ANISOTROPY IN THE ADSORPTION OF H2O AT LOW COORDINATION SITES ON PT(111)

The H2O adsorption on Pt(111) at 140 K is investigated by temperature- variable scanning tunneling microscopy. H2O adsorbs preferentially at the upper side of step edges. At these low coordination sites the adso rbates are bound stable as quasi-one-dimensional chains up to 160 K. I n contrast, the desorption from the two-dimensional H2O islands on the terrace has already started at 145 K. The occupancy of the sites at t he upper side of step edges is different at the two types of dense pac ked steps. This adsorption anisotropy is correlated with the different dipole moment of the two types of steps.