Adsorption of sodium and potassium on a gold(100) surface: An example of alkali-metal-induced deconstruction

Abstract

We have studied the interaction of Na and K with a (5\ifmmode\times\else\texttimes\fi{}20) reconstructed Au(100) surface between 130 and 1150 K using low-energy-electron-diffraction (LEED), x-ray-photoelectron spectroscopy (XPS), work-function change (\ensuremath{\Delta}\ensuremath{\Phi}), and thermal-desorption spectroscopy (TDS) measurements. Our results indicate a strong temperature dependence of the interaction phenomena: At 300 K, we find for both Na and K adsorption the lifting of the inherent (5\ifmmode\times\else\texttimes\fi{}20) reconstruction at a critical coverage ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{crit}}$\ensuremath{\approxeq}0.2 ML, and, at somewhat larger \ensuremath{\Theta}, a thermally activated transformation to a (likewise reconstructed) (1\ifmmode\times\else\texttimes\fi{}2) phase of the missing-row (MR) type. Higher coverages lead to different LEED structures depending on the alkali metal and the deposition temperature. All structural changes are sensitively mirrored in the \ensuremath{\Delta}\ensuremath{\Phi}-\ensuremath{\Theta} relations, which reveal intermediate work-function maxima right at ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{crit}}$. These maxima are superimposed on the typical descending \ensuremath{\Delta}\ensuremath{\Phi}-\ensuremath{\Theta} curves usually observed with alkali-metal adsorption on metal surfaces (\ensuremath{\Delta}${\mathrm{\ensuremath{\Phi}}}_{\mathrm{m}\mathrm{a}\mathrm{x},\mathrm{N}\mathrm{a}}$=-1.9 eV; \ensuremath{\Delta}${\mathrm{\ensuremath{\Phi}}}_{\mathrm{m}\mathrm{a}\mathrm{x},\mathrm{K}}$=-2.6 eV). At 130 K, the formation of the (1\ifmmode\times\else\texttimes\fi{}2) MR reconstructed phases cannot be observed anymore, and accordingly no intermediate \ensuremath{\Delta}\ensuremath{\Phi} maxima appear.The Na and K binding states as mapped by TDS exhibit an extremely strong coverage dependence and extend over the remarkable T range of \ensuremath{\sim}700 K. Within the monolayer coverage at least three different binding states appear that reflect strong lateral interactions between neighboring alkali-metal atoms and marked changes in the alkali-metal---substrate bonding character: At low coverages, the ionic contribution dominates. There follows an ionic-to-neutral transition at medium coverages, and close to the monolayer the alkali-metal atoms clearly possess metallic character. The same conclusions can be drawn from our XPS data, which exhibit a clear core-level shift in the medium-coverage range associated with the ionic-to-neutral transition. The results are discussed and compared with other data recently obtained with Na and K interactions with noble-metal surfaces.