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-r ay absorption spectroscopy. These techniques allow us to resolve the b enzene p contributions involved in the chemical bond. Symmetry selecti on rules observed in resonant inelastic x-ray scattering are applied f or the adsorbate case to identify the symmetry of new pi states formed in the chemical bond. Based on x-ray absorption results we conclude t hat 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 pie(2u) x-r ay absorption intensity is strongly reduced. This is attributed to a s plitting of the previously unoccupied e(2u), orbital into bonding and antibonding states due to adsorbate pi-substrate 3d interaction leadin g to a pi backdonation bond. Moreover, a broad distribution of sigma-s ymmetric states is observed all the way up to the Fermi level, indicat ing that despite of rehybridization sigma states also contribute to th e chemical bond. On Cu(110) in contrast a new, third pi state is cut b y 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 attrib uted to weak hybridization in the chemisorbed state. The presented res ults give insights in the bonding of benzene to metal substrates and s uggest that the usual pi-donation bonding model of benzene to metal su rfaces has to be extended.