Importance of vibronic coupling in core level photoemission spectra of polycyclic aromatic hydrocarbon molecules

The electronic structures of fivepolycyclic aromatic hydrocarbons (PAHs) (antracene, fluorene, phenantrene, cyclopentaphenantrene and coronene) have been studied by means of synchrotron radiation spectroscopies at the carbon K-edge, XPS (X-ray Photoelectron spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structure), experimental data have been interpreted by comparison with theoretical calculations. NEXAFS spectra have been calculated with the Transition Potential (TP) scheme in the framework of the Density Functional Theory (DFT). Namely the Kohn-Sham equations are solved employing a basis set of Slater Type Orbitals with an occupation number of the C1s core orbital reduced by 0.5 electrons. Intensity is calculated as electric dipole transition moment. Carbon 1s photoelectron spectra have been assigned by comparison with theoretical calculations. But it is also possible to unveil important discrepancies between theory and experimental data, which can be ascribed to electronic correlation effects or to vibronic coupling. Vibronic coupling effects have been explicitely studied in the case of the C K-edge NEXAFS of phenantrene and coronene. Their inclusion even within the Franck-Condon approximation appear to improve considerably the agreement between theory and experiment.