The electronic structure of aromatic molecules adsorbed on Pd(111)

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Abstract
Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and electron energy loss spectroscopy (ELS) have been used to study the electronic energy level structure of aromatic molecules such as benzene, m-xylene, pyridine, and 2,6-dimethylpyridine adsorbed on Pd(111) at room temperature. Angle resolved He i and He ii spectra were investigated to assign adsorbate derived bands in terms of molecular orbitals of the gas-phase molecules, whereby He ii radiation proved particularly useful for this assignment because deeper lying σ orbitals can be observed. Angle resolved photoemission behavior of benzene is satisfactorily described by an adsorbate complex of C6v symmetry, i.e., consisting of benzene molecules with their ring plane parallel to the surface. It appears that the geometrical structure of the surface does not greatly influence the symmetry properties of the surface molecule. Reduction of the free-molecule symmetry C2v to Cs in the adsorbed state is necessary for m-xylene, pyridine, and dimethylpyridine to account for the relaxation of symmetry based restrictions in normal emission. This can be interpreted for m-xylene and dimethylpyridine in terms of adsorption parallel to the surface, but for pyridine an adsorbate phase of molecules in an inclined geometry, with their molecular plane tilted with respect to the surface, is proposed. The initial state information of UPS is used to discuss the electronic excitation spectra of aromatic molecules on Pd(111). The intramolecular π→π* excitations of the free molecules are not clearly observed in ELS of the adsorbed molecules, but strong charge transfer bands are seen in the loss energy region 2.5–5 eV. Possible reasons for the absence of distinct intramolecular features in adsorbate ELS are discussed. These include pronounced modification of the π orbital structure for strong adsorbate–surface interaction, reduction of transition probabilities due to the dielectric response of the substrate, and interference of the molecular transition with surface plasmon excitation.