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

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measu red using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with th e calculated density of unoccupied states (DOUS), indicating significant co re-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical ca lculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which aff ects both transition energy and intensity. During the course of this analys is, the core-hole effect in benzene was also analyzed. It was found that th e magnitude of the core-hole effect is strongly dependent on the combinatio n of the excited site and final vacant orbital. This dependence could be se miquantitatively explained in terms of the combination of the excitation si te 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 c ore-hole effect, with additional effect by the site-dependent core ionizati on energy. In some cases, even the transitions to the same vacant orbital c ontribute to different spectral features, or an apparently single spectral feature consists of transitions to different vacant orbitals. Thus direct i nformation about DOUS of a molecule with such inequivalent sites is not obt ainable from the NEXAFS spectrum, although smaller deviation is suggested f or other systems with single site, such as the C atoms in C-60 and graphite , and the N atoms in Zn tetraphenyl porphyrin. (C) 1998 American Institute of Physics. [S0021-9606(98)00147-0].