Adsorption Sites in the High-Coverage Limit of CO on Cu(111)

The development of a comprehensive theory describing gas-surface interactions requires accurate experimental data for benchmarking. The adsorption of CO to Cu(111) is such a benchmark system. While state-of-the-art calculations still erroneously predict the favored adsorption site at low coverage to be the 3-fold hollow site, experimental studies have not yet definitively identified adsorption sites for all overlayer structures. Using a new combination of well-established techniques, we have reinvestigated CO adsorption, its ordering, and desorption on Cu(111). Our results support earlier suggestions for various overlayer structures for different coverages. For the intermediate-coverage 1.5×1.5 structure, we show that only on-top sites are occupied. Bridge site adsorption occurs only beyond a coverage of 0.42 monolayer (ML) in the 1.4×1.4 structure. Occupancy of this site is associated with a shift of atop CO molecules to off-centered atop with a characteristic IR absorbance frequency. These findings indicate a complex balance of coverage-dependent adsorbate interactions and binding energies that results in nonintuitive ordering and requires improvements in theory to understand.