Lattice parameter and orientation of xenon on graphite at low pressures

A study of xenon adsorbed on (0001) graphite has been performed using transmission high-energy electron diffraction (THEED). The single crystal diffraction patterns have been obtained at a range of pressures 1.8×${10}^{\mathrm{-}8}$<p<1.8×${10}^{\mathrm{-}5}$ Torr and temperatures 50<T<80 K, which covers the incommensurate monolayer, commensurate monolayer, and bilayer phases. The lattice-parameter misfit, m, has been measured to ±0.1%, and the misorientation θ to ±0.1° accuracy. The misfit in the incommensurate phase is parametrized as m=B(T-$T_0$ ${)}^{\beta }$, with β=0.80±0.03 and B and $T_0$ as functions of ${\log }_{10}$(p). A tricritical point (I-C monolayer, bilayer) is observed at p=2.0±0.5×${10}^{\mathrm{-}6}$ Torr, T=62.5±1 K. The small misorientation (0≤θ<0.4°) is a well-defined function of m in the monolayer phase, with an aligned-rotated transition at m=1.5±0.5%. These features are compared with other experiments and with theoretical predictions.