Site selectivity of benzene adsorption on Ni(111) studied by high-resolution x-ray photoelectron spectroscopy

Abstract

The adsorption and thermal evolution of deuterated and nondeuterated benzene on Ni(111) has been studied by in situ high-resolution x-ray photoelectron spectroscopy. The recorded C $1s$ spectra show vibrational fine structure due to vibrational excitations of the C-H bonds in the ionic final state. Furthermore, different adsorption sites can be identified, since carbon atoms within the same molecule, but with different positions relative to the substrate have different C $1s$ binding energies. At $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for both ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$ a well ordered $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7)}$ $R19.1\ifmmode^\circ\else\textdegree\fi{}$ superstructure is observed at saturation of the chemisorbed layer, with a nominal coverage of 0.143 ML. From a quantitative analysis of the XP spectra collected during adsorption one finds that up to a coverage of 0.10 ML benzene occupies only bridge adsorption sites. At higher coverage, a change to hcp hollow sites, accompanied by a rotation of the molecules by 30\ifmmode^\circ\else\textdegree\fi{}, occurs for ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ and ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$. In contrast to these results, an isotopic effect is found for adsorption at $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. While the adsorption behavior for ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$ is similar as at $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ no change in adsorption site is observed, i.e., up to saturation of the chemisorbed layer bridge sites are occupied. Only upon heating, a site change from bridge to hcp hollow of the adsorbed ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$ is induced, starting around $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A simple explanation for the observed behavior is proposed.