Orientational order in nitrogen monolayers adsorbed on graphite at low temperature
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Elastic neutron diffraction has been used to study the structure of N2 films adsorbed on the (0001) surfaces of an exfoliated graphite substrate at coverages CTHETA between 1.0 and 1.67 layers and at temperatures below 11 K. For CTHETA=1.0, the diffraction patterns can be fit by the in-plane rectangular √3 ×3 herringbone structure denoted C (commensurate), previously inferred from low-energy electron diffraction (LEED) experiments. Analysis of the relative Bragg-peak intensities in the neutron diffraction pattern of the C phase extends the LEED results by yielding a value of φ=45°±5° for the angle between the N—N bond and the short axis of the unit cell. Also, we find a substantially smaller Debye-Waller factor for this phase than previously inferred from x-ray experiments. At coverages CTHETA=1.13 and 1.27, the diffraction patterns are consistent with the uniaxial incommensurate (UI) phase seen by LEED. The patterns can be fit with the same molecular orientational parameters as for the C phase. We find the compression of the monolayer to be complete at CTHETA=1.67 where the film density is ∼10% greater than for the C phase. Fits to the diffraction pattern at this coverage indicate a slight oblique distortion of the unit cell from hexagonal symmetry. For this nearly triangular incommensurate (TI) phase, we obtain orientational parameters in the ranges 30°<φ<45° and 10°<β<20° where β is the tilt angle of the N—N bond with respect to the surface. At coverages between CTHETA=1.27 and 1.40, there is evidence of coexistence of the UI and TI phases.
Phys. Rev. B 35, 5841-5848 (1987)