Abstract
The first Low-Energy Electron Diffraction (LEED) intensity analysis of molecular adsorption in multiple sites is presented. A 3 4 monolayer CO overlayer on Rh(111) is found to involve one bridge site and two near-top sites in each unit cell, which supports the bridge and top assignment based on earlier High-Resolution Electron Energy Loss Spectroscopy (HREELS) data. The near-top site is asymmetrical, with the CO axis close to the surface normal (but possibly tilted about 5°), yielding a bent Rh-C-O species. The near-top CO molecules are forced sideways from the ideal top site by ~ 0.53 Å due to bridge-bonded CO molecules located at a distance of 2.85 Å. This short CO-CO separation is somewhat smaller than corresponding intermolecular separations found in metal carbonyl clusters and crystalline CO and CO 2. The resulting CO overlayer has a hexagonal geometry that is distorted both parallel and perpendicular to the surface due to the strong metal-CO bonding. The Zanazzi-Jona and Pendry R-factors for the best structure are 0.25 and 0.47, respectively. The LEED analysis exhibits the use of different levels of approximation in the multiple scattering theory in order to show the value of approximate but fast calculations in preliminary structural searches. A new algorithm to obtain experimental I– V curves from digitized images of LEED patterns is described.
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