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 (111) 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(111) to be very complex. Three different regimes of CO K interaction can be distinguished. (1) At very low Θ K (0.0–0.02) TDS measurements show a shift of the CO desorption maximum (for low CO coverages) towards lower temperature. ΔΦ indicates a decrease and finally a vanishing of the effective negative CO dipole moment. (2) At Θ K = 0.02−0.155 a new CO desorption maximum appears, shifting continuously from 165 to 270 K with increasing K coverage. ΔΦ indicates a positive effective dipole moment (increasing 2π∗ backdonation). Between Θ 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 Θ 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. Under these conditions several additional desorption peaks are detected between 400 and 800 K. Desorption of CO 2 was detected under these Θ K conditions, probably due to a partial dissociation and recombination reaction.