Long and short range effect of alkali promoters on metal surfaces : K on Rh(111)

Photoemission of physisorbed noble gases was used to study the variation of the surface potential on a K-promoted Rh (111) surface. The predominant effect of K is to lower the potential on nearest neighbor sites by 1-1.5 eV, but indicate also that the potential on next nearest neighbor sites is still 0.4-1 eV lower than on unpromoted Rh, depending on the K coverage. The latter values qual. agree with the cumulative effect of K/Rh dipoles which form an ordered array on the surface. [on SciFinder (R)

Surface potential around potassium promoter atoms on Rh(111) measured with photoemission of adsorbed Xe, Kr, and Ar

The effect of K promoter atoms on Rh (111) on the surface potential was studied with photoemission of adsorbed Xe, Kr, and Ar. By using probe atoms of different sizes, the authors have measured the variation of the surface potential in the vicinity of a K atom. The surface potentials were probed on Rh (111) surfaces, covered with 2.7, 4.1, and 5.0 at. % K. At all these coverages, the authors find a decrease in potential close to the K atoms (4 .ANG.) on the order of 1-2 eV, which depends on the lateral distance from the K atom. Further away from the K atoms, the authors find a nearly const. potential that is 0.4-1.0 V lower than on unpromoted Rh (111). This implies that all available absorption sites on the promoted surface are affected by the presence of K. The measured potentials depend on the K coverage, indicating that the potentials result from a cumulative effect of all K atoms on the surface. The obsd. 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. [on SciFinder (R)