Pyrene molecular anion and its homogeneous clusters, (pyrene) n- (n=2-20), have been studied by anion photoelectron spectroscopy. The adiabatic electron affinity (EAa) of pyrene was directly determined to be 0.406±0.010 eV. This EAa value agrees with that calculated using the method of density functional theory, showing that the excess electron occupies the lowest unoccupied molecular orbital of pyrene molecule. Photoelectron spectra of its cluster anions suggest that the excess electron in the pyrene dimer anion is delocalized over the two pyrene moieties, whereas in larger clusters the excess electron is predominantly localized on a single pyrene molecule.
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