The structure of the Cu(111)(root 7 x root 7)R19 degrees-S surface phase: a new normal-incidence X-ray standing wave study

The local geometry of the adsorbed sulphur atoms in the Cu(111)(root 7 x ro ot 7)R19 degrees-S surface phase has been investigated with normal-incidenc e X-ray standing wavefield (NIXSW) absorption using the (111), ((1) over ba r 11) and (200) Bragg reflections. The results are compared with the predic tions for previously proposed models based on earlier investigations using NIXSW [but only the (111) reflection], surface extended X-ray absorption fi ne structure, quantitative low energy electron diffraction, surface X-ray d iffraction (SXRD) and scanning tunneling microscopy. The best agreement is found for the model deduced from SXRD based on Cu4S adclusters, but for the specific overlayer substrate registry in which these clusters are centred atop an outermost substrate layer Cu atom. All of the other models show sig nificant inconsistencies with these new data. (C) 2000 Elsevier Science B.V . All rights reserved.