Incident ions, from a Van de Graaff accelerator, in the MeV energy range, deposit their energy into the near surface of a sample. This, in turn, causes atomic, molecular, cluster and fragment ion species to be desorbed and ionized, while simultaneously emitting characteristic elemental X-rays. The multielemental X-rays provide qualitative elemental information, which may be deconvoluted and fit to a theoretical X-ray spectrum, generated by a quantitative analysis program, GUPIX, while the atomic, molecular, cluster, and fragment ion species are identified using a quadrupole mass spectrometer. This methodology directly links elemental determinations with chemical speciation.The development of this particle desorption ionization particle induced X-ray emission mass spectrometer, the PDI-PIXE-MS (or PIXE-MS) instrument, which has the ability to collect both qualitative multielemental X-rays and mass spectral data is described. This multiplexed instrument has been designed to use millimeter-sized MeV particle beams as a desorption ionization (PDI) and X-ray emission (PIXE) source. Two general methods have been employed, one simultaneous and the other sequential. Both methods make use of a novel X-ray/ion source developed for use with the quadrupole mass spectrometer used in these experiments. The first method uses a MeV heavy ion particle beam, typically oxygen, to desorb and ionize the sample, while simultaneously producing characteristic multielemental X-rays. The resulting molecular, cluster, and fragment ions are collected by the mass spectrometer, and the X-rays are collected using a Si-PIN photodiode detector in conjunction with a multichannel analyzer (MCA). Heavy ions of N+, O+, O+2, Ar+, and Kr+ have been investigated, although heavy ion X-ray and mass spectra have focused on the use of oxygen particle beams. The second method is performed by first collecting the X-ray data with a MeV ion beam of He+ ions, then desorbing and ionizing the sample species with a MeV particle beam of heavy ions, producing good ion yields, for mass spectral data collection. The potential development of a scanning microprobe instrument, that would provide micron-scale, imaged, multielemental, and molecular and fragment ion chemical information is being investigated through the development of this prototype PIXE-MS instrument.

La proximité omniprésente de l’ordre magnétique et supraconducteur dans les supraconducteurs non conventionnels implique l’importance de comprendre le magnétisme dans ces matériaux. Dans ce contexte, cette thèse porte sur l’étude du magnétisme dans trois supraconducteurs non conventionnels. Les excitations magnétiques dans le système d’oxychlorure de cuivre de l’élément léger Ca2CuO2Cl2 ont été étudiées en fonction du dopage et de la température en utilisant principalement la diffusion inélastique résonante aux rayons X. L’effet de la pression sur le magnétisme dans epsilon-fer et le beta-FeSe a été étudié en utilisant la spectroscopie d’émission des rayons X et la diffraction des neutrons sur poudre. / The ubiquitous proximity of magnetic and superconducting order in unconventional superconductors implies the importance of understanding magnetism in these materials. In this context, this thesis concerns the study of magnetism in three unconventional superconductors. The magnetic excitations in the light element copper oxychloride system Ca2CuO2Cl2 were studied as a function of doping and temperature using primarily resonant inelastic x-ray scattering. The effect of pressure on magnetism in epsilon-iron and beta-FeSe was studied using x-ray emission spectroscopy and neutron powder diffraction.

The general application of soft X-ray spectroscopy in the study of the electronic structure of materials is discussed, with particular emphasis on broad band materials. Several materials are studied using both soft X-ray spectroscopy and density functional theory to provide experimental and theoretical electronic structures, respectively. In particular, bonding, cation hybridization, and band gaps for several binary oxides (the alkali oxides: BeO, MgO, CaO, SrO, BaO; the post-transition metal oxides: ZnO, CdO, HgO; and the period 5 oxides In2O3, SnO, SnO2, Sb2O3, Sb2O5, and TeO2) are studied. The technique of using the peaks in the second derivatives of an X-ray emission and an X-ray absorption spectrum to estimate the band gap of a material is critically analyzed, and a more accurate ``semi-empirical'' method that involves both measured spectra and theoretical calculations is proposed. The techniques used in the study of binary oxides are then applied to a more interesting (and industrially relevant) group of ternary oxides based on TiO2 (PbTiO3, Sn2TiO4, Bi2Ti4O11, Bi4Ti3O12, and ZnTiO3), and a general rule for the band gaps of these materials is suggested based on empirical data. This research may help direct efforts in synthesizing a hydrogen-producing photocatalyst with a band gap that can efficiently harness the bulk of the solar spectrum. Finally, several layered pnictide superconductors and related compounds (CaFe2As2, Co-, Ni- and Cu-doped BaFe2As2, LiFeAs, LiMnAs, CaCu1.7As2, SrCu2As2, SrCu2(As0.84Sb0.16)2, SrCu2Sb2, and BaCu2Sb2) are studied. The X-ray spectra provide rather strong evidence that these materials lack strong on-site Hubbard-like correlations, and that their electronic structures are almost entirely like those of a broad band metal. In particular, it is shown that the notion that the transition metals are all divalent is completely wrong for copper in a layered pnictide, and that at best in these systems the copper is monovalent.

In order to elucidate the effects of nanostructuring on electron behavior in vanadium oxides, a suite of x-ray spectroscopy techniques was employed to comprehensively characterize the electronic structures of V2O5 and VO2 nanoparticles and compare them to their bulk counterparts. V2O5 and VO2 nanoparticle powders were characterized via PXRD, TEM, and HR-TEM to confirm size, purity, and crystallinity. Additionally, DSC and temperature-varied PXRD measurements on both VO2 samples confirmed the structural aspect of the monoclinic to rutile metal-insulator phase transition, and UV-Vis measurements allowed for Kubelka-Munk analysis on the V2O5 samples. XAS measurements enable the comparison of unoccupied conduction band states, while XES and RIXS measurements reveal occupied valence band states and the individual vanadium and oxygen PDOS below the Fermi level. XPS measurements of both core and valence band states both confirmed the valence band structure revealed by XES and also provide information on core-state energy levels. In the case of V2O5, the valence band O 2p states are upshifted in the nanoparticle sample, while the lowest V 3d conduction band states are unshifting but provide more available unoccupied states for excitation. These changes produce a shrunken bandgap in the V2O5 nanoparticles that is in line with much previous computational work, but unexpected from previous experimental results and defies the Moss-Burstein effect usually observed in V2O5. The resulting changes in band structure are attributed to a higher concentration of oxygen vacancy defects in the nanoparticle sample. Additionally, electron correlation effects in V2O5 nanoparticles are found to be enhanced relative to the bulk, likely due to added electron presence in the V 3d split-off band. In the case of VO2, dramatic changes in both the valence band and conduction band states are observed both below and above the structural phase transition temperature. These changes (lowered unoccupied conduction band states coupled with broadened and upshifted occupied valence band states) also lead to nanoparticle bandgap reduction and enhanced metallicity. The enhanced metallic nature of the VO2 nanoparticles is again attributed to the increased presence of surface oxygen vacancy defects, as well as a V2O3-like surface reconstruction. Additionally, electron correlation effects are found to be reduced in the VO2 nanoparticle samples relative to the bulk, unlike in the case of V2O5.

Transition-metal oxides (TMOs) display numerous fascinating and complex properties, such as mixed-valency, low dimensionality, lattice distortion, and phase transition, etc. These properties arise from the partially filled d- or f-electron shells of TM cations and are often accompanied by the intriguing interplay between degrees of freedom. To understand the complexity of d-electron TMOs, this thesis is primarily focused on studying their underlying electronic structure using X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), X-ray photoemission spectroscopy (XPS), and resonant inelastic X-ray scattering (RIXS). The measurements at the O K- and TM L-edges are achieved by taking advantage of high-flux and high-resolution synchrotron radiation light with tunable monochromatic photon energy. Four electronically and structurally distinctive oxides are selected as representative TMOs for investigation in this thesis. To begin with, through a comparative study of WO3 and Na0.67WO3 crystals, the narrowing of the conduction band is observed with Na doping and the core-hole energy shift in the O K-edge XAS process is experimentally determined. Indirect and direct band gaps of photoanode WO3 are measured from the resonant XES with polarization-dependent experimental geometry. The other sodium bronze studied is quasi-one-dimensional β-Na0.33V2O5 polycrystalline film. The film stoichiometry, preferential orientation, and orbital anisotropy are well characterized by a variety of photon and electron techniques and compared to density-functional theory (DFT) calculation. The V 3d orbital splitting of β-Na0.33V2O5 is surveyed by the V L-edge RIXS and compared with isoelectronic β-Sr0.17V2O5 regarding distortions to VO6 octahedra. Furthermore, the complex electronic structure of Mott insulators La1-xLuxVO3 is investigated to understand their spin-orbital phase diagram. The effects of rare-earth size on the O 2p hybridization states and the local crystal field of VO6 octahedron are found to agree with the prediction of DFT calculation and the evolution of crystal structure. The changes of experimental spectra with temperature are associated with Jahn-Teller distortion and orbital ordering due to structural phase transition. Lastly, the band structure and low-energy excitations of spinel MnV2O4 are explored using soft x-ray spectroscopies and theoretical calculations. The presence of Hubbard bands and the mixing between V and Mn 3d states are suggested both experimentally and theoretically.

<p>Metalloproteins, or proteins with a metal ion cofactor, are essential for biological function of both lower and higher level organisms. These proteins provide a multitude of functions from molecular transport, such as the hemoglobin transport of oxygen, to biologically important catalytic processes. As an example case, photosystem II (PSII) is studied as a representative metalloprotein. It was chosen based on the potential impact in the energy sector due to its ability to perform water oxidation using solar based energy. Understanding mechanisms by which the Mn₄Ca cluster inside PSII, also known as the oxygen evolving complex (OEC), can store energy as redox equivalents for splitting water will be essential for future development of analogous artificial systems. By using time resolved x-ray spectroscopy, the electron structure of the metal in the protein was probed through the catalytic cycle. While the applications mentioned herein are based on PSII from spinach, the developments in time-resolved x-ray spectroscopy techniques are also applicable to other metalloproteins.</p><p></p><p>By creating a new x-ray spectrometer we were able to capture the difference in x-ray emission spectra between two compounds differing in a single metal bound ligand, i.e. Mn^IV-OH and Mn^IV=O. This both establishes the functionality of the x-ray emission spectrometer and provides useful insight into the expected changes upon an oxygen double bond formation. This change in spectroscopic signal is discussed in context of the OEC which has been hypothesized to form a Mn^IV=O state.</p><p></p><p>A new sample delivery system and further developments to the x-ray spectrometer enabled both time-resolved x-ray absorption and time-resolved x-ray emission of PSII. These experiments show the potential of synchrotron sources for time-resolved x-ray spectroscopy. From our x-ray absorption measurements we were able to follow the electronic structure changes in time using a single incident photon energy. From the kinetic traces obtained, we show possible alternative interpretations of previous results showing a delay in reduction during the final step in water oxidation. From the x-ray emission spectroscopy (XES) measurements of PSII we were able to reproduce previous results within a limited collection time and give estimates for data size requirements for metalloproteins using this spectrometer. Between the results of both these measurements, we show the improved capability for time resolved measurements at synchrotrons.</p><p>The development of x-ray free electron lasers (XFELs) has also opened many opportunities for understanding faster electronic dynamics by providing femtosecond x-ray pulse durations with ~10¹² photons per pulse. While theoretical modeling of distortions to crystallographic data have been performed, little to no work has been done to understand under what conditions such an intense pulse will have on an impact on emission spectra. Here an atomistic model was developed, and data collected, to clarify the effects of sequential ionization, i.e. two single photons absorbed by the same atom at different times during a single pulse. Experimentally we found that XFELs easily achieve flux densities that invoke a different response than is classically observed for single photon absorption and emission for Mn^II which was used as a representative case for 3d transition metals in general. We also give parameters by which the onset of this damage can be predicted and an approximation to its effect on 3d transition metals. Additionally this work guides the work of future XFEL facilities as it shows that shorter pulses, currently believed to be able to escape x-ray induced distortions to crystallography data, is not a viable method for overcoming changes in x-ray emission spectra.</p><div><br></div>

A core level spectroscopy study of ice and water is presented in this thesis. Combining a number of experiments and spectrum calculations based on density functional theory, changes in the local valence electronic structure are shown to be sensitive to the local H-bonding configurations. Exploiting this sensitivity, we are able to approach important scientific problems for a number of aggregation states; liquid water, the water-metal interface, bulk and surface of hexagonal ice. For the H-bonded model system hexagonal ice, we have probed the occupied valence electronic structure by x-ray emission and x-ray photoelectron spectroscopy. Stepwise inclusion of different types of interactions within density functional theory, together with a local valence electron population analysis, show that it is essential to include intermolecular charge transfer together with internal s-p rehybridizations in order to describe the changes in electronic structure seen in the experiment. The attractive electrostatic interaction between water molecules is enhanced by a decrease in Pauli repulsion. A simple electrostatic model due to charge induction from the surrounding water is unable to explain the electronic structure changes. By varying the probing depth in x-ray absorption the structure of the bulk, subsurface and surface regions is probed in a thin ice film. A pronounced continuum for fully coordinated species in the bulk is in sharp contrast to the spectrum associated with a broken symmetry at the surface. In particular molecular arrangements of water with one uncoordinated OH group have unoccupied electronic states below the conduction band that are responsible for a strong anisotropic pre-edge intensity in the x-ray absorption spectrum. The topmost layer is dominated by an almost isotropic distribution of these species, which is inconsistent with an unrelaxed surface structure. For liquid water the x-ray absorption spectrum resembles that of the ice surface, indicating a domination of species with broken hydrogen bond configurations. The sensitivity to the local hydrogen bond configuration, in particular the sensitivity to broken bonds on the donor side, allows for a detailed analysis of the liquid water spectrum. Most molecules in liquid water are found in two-hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond. The results, consistent with diffraction data, imply that most molecules are arranged in strongly H-bonded chains or rings embedded in a disordered cluster network. Molecular dynamics simulations are unable to describe the experimental data. The water overlayer on the close-packed platinum surface is studied using a combination of core-level spectroscopy and density functional theory. A new structure for water adsorption on close-packed transition metal surfaces is found, where a weakly corrugated non-dissociated overlayer interacts via alternating oxygen-metal and hydrogen-metal bonds. The latter results from a balance between metal-hydrogen bond formation and OH bond weakening. The ultrashort core-hole lifetime of oxygen provides a powerful probe of excited state dynamics via studies of the non-radiative or radiative decay following x-ray absorption. Electrons excited into the pre-edge state for single donor species at the ice surface remain localized long enough for early time solvation dynamics to occur and these species are suggested as strong pre-existing traps to the hydrated electron. Fully coordinated molecules in the bulk contribute to a strong conduction band with electron transfer times below 0.5 femtoseconds. Upon core-ionization, both protons are found to migrate substantial distances on a femtosecond timescale. This unusually fast proton dynamics for non-resonant excitation is captured both by theory and experiment with a measurable isotope effect.