Vibronic coupling effect in core level photoemission spectra of polycyclic aromatic hydrocarbon molecules

The electronic structures of some polycyclic aromatic hydrocarbons (PAHs) (antracene, phenantrene, cyclopentaphenantrene, coronene, crysene and tetracene) 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 at the spin-restricted DFT (Density Functional Theory) level. The core hole at each nonequivalent carbon center is modeled by the half core hole (also referred as the Transition Potential, TP) approximation. 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 [1]. Their inclusion even within the Franck-Condon approximation appear to improve considerably the agreement between theory and experiment[2]. The comparison with NEXAFS data from PAH deposited on surfaces, demonstrate that the vibronic coupling is essential for the correct interpretation of data also in the solid state [3,4]. References [1] Giovanna Fronzoni, Oscar Baseggio, Mauro Stener, Weijie Hua, Guangjun Tian, Yi Luo, Barbara Apicella; Michela Alfé, Monica de Simone, Antti Kivimäki, Marcello Coreno, J. Chem. Phys. 141 (2014) 044313; doi: 10.1063/1.4891221. [2] J. Lüder, M. de Simone, R. Totani, M. Coreno, C. Grazioli, B. Sanyal, O. Eriksson, B. Brena, C. Puglia J. Chem. Phys. 142 (2015) 074305; doi: 10.1063/1.4907723 [3] H. Oji, R. Mitsumoto, E. Ito, Y. Ouchi, K. Seki, T. Yokoyama, N. Kosugi J.Chem.Phys. 109 (1998) 10409 [4] F. Roth , J. Bauer, B. Mahns, B. Buchner Phys.Rev.B 85 (2012) 014513