HE-SCATTERING STUDIES OF ALKALI-METAL OVERLAYERS ON GRAPHITE

Hc-scattering studies show that when alkali metal overlayers are present on a graphite surface the He-surface interaction has an extremely small corrugation, so small that no He diffraction was observed. He atoms do, however, couple to the overlayer phonons, and essentially dispersionless, low-energy modes were observed for K, Rb and Cs overlayers at submonolayer and monolayer coverages. Density-functional calculations for K and Cs on graphite indicate that the small surface corrugation experienced by the He atoms is due primarily to the large spatial extent of the alkali metal valence electrons, causing the turning point of the He atom to be significantly farther from the surface than for other materials. Also, at near-monolayer densities, the metallization of the overlayer produces an extremely smooth charge density. The vibrational modes observed with He-atom scattering are identified as modes polarized parallel to the surface. Modes which would correspond to a polarization perpendicular to the surface surprisingly are not observed. Growth of two and three layers were observed for Rb and Cs, respectively, their phonon energies decreasing and the amount of dispersion increasing for successive layers.