Benzene adsorption on a pseudomorphic Cu monolayer on Ni(111) - a combinedTPD and ARUPS study

The adsorption and desorption of benzene on a pseudomorphic Cu monolayer on Ni(111) has been investigated by temperature programmed desorption (TPD) a nd angle-resolved ultraviolet (UV) photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. The experimental results are com pared to those of an earlier investigation of benzene adsorbed on Ni(111). Benzene is weakly chemisorbed on the pseudomorphic Cu monolayer on Ni(111): Desorption of the first layer occurs in the temperature range between 155 and 230 K with no sign of dissociation; the analysis of the binding energie s of the molecular benzene levels reveals small bonding shifts of 0.2-0.30 eV of the benzene pi levels as compared to 1.4 eV for the strongly chemisor bed saturated benzene layer on Ni(111). The orientation of benzene on Cu/Ni (111) is determined from the ARUPS spectra in combination with symmetry sel ection rules. For low coverages, the molecular plane is oriented parallel t o the surface, as is also the case for benzene on Ni(111). However, no well defined azimuthal orientation is observed, which is attributed to frustrat ed rotations and/or the high mobility of the adsorbed molecules at the temp erature of investigation (80 K). For the saturated first benzene layer on C u/Ni(111), a tilting of the adsorbed molecules occurs due to lateral intera ctions. (C) 1999 Elsevier Science B.V. All rights reserved.