Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene

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Abstract

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final state. The severe deviation of the NEXAFS spectra from the DOUS is ascribed mainly to such variation of the core-hole effect, with additional effect by the site-dependent core ionization energy. In some cases, even the transitions to the same vacant orbital contribute to different spectral features, or an apparently single spectral feature consists of transitions to different vacant orbitals. Thus direct information about DOUS of a molecule with such inequivalent sites is not obtainable from the NEXAFS spectrum, although smaller deviation is suggested for other systems with single site, such as the C atoms in C60 and graphite, and the N atoms in Zn tetraphenyl porphyrin.

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