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

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.

Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study

“…This substantiates the importance of core hole screening which influences the NEXAFS spectra differently depending on the location and chemical environment of the excitation center. It is consistent with earlier work by by Oji et al [18] and may callenge the validity of a constant energetic shift used in the static exchange approach [16,26].…”

“…It has been demonstrated that in many cases obtaining molecular NEXAFS signatures based on total densities of unoccupied states (DOUS) only is not fully conclusive [18]. The reason for this is the missing consideration of initial state effects which are due to differences in the C1s binding energies of differently coordinated carbon atoms within the molecule.…”

“…by single C-C bonds [12][13][14][15]. However, this approach is not applicable to larger fully conjugated aromatic molecules which, instead, exhibit a number of additional π*-resonances according to the size of the aromatic electron system [16][17][18]. This situation bears resemblance to the vibrational spectra of extended aromatic molecules, where collective, almost phononic vibrations appear instead of localized modes that are commonly used to identify functional units by fingerprints [19].…”

High resolution NEXAFS of perylene and PTCDI: a surface science approach to molecular orbital analysis

“…[12][13][14] From the experimental point of view, the spectroscopic characterization of the molecular orbitals of the free PTCDI molecule, as well as of perylene, is largely outdated or limited. [15][16][17][18] In this context, the knowledge of the molecular orbital symmetry and energy, as measured in NEXAFS, is fundamental to understand the key parameters that govern the performances of an archetypal device, such as the orientation of the molecules on the surfaces 19 and the mechanism of charge transfer to the substrate. 20 In their extended review, Zahn and coworkers reported a comparative study of the electronic and vibrational properties of PTCDA and dimethyl-PTCDI, 2 however their analysis of NEXAFS spectra was limited to the comparison with previous calculations for naphthalene-tetracarboxylic dianhydride (NTCDA), 21 thus a clearcut disentanglement between initial and final state effects in the PTCDI NEXAFS is still missing.…”

Spectroscopic characterization of the on-surface induced (cyclo)dehydrogenation of a N-heteroaromatic compound on noble metal surfaces

“…In the π* region, we observe five peaks, denoted as A, B, C, D and E. The most intense peaks at 284.9 and 285.3 eV are due to de C1s transitions to the LUMO π* states [20][21][22][23] of non-equivalent C sites in the molecules. 24,25 There is a large dichroism between the s-and p-polarised signals, even though the s-polarised signal does not vanish. The π* region is less intense for s-polarised light and the σ* region is less intense for p-polarises light.…”