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 π 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 π 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 π-substrate $3d$ interaction leading to a π backdonation bond. Moreover, a broad distribution of σ-symmetric states is observed all the way up to the Fermi level, indicating that despite of rehybridization σ states also contribute to the chemical bond. On Cu(110) in contrast a new, third π 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 σ density of states is observed that tracks π 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 π-donation bonding model of benzene to metal surfaces has to be extended.