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.

The chemical bonding and the electronic structures of C 60 F x and C 70 F x were investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and UV photoemission spectroscopy (UPS), which are useful methods for examining the unoccupied and the occupied states, respectively. With these results and XPS measurements, we derived the electronic energy diagram of C 60 F x and discussed the change of the electronic structure from that of C 60 by fluorination. The energies of the LUMO and the Fermi level of solid C 60 F 48 were estimated to be -5.0 and -5.4 eV below the vacuum level, indicating that highly doped C 60 F x is a strong electron acceptor. The electronic absorption spectra of C 60 F x solutions deep into the vacuumultraviolet region were also measured, and the isomerism of C 60 F x was discussed by comparing the observed results with theoretical simulations.

Near edge X-ray absorption fine structure (NEXAFS) spectra were obtained using total electron yield detection for poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) and their model compounds 4,4′-biphenyldicarboxylic acid dimethyl ester (PAM), 4,4′-biphenyldicarboxylic acid (PCA), and terephthalic acid (TPA). The spectra of PBT and PET were interpreted with the comparison of the spectral features of model molecules and the polarized NEXAFS spectra of oriented films of PAM, PCA, and PBT. From the polarization dependence, the peaks previously ascribed to a C 1s f π*(ring) or π*(ring/CdO) excitation at 289.8 eV and an O 1s f 3p/Rydberg or f π*(CdO) transition at 536.4 eV were reassigned to a C 1s f σ*(O-CH x ) transition and an O 1s f σ*(O-CH x ) transition, respectively. The present results demonstrate that the polarization dependence of the NEXAFS spectra of oriented small analogous molecules are useful for the assignment of the NEXAFS spectra of polymers.

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