Small-cluster analogs of CO adsorption on Cu/Ru(0001): Total-energy and carbon-metal stretch frequency calculations

Total energies as a function of carbon-metal-layer separation and carbon-metal perpendicular stretch frequencies are calculated for small clusters configured to be analogs for the chemisorption system CO on Cu/Ru(0001). The clusters (with 7--10 metal atoms) include analogs of CO/Cu(111) and CO/Ru(0001) with on-top and threefold binding sites. The latter are more stable in both cases. Mixed-metal clusters are also studied (i) with CO bound to Cu in proximity to Ru and (ii) with CO bound to Ru in proximity to Cu. The former, case (i), show enhanced CO binding with respect to CO on pure Cu, and the latter, case (ii), show weakened CO binding with respect to CO on pure Ru. Possible precursor intermediate states are described for commensurate and incommensurate epitaxy. Bonding to Ru sublayers through a Cu adlayer is also described. The results are discussed in conjunction with previous experimental findings, and anticipate future experimental vibrational characterizations.