The role of bond orientational order and the nature of the melting transition in an incommensurate physisorbed monolayer: argon on graphite at completion

A constant-temperature, constant density molecular-dynamics (MD) method is utilized to study the behavior of bond-orientational, translational and thermodynamic parameters in the solid, melting transition and fluid regimes of argon physisorbed onto graphite (Ar/gr) at complete coverage. For Ar/gr, completion occurs near r=1.26 in units of 0.0636 atoms/Å2, where r=1 corresponds to the complete E3×E3 R30° commensurate adlayer. Several bondorientational and newly introduced pair-distribution order parameters and effective densities prove especially useful in helping evaluate the temperature behavior of the system’s structural order. A broad melting transition accompanied by vacancy formation through large vertical fluctuations in the adsorbate atoms is observed at Tm=135±5 K, spanning a temperature range of roughly 40 K. The melting transition accompanies a rapid formation of a lowdensity second layer. Substantial bond-orientational order persists into the high-temperature fluid (T#300 K), which is not hexatic in nature and which exhibits infrequent desorption from the first layer. The behavior and interaction of the neighbor shells are compared and contrasted to those observed in Kr/gr. © 1999 Elsevier Science B.V. All