Interaction of acetylene molecules with the MgO(100) surface: LEED experiments and potential-energy calculations

Structural and thermodynamical low-energy electron diffraction experiments were performed on MgO(100) single crystals cleaved in-situ under ultra-high vacuum conditions. Acetylene adsorption experiments were conducted within the temperature range 40-94 K and the C2H2 pressure range 10(-9)-10(-6) Torr. We have determined experimentally a commensurate (2x2) herringbone structure containing two molecules and having two glide planes. Bulk condensation of acetylene has been studied and the latent heat of sublimation Delta H-sub=5.08+/-0.3 kcal mol(-1) determined. The isosteric heal of adsorption Q(st)=6.9+/-0.4 kcal mol(-1) and lateral interaction energy Q(l)=3.1+/-0.4 kcal mol(-1) have been measured and compared with semi-empirical potential calculations which detail the nature of the various contributions to these physical quantities. The stable adsorption sites are the Mg atoms, and the calculated geometry is in good agreement with our LEED observations. Moreover, the influence of the molecular motions on the isosteric heat of adsorption is evaluated to take temperature effects into account. (C) 1997 Elsevier Science B.V.