The structure of the Pd(110)(2×1)-CO surface

The structure of the Pd(110)(2×1)-CO ordered adsorption phase has been determined by scanned-energy mode photoelectron diffraction. The CO molecules are adsorbed close to short-bridge sites with alternating tilts along the close-packed 〈110〉 surface rows. This local geometry is consistent with that found in previous theoretical total energy calculations and an earlier X-ray photoelectron diffraction study, but is in direct contradiction to the results of an earlier quantitative low energy electron diffraction investigation. While the best-fit model structure involves some twist of the CO molecules out of the 〈100〉 mirror planes of the surface creating a surface phase of p1g1 symmetry, the more symmetric p2mg falls within the estimated limits of precision of the analysis.