Simulation studies of bilayers of N 2 adsorbed on graphite at 25 and 35 K

Abstract

Abstract Molecular dynamics simulations of bilayers of model nitrogen molecules adsorbed on the graphite basal plane at 25 and 35 K are reported. Systems considered include commensurate bilayers and three uniaxially compressed films, namely, two with different compressions along the X -axis (perpendicular to the herringbone lattice glideline) and one with a compression along the Y -axis, relative to the commensurate spacing. For comparison, commensurate and X -compressed monolayer films were also simulated. Properties simulated include: center-of-mass (COM) molecule-surface densities as a function of separation distance: in-plane COM pair correlation functions; in-plane and out-of-plane distributions of the molecular axis orientations; average potential energies for N 2 -surface and for N 2 -N 2 interactions; and velocity time-correlation functions for translational and orientational motion perpendicular to and parallel to the surface. These results indicate that the N 2 layers at these temperatures are nearly harmonic oscillator solids, with considerable orientational freedom. Molecules tend to be coplanar with the surface in both layers. The in-plane orientations for the compressed films form a herringbone lattice, in both layers, with variable angles between the molecular axis and the crystal glidliines; the commensurate film does not show herringbone ordering for the potential model used in this work. It is concluded that both the orientational and translational the degrees of freedom strongly coupled, between layers as well as within the layer. It is also shown that the Y -compressed film is unstable and that both layers in the bilayer spontaneously rearrange into a (rotated) uniaxially X -compressed structure.