Chemical-shift X-ray standing wavefield determination of the local structure of methanethiolate phases on Ni(1 1 1)

By monitoring the X-ray absorption through the chemically-shifted components of the S 1s photoemission signal, normal-incidence X-ray standing wavefield absorption at the (1 1 1) and (1̄ 1 1) scatterer planes has been used to determine the local adsorption geometry of the two distinct methanethiolate (CH3S–) species which occur on Ni(1 1 1) following exposure to methanethiol. The species which is favoured at low temperatures is found to occupy either mixed hollow or bridge sites on a non-reconstructed Ni(1 1 1) surface, whereas that seen at higher temperatures is shown to involve Ni surface layer reconstruction and the data are consistent with hollow site adsorption on a reduced density outermost Ni layer. The relative merits of alternative reconstruction models based on that which occurs due to methanethiolate adsorption on Cu(1 1 1), or the (5√3×2)rect. phase formed by atomic S on Ni(1 1 1), are discussed. Both of these models are based on local square or `pseudo-(1 0 0)ʹ outermost Ni layers. Co-adsorbed atomic sulphur, to which the methanethiolate species decompose at higher temperatures, appears to occupy mainly fcc hollow sites at low temperatures, but is partially converted to the local geometry of the ordered reconstructed (5√3×2)rect.-S phase after higher temperature annealing.