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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.
Kjems et al. (Sun,) studied this question.