Electronic structure, orientation and symmetry of benzene and benzene coadsorbed with CO and NO on NI(111) and RU(001)

The adsorption of benzene on Ni(111) and Ru(001) was studied by ARUPS using linearly polarized synchrotron radiation for several pure benzene layers and for benzene coadsorbed with CO or NO. On both surfaces the benzene molecules are adsorbed parallel to the surface; the symmetry is reduced to C_3v for Ni(111) and to C_6v for Ru(001), in both cases independent of benzene coverage and the coadsorption of CO or NO. Overall the electronic structure of chemisorbed benzene is very similar for different benzene coverages and coadsorption of CO: only small uniform shifts in the electronic binding energies are observed which are attributed to electrostatic initial state changes. In contrast to CO, coadsorbed NO leads to differential shifts of the benzene molecular levels, indicating some additional changes in bonding. A pronounced maximum in the photoionization cross section of benzene adsorbed on Ni (111) at h{stroke}ω = 25 eV with the electron emission centered along the surface normal is attributed to a molecular shape resonance into an a₁ final state.