Neutron Scattering Study of Nitrogen Adsorbed on Basal Plane Oriented Graphite

Thermal-neutron scattering has been used to investigate the structure of nitrogen films adsorbed on Grafoil, a basal-plane-oriented graphite. Diffraction scans were made at coverages between 1/3 of a monolayer and 7/4 monolayers over a temperature range from 10 to 90 K. The observed line shapes were analyzed and found to be generally consistent with the predictions of two-dimensional diffraction theory. From the data at least three distinct surface phases can be identified: (i) a low-temperature epitaxial phase with a triangular lattice structure, (ii) a more compressed low-temperature phase with the same structure but not in registry with the substrate, and (iii) one or more high-temperature disordered phases. Changes in the slope of the adsorption isotherm are found to correlate well with the observed transitions from one phase to another.