STRUCTURES AND PHASE-TRANSITIONS IN PHYSISORPTION - MOLECULAR-AXIS ORIENTATIONAL ORDERING IN NITROGEN AND CARBON-MONOXIDE MONOLAYERS ON GRAPHITE

Molecular axis-orientational ordering occurs at temperatures below 40 K for monolayers of molecular nitrogen and carbon monoxide physisorbed on the graphite basal plane. The method by which the type of structure has been inferred from the symmetries observed in low-energy electron diffraction (LEED) measurements is briefly reviewed. The information available about the molecular-axis orientational order-disorder transitions from LEED, neutron diffraction, X-ray diffraction, and heat capacity measurements is reviewed and quantatively compared with theoretical expectations.