Photoemission study of CO adsorption on ordered Pb ∕ Ni ( 111 ) surface phases

$\mathrm{Pb}∕\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}(111)$ surface phases were investigated by synchrotron radiation photoemission and low-energy electron diffraction. For room temperature deposition of Pb, two surface ordered layers, $(3\ifmmode\times\else\texttimes\fi{}3)$ and $(4\ifmmode\times\else\texttimes\fi{}4)$, were observed. The $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}5d$ and $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}4f$ core levels as well as valence band spectra indicated a weak chemical interaction between Pb and Ni, and the formation of a close-packed overlayer with Pb atoms in two different adsorption sites. Annealing of the $\mathrm{Pb}∕\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}(111)$ surface led to the formation of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction, characterized by a topmost layer consisting of a substitutional alloy. The transformation to the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure was accompanied by the appearance of a strong photoelectron diffraction effect, confirming embedding of Pb atoms in the $\mathrm{Ni}(111)$ first surface layer. $\mathrm{CO}$ adsorption results showed that lead simply blocked the $\mathrm{CO}$ adsorption sites for the unannealed surface while the surface alloy exhibited a chemical effect of weakening of the $\mathrm{CO}\text{\ensuremath{-}}\mathrm{Ni}$ bond. The $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}5d$ core-level shift indicated charge transfer from Pb to the surface, particularly for $(4\ifmmode\times\else\texttimes\fi{}4)$ and $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structures.