Orientational Ordering of Nitrogen Molecular Axes for a Commensurate Monolayer Physisorbed on Graphite

Low-energy electron diffraction has been used to study the orientational ordering of the molecular axes of nitrogen molecules physisorbed on graphite single crystals. A 2 × 1 superlattice structure with in-plane orientations is inferred for the monolayer where the molecular centers have the commensurate ($\sqrt{3}\times \sqrt{3}$) 30° structure. The superlattice intensity decreases significantly near 30 K but persists up to 40 K, consistent with a first-order transition with finite-size effects.