Helium Atom Diffraction from Clean and Krypton Film-Covered Graphite Surfaces.

Helium atom scattering was used to study the basal plane of graphite and the commensurate-incommensurate transition of monolayer krypton films adsorbed on graphite. A 17 meV energy beam was used in both works. Bound state energies of ('4)He atoms physisorbed on graphite (0001) surfaces were re-determined with improved resolution and precision. The energy levels were all slightly deeper than was previously thought, and an additional high -lying level was resolved. These results were in good agreement with recent theoretical calculations. The commensurate-incommensurate transition of monolayer krypton films on graphite was observed for transition temperature in the range 50.5 to 60 K. The change in lattice spacing appears continuous. The slightly incommensurate phase was disordered but still apparently solid. A possible precursor of the transition was observed as a decrease of specular and diffracted beam intensities while the film was still commensurate. The rotational transition of the incommensurate Kr monolayer was also observed. Our results were in qualitative agreement with Shiba's predictions. In the theoretical work the potential energy of a He atom above an ordered overlayer of Kr, Xe, or Ar on either a graphite or a Ag surface was calculated. The dominant term in the interaction is the pair interactions with the adatoms; non-negligible contributions arise also from the substrate and from three-body interactions involving both adatoms and the substrate. The dependence on coverage was explored. Agreement with bound state resonance data for the graphite case was excellent.