Reorientation of N₂ adsorbed on graphite in various computer simulated phases

Abstract

The reorientational motion of nitrogen molecules adsorbed on graphite was investigated in simulations of four phases. In the low temperature orientationally ordered commensurate and uniaxial phases the amplitude of reorientation is small. The out-of-plane motion is nearly harmonic. The in-plane motion is somewhat more complex, but we show that it can be related to the relevant angular velocity autocorrelation function via a cumulant expansion. The complexity is due to interactions between nitrogen molecules which do not affect the out-of-plane motion. In the simulations of the orientationally disordered phases the amplitudes of reorientation are larger and the simple cumulant approximation is no longer valid. The out-of-plane motion, while still harmonic, is more strongly damped, and the in-plane motion shows features similar to those seen in liquids. There is surprisingly little qualitative difference between the reorientational motion observed in the commensurate and uniaxially compressed phases at corresponding temperatures.