Ethylene Monolayers on Graphite: a Low Energy Electron Diffraction Study

Several phases of monolayer ethylene on single crystal graphite have been studied using LEED. The unit cells 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, have been determined. The OLD phase at 15K is shown to have an incommensurate oblique unit cell with unit vectors of length 5.15 and 7.38A with an angle between them of 94.3^ circ and the short side rotated 5.6 ^circ from the graphite unit vector. Based on published neutron scattering results a basis is proposed for the OLD phase in which molecules are oriented in high symmetry directions of the substrate. This basis is not a herringbone structure as was predicted by molecular dynamics simulations and structure calculations. The DLD phase is shown to have an incommensurate triangular unit cell in agreement with earlier results. It is also shown that the DLD phase appears to under go an incommensurate-commensurate transition to a higher order commensurate structure between 40 and 55K. This transition was always observed before the onset of melting, which was previously believed to be continuous based on heat capacity and neutron scattering experiments. Based on symmetry arguments, it can be shown that for such a higher order commensurate phase, the melting transition cannot be continuous. Several high density phases in which molecules stand on end were observed. Commensurate phases included an orientationally disordered (surd3 times surd3)R30^circ, an orientationally ordered (surd3 times 2surd3)R30^ circ and possibly a (2 times 3)R30^circ phase. The rotation of the highest density incommensurate triangular phase was shown to lie close to that predicted by the symmetry principle of Grey and Bohr, and far from the expected Novaco -McTague angle. Details of the experimental methods are given including the LEED apparatus; computer controlled video image acquisition and analysis; a description of the computer controlled beam switch used to minimize irreversible damage caused by the electron beam; and a description of the analysis algorithm for off-normal incidence calculations.