The electronic structure and molecular symmetry of pure benzene and benzene coadsorbed with CO on Ni(111)

Benzene adsorption on Ni(111) was studied by ARUPS using linearly polarized synchrotron radiation at various benzene coverages and for benzene coadsorbed with CO. Overall the electronic structure of chemisorbed benzene is found to be very similar for the different benzene coverages and in the presence of coadsorbed CO: only small shifts in the electronic binding energies are observed which are attributed to electrostatic initial state changes. Using dipole selection rules the orientation of the adsorbed benzene molecules is determined to be parallel to the surface for all layers investigated. The molecular symmetry is reduced to C_3v in all cases. The photon energy dependence of the photoionization cross section shows a pronounced resonance of the 2e_1u level at a photon energy of 25 eV (E_kin ≈ 13 eV); the corresponding electron emission is centered along the surface normal. This resonance is attributed to a shape resonance into an a₁ final state.