Mix of Molecular Adsorption Sites Detected for Disordered CO on Pt(111) by Diffuse Low-Energy Electron Diffraction

Complete local-bonding geometries in a disordered molecular overlayer have been determined for the first time, together with the relative occupation of different coexisting adsorption sites. Diffuse-LEED intensities were measured with a new, highly sensitive, low-noise, digital-LEED detector, and calculated with an extension of the efficient beam-set-neglect theory. At one-third of a monolayer of carbon monoxide on Pt(111) at 160 K, (88 \ifmmode\pm\else\textpm\fi{} 5)% of the molecules are found to occupy top sites and (12 \ifmmode\pm\else\textpm\fi{} 5)% bridge sites, with bond lengths consistent with earlier results for ordered layers.