Coadsorption of Cs with O and CO on Ru(0001): relation between structural and electronic properties

We have studied structural, electronic and energetical aspects of ordered overlayers which were prepared by depositing Cs and CO on the (2 x 2)-O-precovered Ru(0001) surface. This ternary system may serve as a model system to study the Cs promoted CO oxidation reaction over Ru(0001), The electronic properties of the Cs subsystem, in particular the delocalized character of the Cs-6 s state, are only little affected by the (2 x 2)-O overlayer, while the addition of CO leads to a demetallization of the Ca overlayer. The ternary system reveals two metastable configurations (besides a stable one) which are produced at low sample temperatures, In the first one, Cs and CO have changed their adsorption sites if compared to the stable ternary (2 x 2)-O+Cs+CO phase, i.e. Cs sits in hcp and CO in on-top position. Our studies emphasize the importance of the threefold hollow position of CO to strengthen the back bonding of CO. The surface structure of the second metastable (ternary) arrangement is still elusive. (C) 2000 Elsevier Science Ltd. All rights reserved.