Local structure determination for low-coverage CO on Ni(111)

Scanned-energy-mode photoelectron diffraction has been used to determine the local adsorption structure of CO on Ni(lll) at low temperature at a coverage of approximately 0.25 Mi,, corresponding to a nominal (2 x 2) phase. CO molecules are found to occupy the same two ('hcp' and 'fcc') inequivalent hollow sites as in the 0.5 ML c(4 x 2)-CO phase, and while the results do indicate a preference for the hcp hollow site directly above a second-layer Ni atom in the low-coverage state, this preference (65 +/- 20%) is of marginal statistical significance. The similarity of this parameter in both the (2 x 2) and c(4 x 2) phases can only be reconciled by assuming significant local disorder. It does, however, indicate that the local binding energies at the two hollow sites are very similar and that the presence of both sites in the c(4 x 2) phase is not simply a consequence of near-neighbour adsorbate-adsorbate repulsion.