Surface adsorption structures in 1-octanethiol self-assembled on Cu(111)

A normal incidence X-ray standing wave (NIXSW) and near-edge X-ray absorption fine structure (NEXAFS) study was performed on saturated monolayers of 1-octanethiol on Cu(111). NEXAFS around the S K-edge taken in normal and grazing incidence revealed that the S-C bond was oriented with respect to the Cu(111) surface to give a calculated orientation of the pendant hydrocarbon which was almost normal to the surface, similar to self-assembled alkanethiols on gold surfaces. Modulation in the S(1s) photoemission with variation in the energy of the X-ray standing wave about the (111) and ((1) over bar 11) Bragg energies was modelled to yield values of structural parameters: the coherent position, d(hkl), and coherent fraction, f(hkl), for the adsorbate with respect to the bulk Cu lattice planes. For the (111) reflection, d(111)=1.19+/-0.10 Angstrom, f(111)=0.70+/-0.03 but for the ((1) over bar 11) reflection, f((1) over bar 11) was close to zero (f((1) over bar 11)=0.10+/-0.05) and d((1) over bar 11) was indeterminate. The coherent fraction for the Cu substrate was high for both reflections studied (f(111)=0.95+/-0.03; f(111)=0.92+/-0.02). The structural parameters are rationalised in terms of penetration of the Cu by thiolate sulfur atoms to cause a surface layer reconstruction. The resultant adsorbate structure is either incommensurate or else consists of a large-mesh commensurate superlattice in which the S atoms exist at a number of different positions with respect to the ((1) over bar 11)planes of the bulk. (C) 1997 Elsevier Science B.V.