Orientational order in nitrogen monolayers adsorbed on graphite at low temperature

Elastic neutron diffraction has been used to study the structure of ${\mathrm{N}}_2$ 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.