Structure and thermodynamics of acetylene C2H2 on MgO(100)

The structure and thermodynamics of C2H2 monolayers adsorbed onto MgO(100) single crystal surfaces have been studied by LEED experiments between 40 and 94 K. The surface was prepared by cleaving a crystal in situ under ultrahigh vacuum conditions. The monolayer presents an ordered commensurate (2 X 2) structure. We propose a unit cell containing two molecules and having two glide planes. Furthermore, adsorption isotherms and adsorption isobars were performed in order to establish a phase diagram and to determine the isosteric heat of condensation at half monolayer coverage.