Remarkably long-ranged repulsive interaction between adsorbed CO molecules on Pt-modified Ge(001)

We have studied the adsorption of CO molecules on a Pt modified Ge(001) surface at 77 K. The CO molecules preferentially adsorb on the self-organized Pt chains rather than on the terraces. We have identified two different adsorption sites at 77 K: one either on the short-bridge site or on top of one of the atoms of a buckled Pt dimer, and the other one on the long-bridge site in between two Pt dimers. The CO molecules are mobile at room temperature but immobile at 77 K. A statistical analysis of the nearest-neighbor spacing of the CO molecules reveals that adsorbed CO molecules interact repulsively along the Pt-dimer chains. This repulsive interaction, composed of electrostatic terms and a strain-mediated term, gradually fades away over, for simple molecules, a remarkably long distance of 3–4 nm.