Structure determination of an alkali metal-CO coadsorption phase: Ni(111)-K/CO.

The structure of a Ni(111)- $(2\ifmmode\times\else\texttimes\fi{}2)$--K $/n$CO $(n\ensuremath{\approx}2)$ coadsorption phase has been determined using K $2p$, C $1s$, and O $1s$ scanned energy mode photoelectron diffraction, and compared with results from pure CO and K adsorption phases having similar coverages. Coadsorbed K has little influence on the local hollow site geometry of the adsorbed CO molecules; the K remains in the atop site of the pure K layer, but the K-Ni bond length increases substantially (by $0.15\ifmmode\pm\else\textpm\fi{}0.05$ \AA{}), as does the Ni-Ni outer layer spacing. The results conflict with aspects of current interpretations of spectroscopic data from such systems.