Orientational order in nitrogen monolayers adsorbed on graphite at low temperature

Abstract

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 \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{}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 \ensuremath{\varphi}=45\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{} 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 \ensuremath{\sim}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\ifmmode^\circ\else\textdegree\fi{}<\ensuremath{\varphi}<45\ifmmode^\circ\else\textdegree\fi{} and 10\ifmmode^\circ\else\textdegree\fi{}<\ensuremath{\beta}<20\ifmmode^\circ\else\textdegree\fi{} where \ensuremath{\beta} 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.