Adsorption of simple molecules on graphite: A computer simulation study

Abstract

<p>The structure and dynamics of monolayers of the simple hydrocarbon molecules CH₄, C₂H₄ and C₂H₆, physisorbed on graphite, have been explored by computer simulation, along with the behavior of corrugated bilayer systems of the dipolar molecules CH₃F and CH₃Cl on Xe/graphite.</p> <p>The simulations found that the corrugation of the surface can have a strong effect on the monolayer structures of these molecules. The experimentally observed phases of C₂H₆ and CH₃F were found to be stabilized by the surface corrugation, though C₂H₄ and CH₃Cl were not. The structure of CH₃Cl/Xe/graphite was found to be an array of vertically aligned anti-ferro ordered pairs of molecules arranged in rows on the surface, while CH₃F/Xe/graphite formed zig-zag chains lying flat on the surface.</p> <p>The dynamics of these layer systems was also examined. In each case, the calculated power spectra agreed well with experiment, and showed strong evidence for translation-rotation coupling in the monolayers. The motions of CH₃F and CH₃Cl were particularly coupled to the motions of the underlying Xe layer.</p> <p>Simulations of the melting of ethylene suggest a novel mechanism: the molecules create locally enhanced density fluctuations necessary for melting by standing up from the surface, smearing the transition, and making it appear continuous.</p> <p>Calculated diffusion constants in the liquid phase of methane on graphite agreed well with experiment. Similar calculations of liquid ethylene did not agree as well with experiment, possibly because of an inadequate potential.</p>