Adsorption of CO on Ni_3Al(111) investigated using high-resolution photoemission spectroscopy and density functional theory

摘要

The adsorption of CO on Ni_3Al(111) has been studied using high-resolution photoemission spectroscopy and density functional theory. Despite the fact that CO binds to Ni dominated sites only at this surface, CO adsorption induces a shifted contribution in the Al 2p core-level spectra. This contribution moves toward higher binding energy upon increasing CO coverage. The calculations give Al 2p core-level binding energy shifts in good agreement with the experimental values and show that adsorption of CO in the Ni sites induces core-level binding energy shifts for nearby Al atoms located in the two outermost surface layers. The surface Al atoms relax inward upon CO adsorption. At low CO coverage only one peak is observed in the C 1s spectra. This contribution is assigned to CO adsorbed in Ni threefold hollow sites. The calculations predict that CO adsorbs in the hollow sites for coverages up to 0.50 ML with a strong preference for the hcp site above a second layer Al atom at low coverage. At higher CO coverage, an additional contribution appears in the C 1s spectra whereas the other contribution shifts toward higher binding energies. The theoretical results suggest that this behavior is originating from the occupation of Ni on top and Ni bridge sites in addition to hollow sites.