The properties of a two-dimensional water layer on MgO(001)

Elastic helium-atom scattering (HAS) has been employed to study the adsorption behavior of water on the in-situ cleaved MgO(001) surface at 100 less than or equal to T(crystal)less than or equal to 180 K. At these temperatures, water forms a dense phase which consists of orthogonal domains each with a c(4 x 2) unit mesh. No differences in the structures of the H2O and D2O low-temperature phases were observed. Heating of the crystal covered with the c(4 x 2) water phase to T-crystal>185 K results in the partial desorption of the water layer and the formation of a less dense p(3 x 2) water phase. A thermodynamic study of the high-temperature water phase using low-energy electron diffraction (LEED) to measure the equilibrium adsorption isobars and isotherms enabled us to determine the isosteric heat of adsorption Q(st)=85.3+/-2.1 kJ mol(-1) as well as the lateral interaction energy Q(1)=35.1+/-9.6 kJ mol(-1).