Anisotropy in the Adsorption of ${\mathrm{H}}_2$O at Low Coordination Sites on Pt(111)

The ${\mathrm{H}}_2$O adsorption on Pt(111) at 140 K is investigated by temperature-variable scanning tunneling microscopy. ${\mathrm{H}}_2$O adsorbs preferentially at the upper side of step edges. At these low coordination sites the adsorbates are bound stable as quasi-one-dimensional chains up to 160 K. In contrast, the desorption from the two-dimensional ${\mathrm{H}}_2$O islands on the terrace has already started at 145 K. The occupancy of the sites at the upper side of step edges is different at the two types of dense packed steps. This adsorption anisotropy is correlated with the different dipole moment of the two types of steps.