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Diese Arbeit untersucht den Zusammenhang von elektronischer und nukleare Anregung beim Raman-Effekt mit der Methode der zeitaufgelösten harten UV-Spektroskopie. Wir verwenden kohärente stimulierte Raman-Streuung, ein Spezialfall der Standard-Raman-Streuung. Bei dieser regt ein hinreichend kurzer kohärenter Lichtimpuls Schwingungen der Kerne an, bei denen die Kerne messbar ausgelenkt werden, wohingegen die Auslenkungen bei normaler Raman-Streuung, wegen deren inkohärenten spontanen Natur, nicht messbar sind. Wir konnten Auslenkung kleiner als 10^-4 A in Echtzeit durch ihren Effekt auf das harte UV-Spektrum nachweisen. Diese Ergebnisse konnten mit Lithiumborhydrid als Probe und nicht-resonanter naher Infrarotstrahlung als Anrege-und harter UV-Strahlung als Abfrageimpuls erzielt werden. Zum Nachweis dieses Prozesses verwenden wir harte UV-Absorptionsspektroskopie an der Lithium K-Kante von Lithiumborhydrid bei 60 eV. Das Absorptionsspektrum besteht aus einem starken exzitonischen Anteil zu Beginn der Absorption und einem Plateau bei höheren Energien. Bei Anregung durch einen NIR-Impuls beobachteten wir eine oszillatorische Änderung des Absorptionsspektrums mit einer Frequenz von 10 THz, was wir der Modulation der interatomaren Abständen durch kohärente Phononen, und die damit einhergehende Modulation der chemischen Umgebung des absorbierenden Atoms, zuschreiben. Harte UV-Spektroskopie, insbesondere bei niedrigen Energien und nahe der Kante, ist hoch sensitiv auf die chemische Umgebung des jeweiligen absorbierenden Atoms. Unsere Resultate erlauben einen faszinierenden, neuen Einblick in die mikroskopische Natur des Raman-Effekts. Sie verbinden einen direkten Nachweis des antreibenden Mechanismus, der induzierten Polarisation, mit einer direkten Beobachtung des Resultats, die oszillatorische Auslenkung der Kerne. Dabei konnten mit harter UV-Spektroskopie nukleare Auslenkungen in der Größenordnung von 10-4 A mit Subpicosekundenzeitauflösung aufgelöst werden. / This thesis explores the subtle interplay between electronic and nuclear excitation in the Raman effect with time resolved XUV absorption spectroscopy. Coherent stimulated Raman scattering, the type of Raman interaction we induce, is a variant of the well known Raman scattering, where a sufficiently short pulse excites nuclear vibrations coherently, i.e. with actual displacement of the nuclei. In standard Raman scattering, due to its incoherent, spontaneous nature, there is no displacement of nuclei. We were able to observe nuclear displacements as small as 10^-4 in real time by their effect on the XUV absorption spectrum. Specifically we studied non-resonant NIR pump XUV probe absorption spectroscopy on lithium borohydride (LiBH_4). In the XUV absorption experiments in this thesis we concentrate on the Lithium K-edge absorption spectrum around 60 eV which consists of a strong excitonic peak at the onset of absorption and a plateau at higher energies. Upon excitation with a NIR pulse we observe oscillatory changes in the absorption spectrum with a frequency of 10 THz, which we identify as the effect of coherent phonon excitations of an external A_g phonon mode. The coherent oscillation changes the distance between Li^+ anions and BH_4^- cations, which modifies the electronic environment around the Li anion. XUV absorption spectroscopy, especially XANES, is highly sensitive to such changes of the chemical environment around the absorbing atom. We use two different approaches to derive the absolute displacement, which are observed in the experiment. Our results allow for a fascinating new insight into Raman scattering as they connect a direct observation of the driving mechanism, the induced polarization, with a direct observation of the outcome the oscillatory nuclear displacement. With XUV absorption spectroscopy nuclear displacements in the order of 10^-4 A were resolved with sub picosecond accuracy in the time domain.

We present the first high-resolution X-ray spectrum of a putatively single Wolf–Rayet (WR) star. 400 ks observations of WR 6 by the XMM-Newton telescope resulted in a superb quality high-resolution X-ray spectrum. Spectral analysis reveals that the X-rays originate far out in the stellar wind, more than 30 stellar radii from the photosphere, and thus outside the wind acceleration zone where the line-driving instability (LDI) could create shocks. The X-ray emitting plasma reaches temperatures up to 50 MK and is embedded within the unshocked, “cool” stellar wind as revealed by characteristic spectral signatures. We detect a fluorescent Fe line at ≈6.4 keV. The presence of fluorescence is consistent with a two-component medium, where the cool wind is permeated with the hot X-ray emitting plasma. The wind must have a very porous structure to allow the observed amount of X-rays to escape. We find that neither the LDI nor any alternative binary scenario can explain the data. We suggest a scenario where X-rays are produced when the fast wind rams into slow “sticky clumps” that resist acceleration. Our new data show that the X-rays in single WR star are generated by some special mechanism different from the one operating in the O-star winds.

In this dissertation, we used a combination of several synchrotron-based x-ray spectroscopic techniques to investigate the effects of strain, ionic defect formation, and heteroengineering in strongly-correlated electronic systems. First, we introduce a method to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. In our approach, we utilized atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single crystalline CaMnO3 films under coherent tensile strain, varying systematically from +0.8% to +4%. An increase of the oxygen vacancy content in the single-crystalline CaMnO3 thin films with applied in-plane strain was experimentally observed using high-resolution soft x-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES). Our experimental results were verified using first-principles theory and atomic core-hole multiplet calculations. Furthermore, our results highlight the importance of protecting the surfaces of CaMnO3 thin-films with thin Pt layers in-situ in order to stabilize the oxygen vacancy content. Next, we discuss the role of oxygen vacancies in driving the metal-insulator transition in LaNiO3 thin films. Here, we also use atomic layer-by-layer pulsed laser deposition (ALL laser PLD) from two separate targets to synthesize high-quality single-crystalline LaNiO3 films with systematically varying thicknesses, ranging from 1 u.c. to 50 u.c. An increase in the oxygen vacancy content was observed with the decreasing LaNiO3 film thickness using XAS. A higher concentration of oxygen vacancies was observed for the ultrathin insulating films (<1.5 u.c.). The experimental results were compared to first-principles theoretical calculations. We found that LaNiO3 exhibits room-temperature metallic behavior for thicknesses down to 1.5 u.c., which is the lowest value reported to date. Finally, we have investigated an atomically-abrupt interface between the paramagnetic LaNiO3 and the antiferromagnetic CaMnO3 thin films. The interface between these two complex oxides exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Here, we used depth-resolved standing-wave photoemission spectroscopy (SW-XPS), scanning transmission electron microscopy (STEM), and XAS to observe a depth-dependent charge reconstruction occurring at the LaNiO3/CaMnO3 interface. Our elemental standing-wave rocking-curve analysis revealed the depth-dependent changes of the Mn and Ni valence states at the interface, yielding increased amounts of Mn3+ and Ni2+ cations at the interface. These results suggest Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-Mn4+ superexchange as possible underlying causes of the emergent interfacial ferromagnetism. / Physics

The global energy transition, involving the widespread adoption of electric vehicles and grid-scale energy storage, demands energy storage devices made up of abundant, inexpensive minerals. For this to be achieved, the large Co content in conventional Li-ion battery cathodes (e.g., LiCoO₂) must be replaced while also maintaining or improving the energy density of the battery. Alternative low-Co and Co-free materials (e.g., layered LiNixMnyCozO₂, spinel LiNi₀.₅Mn₁.₅O₄, and olivine LiFePO₄) are promising alternatives due to their theoretically higher energy densities or improved safety properties from the industry standards. However, in practice, these materials exhibit both bulk and interfacial instabilities that limit their practical energy density and cycle lifetime. It is well known that reactions between the delithiated (charged) cathode surface with the electrolyte generates electrolyte decomposition species that form an interphase layer called the cathode electrolyte interphase (CEI), where such reactions are concomitant with a crystallographic reconstruction of the surface of the bulk material. The CEI is air sensitive, disordered, nanometers thick and evolves as a function of state of charge and cycle number, making it difficult to fully understand its composition and effect on device performance. The dynamic nature of the CEI necessitates development of chemical characterization tools that can analyze surface reactivity during battery operation. Commercial cathode films are also composites including not just the electrochemically active material but also conductive carbon additive and polymer binder, meaning we need spatially resolved tools to study CEI composition across the film to isolate reactivity by film component. In this thesis, we have developed and applied spatially resolved and operando characterization tools to study the CEI of low-Co and Co-free cathode materials and use these data to pinpoint the degradation reactions at play during battery operation. In the first chapter, we introduce the three most prevalent types of cathode materials (layered, spinels, and olivines) used in Li-ion batteries. We then highlight recent progress in the analytical characterization tools that have been developed to elucidate CEI composition, spatial arrangement, and formation pathways during battery operation while discussing the difference in surface reactivity between each cathode active material as revealed by these techniques. Major findings from my own thesis work, detailed in following chapters, are discussed in parallel within this broader context. Finally, equipped with a deeper understanding of the CEI and the processes that lead to its formation, we discuss what remains to be discovered and enabled by optimizing these complex interfaces. The second chapter investigates the composition of the CEI formed by the Li-rich layered cathode material, Li₂RuO₃, to better understand performance decline in this class of materials. To bridge this gap in understanding, we use solid-state NMR (SSNMR) and surface-sensitive dynamic nuclear polarization (DNP) NMR to achieve high resolution compositional assignment of the CEI. We show that the CEI that forms on Li₂RuO₃, when cycled in carbonate-containing electrolytes, is similar to the solid electrolyte interphase (SEI) that has been observed on anode materials, containing components such as polyethylene oxide (PEO) structures, Li acetate, carbonates, and LiF. The CEI composition deposited on the cathode surface on charge is chemically distinct from that observed upon discharge, supporting the notion of crosstalk between the SEI and the CEI, with Li+-coordinating species leaving the CEI during delithiation. We use electrochemical impedance spectroscopy (EIS) to assess the impedance of the CEI on Li₂RuO₃ as a function of state of charge in connection with the migration of CEI species as identified with NMR. Migration of the outer CEI combined with the accumulation of poor ionic conducting components on the static inner CEI may contribute to the loss of performance over time in Li-excess cathode materials. This work demonstrates the utility of SSNMR for studying electrolyte decomposition at the cathode-electrolyte interface which is then applied in the following chapter to more commercially relevant materials. In the third chapter, we study the CEI and surface reactivity of the Ni-rich layered material LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811). The high specific capacities of Ni-rich transition-metal oxides have garnered immense interest for improving the energy density of Li-ion batteries. However, Ni-rich cathodes suffer from interfacial instabilities that lead to formation of electrochemically inactive phases at the cathode particle surface as well as the formation of a CEI layer on the composite surface during electrochemical cycling. We use a combination of ex situ SSNMR spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information, on the nanometer length scale, on the CEI deposited on NMC811 composite cathode films. XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI and paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in 13C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, we find that LiF, Li₂CO₃, and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ 1H and 19F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows spatially resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films. The in situ solution NMR cell developed here is one of the first of its kind developed specifically for studying electrolyte decomposition products during or directly after battery operation, which is further developed in the next chapter. The fourth chapter focuses on studying the surface reactivity of the high-voltage LiNi₀.₅Mn₁.₅O₄ (LNMO) spinel cathode material. Unfortunately, LNMO-containing batteries suffer from poor cycling performance because of the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando EPR and NMR spectroscopies to study these high voltage reactions, applying the in situ cell design from the previous chapter to operando conditions (characterization during battery charging). We demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (battery charging). Leveraging the temporal resolution (s-min) of magnetic resonance, we find that the LNMO particles accelerate the rate of LiPF6 decomposition and subsequent Mn²⁺ dissolution, possibly due to the acidic nature of terminal Mn-OH groups and protic species generated upon oxidizing the solvents. X-ray photoemission electron microscopy (XPEEM) provides surface-sensitive and localized X-ray absorption spectroscopy (XAS) measurements, in addition to X-ray photoelectron spectroscopy (XPS), that indicate disproportionation is enabled by surface reconstruction upon charging, which leads to surface Mn³⁺ sites on the LNMO particle surface that can disproportionate into Mn²⁺(dissolved) and Mn⁴⁺(s). During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF₂) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF₂ decreases film conductivity by threefold compared to LiF, suggesting that this CEI component may impede both the ionic and electronic properties of the cathode. Ultimately, to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltages like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will likely be necessary. In the fifth chapter, we conduct an in situ X-ray spectroscopy, electron microscopy, and electron diffraction experiment to study the oxidation of the surface of Li metal, which is of critical importance for next generation Li metal batteries. Elemental Li is one of the most promising anode materials for high energy density Li batteries if it can replace graphite because it increases the specific capacity by an order of magnitude. However, Li metal is extremely reactive and is easily oxidized by air and moisture, even under inert conditions (e.g., in argon-filled gloveboxes, ultrahigh vacuum chambers). The industrial production of Li metal anodes, their surface evolution upon contact with the electrolyte, and electrodeposition behavior upon battery cycling all rely on the initial oxidative processes that take place prior to cell assembly. To better understand Li metal oxidation, we deposit pure Li on a Cu substrate and dose the Li deposit with various amounts of oxygen gas. During this experiment, we monitor the surface composition in situ using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), and XPS measurements. We show that by evaporating Li onto Cu substrates, we can bypass long sputtering times needed to study commercial Li foils that usually exhibit alkali metal impurities and thick contamination layers from their external environment. Combined insights from LEED, LEEM and DFT calculations indicate that upon oxygen dosing of this ultrapure Li film, oxygen adsorbs to Li, forming a disordered layer, followed by (111) oriented polycrystalline Li₂O growth. DFT was particularly instrumental in elucidating the precise work function of the surface for the intermediate oxide phases (timescale of seconds) to correlate with trends observed via in situ LEEM imaging experiments. To conclude, we reflect on the overarching insight on cathode degradation that we have learned from these studies and discuss remaining knowledge gaps in the field. We highlight promising future avenues to study for stabilizing the cathode-electrolyte interface of these materials, such as adapting the characterization methods developed here for more high throughput study of next generation electrolyte formulations.

Un-doped, Tb3+ as well as Yb3+ doped ZnO nanocrystals with different concentrations of RE3+ (Tb3+, Yb3+) ions were successfully synthesized via sol-gel method to produce rare earth activated zinc oxide nanophosphors. The phosphor powders were produced by drying the precursor gels at 200˚C in ambient air. Based on the X-ray diffraction results, it was found that the pure and RE3+ doped ZnO nanophosphors were highly polycrystalline in nature regardless of the incorporation of Tb3+ or Yb3+ ions. Moreover, the diffraction patterns were all indexed to the ZnO Hexagonal wurtzite structure and belong to P63mc symmetry group. The Raman spectroscopy confirmed the wurtzitic structure of the prepared samples. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and RE3+ ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and RE3+ doped samples were composed of hexagonal homogeneously dispersed particles of high crystallinity with an average size ranging from 4 to 7 nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. ZnO:Tb3+ PL study showed that among different Tb3+ concentrations, 0.5 mol% Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f-4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1 mol% Tb3+doped ZnO. Optical band-gap was extrapolated from the Ultraviolet Visible spectroscopy (UV-Vis) absorption spectra using TAUC‟s method and the widening of the optical band-gap for the doped samples as compared to the un-doped sample was observed. The PL study of ZnO:Yb3+ samples was studied using a 325 nm He-Cd laser line. It was observed that the ZnO exciton peak was enhanced as Yb3+ions were incorporated in ZnO matrix. Furthermore, UV-VIS absorption spectroscopic study revealed the widening of the band-gap in Tb3+ doped ZnO and a narrowing in the case of Yb3+ doped ZnO system. X-ray photoelectron spectroscopy demonstrated that the dopant was present in the doped samples and the result was found to be consistent with PL data from which an energy transfer was evidenced. Energy transfer mechanism was evidenced between RE3+ and ZnO nanocrystals and was discussed in detail. / Physics / M.Sc. (Physics)

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L-shell x-ray production cross sections have been measured for thin targets of 29Cu, 32Ge, 37Rb, 38Sr, and 39Y. M-shell x-ray production cross sections have been measured for thin targets of 79Au, 82Pb, 83Bi, 90Th, and 92U. All targets were irradiated with a beam of H+ ions with energies in a range from 70 to 200 keV. Experimental cross sections are compared to other measurements at higher energies and to first Born (Plane Wave Born Approximation for direct ionization and Oppenheimer-Brinkman-Kramers-Nikolaev approximation for electron capture) and the ECPSSR (Energy loss, Coulomb deflection, Perturbed Stationary State calculations with Relativistic effects) theoretical cross sections.

Characteristic L-shell x rays of the five rare earths Pr, Sm, Eu, Gd, and Dy were studied in this work. The x rays were produced by ionization from 0.3 to 2.0 MeV protons from the 2.0 MV Van de Graaff at North Texas State University. Total L-shell ionization and x-ray production cross sections were measured for Sm and compared to the BEA, CBEA and PWBA theories. Total L-shell ionization cross sections were measured for Pr, Eu, Gd, and Dy and compared to the BEA, CBEA, and PWBA. The CBEA and PWBA fit the samarium data well for both ionization and x-ray production cross sections. The BEA was generally 40 per cent lower than the data. The CBEA and the PWBA also fit the ionization cross section data for Pr, Eu, Gd and Dy, while the BEA was generally 40 per cent lower than the data.