HREELS investigation of CO-K coadsorption on Ni(111)

The coadsorption of CO and K on Ni(111) has been studied by high-resolution electron energy-loss spectroscopy. We used three precoverages, one of them (0.25 ML) corresponding to an ordered surface overlayer (p(2 x 2)) and the other two (0.3 and 0.47 ML), to an incommensurate and a disordered surface layer, respectively. Different local CO adsorption arrangements are suggested for each K coverage. The vibrational spectra show features at 27, 180 and 210 meV. The loss at 27 meV is related to a vibration of the reconstructed Ni(111) surface due to the p(2 x 2)-K layer and is damped by 3 L of CO. For the other two K precoverages, that loss exists regardless of the amount of CO. The losses at 180 and 210 meV are assigned to the C-O stretching vibration corresponding, respectively, to the occupation of only one or both sites of the same p(2 x 2)-K cell. The CO saturation of the p(2 x 2)-K/Ni(111) surface gives rise to a short-range interaction among CO molecules as well as between each K atom and CO molecules, which causes an overall shift of the C-O stretching frequency towards higher loss energies.