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In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([CnC₁im]⁺[NTf₂]⁻ and [C₄C₁im]⁺[I]⁻). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Furthermore, a detailed assignment of resonance features to excitation centers (carbon or nitrogen atoms) leads to a consistent interpretation of the spectra.
Recently, C K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of graphite (HOPG) surfaces have been measured for the pristine material, and for HOPG treated with either bromine or krypton plasmas (Lippitz et al., Surf. Sci., 2013, 611, L1). Changes of the NEXAFS spectra characteristic for physical (krypton) and/or chemical/physical modifications of the surface (bromine) upon plasma treatment were observed. Their molecular origin, however, remained elusive. In this work we study by density functional theory, the effects of selected point and line defects as well as chemical modifications on NEXAFS carbon K-edge spectra of single graphene layers. For Br-treated surfaces, also Br 3d X-ray Photoelectron Spectra (XPS) are simulated by a cluster approach, to identify possible chemical modifications. We observe that some of the defects related to plasma treatment lead to characteristic changes of NEXAFS spectra, similar to those in experiment. Theory provides possible microscopic origins for these changes.
In Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy X-Ray photons are used to excite tightly bound core electrons to low-lying unoccupied orbitals of the system. This technique offers insight into the electronic structure of the system as well as useful structural information. In this work, we apply NEXAFS to two kinds of imidazolium based ionic liquids ([CnC1im]+ [NTf2]− and [C4C1im]+ [I]−). A combination of measurements and quantum chemical calculations of C K and N K NEXAFS resonances is presented. The simulations, based on the transition potential density functional theory method (TP-DFT), reproduce all characteristic features observed by the experiment. Further, a detailed assignment of resonance features to excitation centers leads to a consistent interpretation of the spectra.
Diese Doktorarbeit ist das Resultat einer intensiven und fruchtbaren Zusammenarbeit zwischen dem Fachbereich 6.1 (Oberflächenanalytik und Grenzflächenchemie) der Bundesanstalt für Materialforschung und -prüfung (BAM) sowie der Arbeitsgruppe Theoretische Chemie von Professor Saalfrank an der Universität Potsdam. Zusammen haben wir die Strukturaufklärung von Systemen in kondensierter Phase unter Verwendung der Röntgenabsorptionsspektroskopie betrieben. Von Seiten der BAM wurden experimentelle Röntgenabsorptionsspektren hoher Auflösung an modernen Synchrotroneinrichtungen (BESSY II, Berlin) aufgenommen. Die Theoretische Chemie liefert, unter Verwendung quantenchemischer Methoden, die Möglichkeit die Spektren zu simulieren. Bei der Röntgenabsorption wird ein Photon hoher Energie von einem molekularen System absorbiert und in einen angeregten Zustand versetzt. Die Intensität der Absorption ist stark abhängig von der eingestrahlten Photonenenergie. Das resultierende Absorptionsspektrum enthält eine Vielzahl von Informationen. Meine Aufgabe bestand darin, die untersuchten Systeme am Computer zu modellieren.