Low-energy electron diffraction study of incommensurateH2, HD, andD2monolayers physisorbed on graphite

Incommensurate (IC) monolayer phases of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$ physisorbed on graphite single crystals are studied by low-energy electron diffraction (LEED). At temperatures near 5 K, a uniaxial IC (UIC) phase and a higher-density rotated IC (RIC) triangular solid phase are observed for monolayers of ${\mathrm{H}}_2$, HD, and ${\mathrm{D}}_2$. The commensurate (C) to UIC phase transition of the ${\mathrm{H}}_2$ monolayer is examined by measuring two-dimensional intensity contours of LEED spots at densities 1.0–1.09 times the C (√3 × √3 )R30° phase density. The orientation angles of the RIC phase approximately agree with the prediction of Novaco and McTague’s calculation for a weakly modulated two-dimensional Cauchy solid. The γ phase, which was first observed in heat-capacity measurements for ${\mathrm{D}}_2$ on graphite, has been determined to be an IC triangular rotated phase with rotation angle greater than that of the higher-density RIC phase. LEED data from the oriented fluid phases are also presented. A phase diagram for ${\mathrm{D}}_2$ is proposed based on LEED, heat-capacity, and neutron-diffraction measurements.