Molecular-dynamics study of the dynamical excitations in commensurate monolayer films of nitrogen molecules on graphite: A test of the corrugation in the nitrogen-graphite potential

The dynamical excitations in a commensurate monolayer solid of ${\mathrm{N}}_2$ molecules adsorbed on graphite have been studied using molecular-dynamics simulations. Velocity and rotational correlation functions as well as coherent intermediate scattering functions and dynamical structure factors have been calculated at several temperatures and wave vectors. Effects of out-of-plane motions and the formation of pinwheel-like structural elements associated with the orientational-disorder transition are observed both in the equilibrium order parameters and in the time-correlation functions. The calculated temperature dependence of the Brillouin-zone-center frequency gap in the acoustic phonon branches up to the melting point agrees well with the experimental observations and gives strong support to a model of the substrate corrugation energy. Diffusive motion in the fluid just above the melting point has been demonstrated from the time dependence of the mean-square displacement of the molecules and the two-dimensional diffusion coefficient is estimated to be 2–3×${10}^{\mathrm{-}5}$ ${\mathrm{cm}}^2$/s at 75–80 K.