Molecular orbital study of the interaction of carbon monoxide and carbon dioxide with copper (100)

A molecular orbital study is made of the structures and energy levels of CO and CO₂ on Cu(100). The importance of self-consistency is discussed. CO is found to occupy fourfold indentations, in agreement with the semiempirical results of Doyden and Ertl. The C-surface distance is 1.0 Å and the CO bond stretches less than 0.1 Å. Large models of the surface show convergence of the electronic structure with four CO molecules on a twelve Cu atom cluster model of the surface. At coverages up to c(2 × 2) half monolayer, calculated energy levels match Demuth and Eastman's phase I photoemission spectrum. Phase II, as observed by them, has no analog in the calculations. No evidence is found for CO deviating from perpendicular to the surface when tightly bound. CO₂ is found to adsorb more weakly to the surface. This molecule rests in a μ bridging position, bonded through mixing of Cu d with CO₂ π¹ orbitals. It bends, with an angle of 120°, which is significantly similar to 122° for the ¹B₂ excited state of free CO₂, which has an electron promoted to the π¹ orbital. On the basis of this molecular orbital study, extrapolations to Ni and Zn surfaces and OCS, CS₂, CSe₂ and CTe₂ are made.