VIBRATIONAL COUPLING AS A PROBE OF ADSORPTION AT DIFFERENT STRUCTURALSITES ON A STEPPED SINGLE-CRYSTAL ELECTRODE

Adsorption of carbon monoxide at step and terrace sites on a Pt(557) = Pt(s)-[6(111) x (100)] electrode was detected with infrared spectrosc opy. Vibrational coupling between adsorbates provided insights into th e assembly of molecules at the different structural sites. The intermo lecular coupling was weak at low coverages as CO ordered along the ste ps. For coverages between 40 and 70% of saturation, separate bands ass ignable to CO on steps and CO on terraces appeared. Coupling across th is coverage range was markedly weaker on Pt(557) than on the structura lly related Pt(335) = Pt(s)-[4(111) x (100)] electrode surface. The re sults indicate that, after the steps fill, CO populates the terraces o n Pt(557) at random rather than by ordering in alignment with the step s. At coverages below saturation, vibrational bands assignable to CO m olecules at step and terrace sites are affected differently by changes in electrode potential. The potential-induced spectral changes for th e terrace CO bands are similar to those of Pt(111)/CO, but the step CO bands show deviations from this trend at hydrogen adsorption potentia ls.