Adsorption of CO on Sm-modified Ru(001): formation of unusual adsorption states of CO

The adsorption behavior of CO over a Sm-preadsorbed Ru(001) surface was studied using photoemission spectroscopy. In addition to the usual CO adsorption upon Ru(001), both more tight and very weak adsorption states of CO appeared at about 105 K. The weakly bound CO was found to yield exceedingly high binding energy (BE) emissions in both core and valence regions compared to CO adsorption on metal surfaces. It is likely that the tightly bound CO attractively interacts with the Sm atom, resulting in the formation of Sm + CO islands over the Ru(001) substrate. This two-dimensional Sm + CO composite produces an adsorption site for the weakly bound state of the CO molecule.