H2O on Pt(111): structure and stability of the first wetting layer.

We study the structure and stability of the first water layer on Pt(111) by variable-temperature scanning tunneling microscopy. We find that a high Pt step edge density considerably increases the long-range order of the equilibrium √37 × √37R25.3°- and √39 × √39R16.1°-superstructures, presumably due to the capability of step edges to trap residual adsorbates from the surface. Passivating the step edges with CO or preparing a flat metal surface leads to the formation of disordered structures, which still show the same structural elements as the ordered ones. Coadsorption of Xe and CO proves that the water layer covers the metal surface completely. Moreover, we determine the two-dimensional crystal structure of Xe on top of the chemisorbed water layer which exhibits an Xe-Xe distance close to the one in bulk Xe and a rotation angle of 90° between the close-packed directions of Xe and the close-packed directions of the underlying water layer. CO is shown to replace H(2)O on the Pt(111) surface as has been deduced previously. In addition, we demonstrate that tunneling of electrons into the antibonding state or from the bonding state of H(2)O leads to dissociation of the molecules and a corresponding reordering of the adlayer into a √3 × √3R30°-structure. Finally, a so far not understood restructuring of the adlayer by an increased tunneling current has been observed.