COADSORPTION OF K AND CO ON CU(111) SURFACES

It is well established that for submonolayer coverages the strong interaction between CO + K is not only influenced by the adsorptives but also by the substrate itself. We report about investigations of CO + K on the Cu(111) surface, where the CO/Cu(lll) interaction is rather weak. The analytical techniques employed include thermal desorption spectroscopy, LEED and work function change measurements. We find the interaction between K + CO on Cu(lll) to be very complex. Three different regimes of CO/K interaction can be distinguished. (1) At very low theta(K) (0.0-0.02) TDS measurements show a shift of the CO desorption maximum (for low CO coverages) towards lower temperature. Delta phi indicates a decrease and finally a vanishing of the effective negative CO dipole moment. (2) At theta(K) = 0.02-0.155 a new CO desorption maximum appears, shifting continuously from 165 to 270 K with increasing K coverage. Delta phi indicates a positive effective dipole moment (increasing 2 pi* backdonation). Between theta(K) = 0.13 and 0.18 around 300 K a very sharp and highly asymmetric CO desorption peak can be detected with no coincident K desorption. (3) At theta(K) > 0.2, where the transition from "ionic-" to "metal-like" K occurs, the strong interaction between CO/K is evidenced by a thermal stabilization of the adlayer to temperatures > 700 K. tinder these conditions several additional desorption peaks are detected between 400 and 800 K. Desorption of CO2 was detected under these theta(K) conditions, probably due to a partial dissociation and recombination reaction.