PHASE-TRANSITIONS OF ETHYLENE MONOLAYERS ON GRAPHITE

Low-energy electron diffraction (LEED) measurements have detected seven solid phases for monolayer ethylene on graphite. Electron-beam induced effects (which presumably include decomposition and/or polymerization of the adsorbed ethylene) were minimized by exposing the adsorbed layer to the nanoamp beam for < 100 s per crystal; data were obtained from four different crystals. We concentrate here on the phase transitions from the molecular-axis-orientationally disordered low density (DLD) phase to the newly discovered commensurate molecular-axis-disordered low density (CDLD) phase and on the melting of the CDLD phase. The change in the principle q vector in the DLD-CDLD phase transition is presented for several different coverages. These data indicate an interesting incommensurate-commensurate (IC-C) transition which has not been suspected from previous studies. This transition preempts the melting transition: as a consequence we can rule out the possibility of a continuous melting transition as suggested by earlier heat capacity and neutron scattering experiments. In addition, LEED images of the fluid phase produced by melting of the CDLD phase are presented which show a modulated-ring-type structure.