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

Abstract

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 \ensuremath{\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 \ensuremath{\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 ${\ensuremath{\pi}}^{*}{e}_{2u}$ 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 \ensuremath{\pi}-substrate $3d$ interaction leading to a \ensuremath{\pi} backdonation bond. Moreover, a broad distribution of \ensuremath{\sigma}-symmetric states is observed all the way up to the Fermi level, indicating that despite of rehybridization \ensuremath{\sigma} states also contribute to the chemical bond. On Cu(110) in contrast a new, third \ensuremath{\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 \ensuremath{\sigma} density of states is observed that tracks \ensuremath{\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 \ensuremath{\pi}-donation bonding model of benzene to metal surfaces has to be extended.