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Physisorption-like Interaction at the Interfaces Formed by Pentacene and Samarium

We have investigated the nature of the interaction between samarium (Sm) and pentacene, and the energy level alignment at the resulting interfaces. The valence electronic structure of in situ prepared samples, i.e., pentacene evaporated onto Sm surfaces and vice versa, was investigated with ultraviolet photoelectron spectroscopy. Pentacene appears to physisorb on the metal surface. Sm also appears to interact weakly when evaporated on the organic material, forming clusters at low coverage. Indications of a valence change of Sm upon evolution from clusters to metallic film are found. The highest occupied molecular orbital of pentacene is measured at 1.85 eV below the metal Fermi level EF for both evaporation sequences. Estimating the energy of the lowest unoccupied molecular orbital of pentacene using the transport gap, we obtain a barrier of only 0.35 eV for the injection of electrons from the metal into the organic material.