Valence shell photoelectron spectroscopy of pyrene and fluorene: photon energy dependence in the far-ultraviolet region.

Inner and outer valence photoelectron spectra (PES) of pyrene and fluorene, two members of the polycyclic aromatic hydrocarbon (PAH) family, were recorded with a high-resolution synchrotron photoelectron spectrometer. Relative photoelectron emission cross sections were measured at photon energies between 15 and 40 eV. Several bands observed in the experimental PES were assigned with the help of OVGF/cc-pVDZ calculations. The first ionization potentials were estimated to be 7.436 ± 0.015 and 7.944 ± 0.055 eV for pyrene and fluorene, respectively. Photoelectron emission cross sections show a clear difference in trend for inner (σ-dominated) and outer (π-dominated) bands. Contrary to the expectation from the trend observed in benzene, the inner bands significantly dominate in the photon energy region from 15 to 27 eV and then are found to contribute uniformly. A pronounced peak in the cross sections is observed at a photon energy of approximately 17 eV for both molecules (irrespective of the difference in symmetry and structure), particularly for the inner valence bands. The feature is observed to be independent of the details of the molecular orbital associated with the outgoing electron. These observations are correlated to a collective excitation in the far-ultraviolet region.