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Ultraviolet Photoelectron Spectroscopy of CO Adsorbed on Palladium Surfaces

Ultraviolet photoelectron spectra (hv=40.8 eV) were recorded from clean and CO covered (110) and polycrystalline Pd surfaces, respectively. Adsorption of carbon monoxide leads to the appearance of two maxima at 7.9 and 10.8 eV below the Fermi level EF which are associated with chemisorption levels derived from the 5σ- and 1π-orbitals of free CO. These states are broadened to 1.5 and 1 eV, respectively, and the vibrational structure present in the UPS data from gaseous CO may no longer be resolved. An observed decrease of the emitted intensity just below EF is interpreted as being caused by an additional level in this energy range which is due to back-donating of metallic d-electrons into the empty 2π*-orbital of CO as predicted theoretically.