Motion in surface layers of N2 on graphite

Abstract

A molecular-dynamics simulation of nitrogen adsorbed on graphite has been used to study the phonons and librons in a molecular adsorbate. Simulations of the commensurate-ordered (16 K), uniaxial (14 K) and commensurate-disordered (40 K) phases of the adsorbed solid are analysed. In addition results on a commensurate simulation in the vicinity of the orientational phase transition (25 K) are also reported. We find significant dispersion of the translational phonons in two orthogonal directions. There is clear evidence of coupling between librational and translational phonons which is responsible for the structure in the librational density of states. The simulation shows the longitudinal phonons inducing rotational disorder in the low-temperature phase and the transverse phonons inducing rotational ordering in the high-temperature phase. These couplings are not strong enough to produce an incommensurate wave of static distortion in the overlayer.