2-DIMENSIONAL WATER CONDENSATION ON THE NACL(100) SURFACE

The adsorption of water on the NaCl(100) surface has been studied by UPS, XPS and LEED, using well-ordered films of NaCl which were grown heteroepitaxially on Ge(100) substrates. In the investigated range of temperature and partial pressure (155-135 K and 1 x 10(-4)-1 x 10(-7) Pa), a first-order phase transition is observed, resulting in the formation of a physisorbed two-dimensional H2O phase (isosteric heat of adsorption q(st) = 65 kJ/mol). This condensed 2D phase is well ordered, the arrangement of H2O molecules can be described by a c(4 x 2) symmetry in two domains, which are rotated by 90-degrees against each other. According to this symmetry, a structural model is proposed with an ice-like, distorted "bilayer"-structure. Additionally, a phase transition from the 2D H2O phase to a three-dimensional solid ice is observed. This solid ice shows a polycrystalline state of order.