The orientation of benzene on bimetallic surfaces

Angle-resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation is used to determine the orientation of benzene molecules on two modifications of the pseudomorphic Ni/Cu(lll) layer, the Ni-terminated adlayer and the Cu-terminated Ni sublayer. It is found that the molecules adsorb with their C-H bonds 30 degrees off the close-packed substrate rows (sigma(v) orientation) on the Ni adlayer whereas no preferential orientation was found for the Cu-terminated Ni sublayer. For these and other close-packed mono- and bimetallic surfaces involving Cu, Ni and Ru, the correlation between adsorption geometry and reactivity of benzene is explored in connection with temperature-programmed desorption (TPD). Like for the Ni/Cu(lll) sublayer, no preferential lateral orientation and a weak bond is found on most other Cu-terminated surfaces at 80 K. This goes along with the absence of any dissociation on these surfaces. Only on the stretched 1 ML Cu/Ru(0001) surface is the substrate-benzene bond strong enough to induce a clear preferential orientation (sigma(d) with C-H bonds parallel to the close-packed rows) but still too weak to induce dissociation. The same orientation was found for the saturated benzene layer on Ni(lll) where the molecules also do not dissociate. Dissociation is, however, observed for the saturation coverage of benzene on the Ni/Cu(lll) adlayer and for low benzene coverages on Ni(lll). On these surfaces the azimuthal orientation of the benzene molecules is sigma(v), i.e. rotated by 30 degrees with respect to the close-packed rows.