Article

# The electronic structure of poly(pyridine-2,5-diyl) investigated by soft X-ray absorption and emission spectroscopies

Authors:
• RISE-Resarch Institutes of Sweden
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## Abstract

The electronic structure of the poly-pyridine conjugated polymer has been investigated by resonant and nonresonant inelastic X-ray scattering and X-ray absorption spectroscopies using synchrotron radiation. The measurements were made for both the carbon and nitrogen contents of the polymer. The analysis of the spectra has been carried out in comparison with molecular orbital calculations taking the repeat-unit cell as a model molecule of the polymer chain. The simulations indicate no significant differences in the absorption and in the non-resonant X-ray scattering spectra for the different isomeric geometries, while some isomeric dependence of the resonant spectra is predicted. The resonant emission spectra show depletion of the {\pi} electron bands in line with symmetry selection and momentum conservation rules. The effect is most vizual for the carbon spectra; the nitrogen spectra are dominated by lone pair n orbital emission of {\sigma} symmetry and are less frequency dependent.

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... Since NEXAFS spectroscopy probes resonant transitions from core levels to unoccupied states, the C 1s chemical shift between carbon-bonded and nitrogen-bonded carbon atoms is the reason of the splitting of the * band, as observed for pure pyridine. 5 Differently from photoemission spectroscopies (XPS and ARPES), NEXAFS probes resonant processes, and the relative shifts of the resonances cannot be directly compared to understand how the empty states move. In fact, nitrogen atoms affect the energy position of both the molecular and atomic states and therefore the relative NEXAFS shift between different polymers reflects the displacement of the C 1s core level with respect to the molecular empty states, see Figure S8. ...
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Article
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By exploring the monosubstituted benzene compound aniline, we demonstrate that resonance inelastic x-ray spectroscopy of chemically shifted species is {ital site} {ital selective}. Core-excited levels with distinct, super-electron-volt shifts can be resonantly excited and their x-ray emission spectra analyzed separately. Core-excited levels referring to sites with small, sub-electron-volt, chemical shifts give resonant x-ray spectra that interfere strongly. It is demonstrated that this interference, which is manifested in the one-step model, can be used to monitor chemical shifts in the sub-electron-volt energy region. We show that in the limit when these chemical shifts go to zero some salient symmetry-selective features of the benzene resonant x-ray emission spectrum are restored in the aniline spectra.
Article
Resonantly excited x-ray fluorescence spectra of the benzene molecule are presented and analyzed in terms of symmetry-selective resonant scattering processes. The sharp frequency dependency that is observed can be understood from strict parity and symmetry-selection rules operating in the full ${\mathit{D}}_{6\mathit{h}}$ point group symmetry. The experimental results prove that the electronic symmetry is not broken for the lowest unoccupied level involved in the resonant x-ray emission process.
Article
Tail excitation and Stokes shifts have strong influence on the appearance of the RIXS spectra, both depending crucially on frequency and form of the spectral functions of the incoming photons and on the vibrational progressions of the core-excited states. The band-gap generated spectra emerge as consequences of Stokes shifts when the absorption energies are detuned from the lowest unoccupied molecular orbital resonance. The polarization and angular dependences of RIXS in C{sub 60} are found to be comparatively weak, something which is rationalized by the highly degenerate electronic structure and the spherical shape of the molecule. The computer simulations in this work rest on transition moments and energies obtained by {ital ab} {ital initio} Hartree-Fock calculations in the full {ital I}{sub {ital h}} point-group symmetry.
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The process of x-ray absorption and emission in solids is considered as an x-ray resonant inelastic scattering process. This coherent inelastic scattering picture has recently been used to interpret the excitation energy dependence in the x-ray emission spectra excited with synchrotron radiation. It also suggests that with near threshold excitation, the x-ray emission spectra should be spatially anisotropic. We consider the validity and implication of this approach and factors affecting the spatial and temporal coherence in the scattering process. Taking into account the relaxation effects such as the electron-electron and the electron-phonon interactions, a significant fraction of the total emission intensity may be attributed to the coherent scattering. This picture of the x-ray absorption and emission process opens up the possibility for momentum-resolved x-ray absorption and emission measurements that can be used for band-structure determination. In addition, it has important implications on the fluorescence yield method of obtaining the absorption spectra.
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