Ethylene on graphite: A low-energy electron-diffraction study

Several phases of monolayer ethylene on single-crystal graphite have been studied using low-energy electron diffraction. We have determined the unit cell and orientation with respect to the graphite substrate of the orientationally ordered and disordered low-density phases (OLD and DLD), in which molecules are believed to lie with the C?C bond parallel to the surface. Based on published neutron-scattering results, we propose a basis for the OLD phase. The low-coverage DLD phase, believed to undergo a continuous melting transition at 70 K, appears to be a higher-order commensurate structure at this transition. We also report observations of ordered and disordered high-density phases in which molecules stand on end, in both commensurate and incommensurate epitaxies.