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Electronic structure of third-row elements studied by high energy resolution x-ray spectroscopy

X-ray inner shell spectroscopic methods are element selective and provide the most direct information on the chemical environment of the central atom. In commonly used x-ray absorption spectroscopy (XAS) the unoccupied states above the Fermi level are probed. On the other hand, in x-ray emission spectroscopy (XES) the relaxation of the core-ionized atom by a radiative transition from occupied electrons state are studied. If the resolution in detection channel is comparable to the natural linewidth of the measured emission line the XES provide information on the electronic structure complementary to XAS. Since spectral features, e.g. energy position and spectral shape are independent on the type of excitation, such analysis is not restricted to synchrotron facilities and can be performed also with laboratory excitation sources. In present work, 2 MeV proton beam and 3 keV monochromatic photon beam were used to induce Kα and Kβ emission spectra of several phosphorus(P), sulfur(S) and chlorine(Cl) containing compounds including various oxidation states, local symmetries, and ligand environments. The high-energy resolution spectrometer in Johansson geometry [1] optimized for tender x-ray energy range was used in the experiment. The consistent interpretation of measured spectra was achieved with implementation of quantum chemical calculations employing the StoBe-deMon molecular/cluster software package, [2] based on density functional theory (DFT). The energy shifts of the most intense Kα diagram line were measured in correlation with formal oxidation state for all three elements. In case of third-row elements, the Kα lines correspond to core-to core (ctc) electron transition and its atomic-like Kα 1, 2 spectral shape allows us a precise and robust analysis of oxidation state. A very high correlation of Kα emission energy with calculated effective charge of central atom is achieved. Therefore, valence electron population affects the screening of the nuclear potential experienced by core electrons, so measured ctc-XES spectra reflect with high sensitivity the local charge of element and can be used for oxidation state analysis of P, S and Cl. Chemical sensitivity is increased further in Kβ emission spectra, which originate from valence-to-core (vtc) transition and reflects directly the chemical environment of the elements. The measured spectra are compared to the results of first-principle calculations based on DFT. The agreement between measured and calculated Kβ emission spectra permits to discuss valence orbitals in term of molecular orbital (MO) picture. The Kβ spectral shape of the XO 4n- and XO 3n- ions (where X is P, S or Cl) are decomposed to atomic orbitals [5]. The effects of ionic and covalent ligand bonding to PO 43- is also discussed.

high energy resolution XES, phosphorus, sulfur, chlorine

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