe, Kr, and Ar. By using probe atoms of different sizes, we have measured the vari ation of the surface potential in the vicinity of a potassium atom. Th e surface potentials have been probed on Rh(111) surfaces, covered wit h 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 t he order of 1-2 eV, which depends on the lateral distance from the pot assium atom. Further away from the potassium atoms we find a nearly co nstant potential that is 0.4-1.0 V lower than on unpromoted Rh(111). T his implies that all available adsorption sites on the promoted surfac e are affected by the presence of potassium. The measured potentials d epend on the potassium coverage, indicating that the potentials result from a cumulative effect of all potassium atoms on the surface. The o bserved changes in surface potential agree well with the potential in a hexagonal network of dipoles, indicating that they are of electrosta tic origin and do not require long-range changes in electronic structu re.