Electronic structure of benzene on Ni(100) and Cu(110): An x-ray-spectroscopy study

The valence electronic structure of benzene chemisorbed on Ni(100) and Cu(110) has been studied using angle-dependent x-ray emission and x-ray absorption spectroscopy. These techniques allow us to resolve the benzene p contributions involved in the chemical bond. Symmetry selection rules observed in resonant inelastic x-ray scattering are applied for the adsorbate case to identify the symmetry of new pi states formed in the chemical bond. Based on x-ray absorption results we conclude that benzene adsorbs with the molecular plane parallel to both surfaces. On Ni a new, third pi state is observed 1.8 eV below the Fermi level. Comparing resonant and nonresonant excitation its symmetry character is identified as being of e(2u), type. The corresponding pi*e(2u) x-ray absorption intensity is strongly reduced. This is attributed to a splitting of the previously unoccupied e(2u), orbital into bonding and antibonding states due to adsorbate pi-substrate 3d interaction leading to a pi backdonation bond. Moreover, a broad distribution of sigma-symmetric states is observed all the way up to the Fermi level, indicating that despite of rehybridization sigma states also contribute to the chemical bond. On Cu(110) in contrast a new, third pi state is cut by the Fermi level. This indicates a resonancelike broadening of the e(2u) orbital due to interaction with the Cu sp band. Additional benzene a density of states is observed that tracks pi emission and is attributed to weak hybridization in the chemisorbed state. The presented results give insights in the bonding of benzene to metal substrates and suggest that the usual pi-donation bonding model of benzene to metal surfaces has to be extended.