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

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 K for both C6H6 and C6D6 a well ordered (root 7 x root 7) R19.1 degrees 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 degrees, occurs for C6H6 and C6D6. In contrast to these results, an isotopic effect is found for adsorption at 125 K. While the adsorption behavior for C6D6 is similar as at 200 K, for C6H6 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 C6H6 is induced, starting around 180 K. A simple explanation for the observed behavior is proposed.