Matjaž Kavčič

Sulfur-metal orbital hybridization in sulfur-bearing compounds studied by X-ray emission spectroscopy.

The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy (XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the Kbeta emission to the cation and the local symmetry is discussed.

Design and performance of a versatile curved-crystal spectrometer for high-resolution spectroscopy in the tender x-ray range

A complete in-vacuum curved-crystal x-ray emission spectrometer in Johansson geometry has been constructed for a 2-6 keV energy range with sub natural line-width energy resolution. The spectrometer is designed to measure x-ray emission induced by photon and charged particle impact on solid and gaseous targets. It works with a relatively large x-ray source placed inside the Rowland circle and employs position sensitive detection of diffracted x-rays. Its compact modular design enables fast and easy installation at a synchrotron or particle accelerator beamline. The paper presents main characteristics of the spectrometer and illustrates its capabilities by showing few selected experimental examples.

Application of the high-resolution grazing-emission x-ray fluorescence method for impurities control in semiconductor nanotechnology

We report on the application of synchrotron radiation based high-resolution grazing-emission x-ray fluorescence (GEXRF) method to measure low-level impurities on silicon wafers. The presented high-resolution GEXRF technique leads to direct detection limits of about 1012 atoms/cm2. The latter can be presumably further improved down to 107 atoms/cm2 by combining the synchrotron radiation-based GEXRF method with the vapor phase decomposition preconcentration technique. The capability of the high-resolution GEXRF method to perform surface-sensitive elemental mappings with a lateral resolution of several tens of micrometers was probed.

Magnetic manipulation of molecules on a non-magnetic catalytic surface

Non-magnetic Pt catalysts, supported on carbon coated magnetic Co nanoparticles, changed catalytic performance in the presence of an external magnetic field. This behavior relates to an electronic change of Pt induced by a localized magnetic field, which modifies the CO adsorption geometry. In situ resonant inelastic X-ray scattering (RIXS) experiments and theory reveal the change of atop CO adsorption geometry on the Pt catalyst to bridged geometry under an external magnetic field. This observation opens the possibility of catalytic control by means of an external magnetic field.

Second order radiative contributions in the Kβ1,3 X-ray spectra of 3d transition metals and their dependence on the chemical state of the element

Abstract Proton induced Kβ1,3 X-ray spectra of mono-elemental targets of 3d transition elements (Ca, Ti, Cr) and chromium compounds (Cr2O3, K2CrO4, K2Cr2O7) were measured by means of high-resolution crystal spectrometry. Kβ′ line, Kβ5 line, KβL1 satellite line and the KMM radiative Auger contributions were clearly resolved and their energies and intensities relative to the parent diagram line determined. The influence of the oxidation state on the second order radiative contributions in emitted spectra of chromium compounds has been demonstrated and feasibility of its potential use for chemical state determination has been discussed.

Chemical State Analysis of Phosphorus Performed by X-ray Emission Spectroscopy

An experimental and theoretical study of phosphorus electronic structure based on high energy resolution X-ray emission spectroscopy was performed. The Kα and Kβ emission spectra of several phosphorus compounds were recorded using monochromatic synchrotron radiation and megaelectronvolt (MeV) proton beam for target excitation. Measured spectra are compared to the results of ab initio quantum chemical calculations based on density functional theory (DFT). Clear correlation between energy position of the Kα emission line and the phosphorus formal oxidation state as well as DFT-calculated number of valence electrons is obtained; measured energy shifts are reproduced by the calculations. Chemical sensitivity is increased further by looking at the Kβ emission spectra probing directly the structure of occupied molecular orbitals. Energies and relative intensities of main components are given together with the calculated average atomic character of the corresponding molecular orbitals involved in transitions.

A database for KL ionization satellites in PIXE

Abstract The experimental database of the intensities of the KαL satellites induced in collisions with protons has been assembled. A detailed analysis of the rearrangement process and the influence of the L-shell spectator vacancy on the Kα fluorescence yield has been carried out, providing the ratios of the double to single ionization cross-section for the collected experimental points. These ratios were compared to the theoretical predictions within the independent particle model employing the semiclassical approximation (SCA). An empirical curve describing the ratio of the experimental and theoretical ionization probabilities has been deduced. This relationship can be used along with computed SCA values within PIXE data-processing packages such as GUPIX, to incorporate double ionization satellites into the model PIXE spectrum.

Chemical speciation via X-ray emission spectroscopy in the tender X-ray range

Kα X-ray emission spectra from a series of phosphorus, sulfur, and chlorine containing compounds covering the full range of oxidation states were measured employing high energy resolution proton induced X-ray emission (PIXE) spectroscopy in the tender X-ray range. Measurements were accompanied by quantum chemistry calculations based on density functional theory (DFT). Clear energy shifts of the Kα lines were measured in correlation with the formal oxidation state for all three elements. This correlation was improved even further using the effective charge based on the DFT calculated valence electron population. Finally, it is demonstrated that the oxidation state analysis based on the Kα energy shifts can be used also for a quantitative analysis of the proportion of separate low-Z species in mixed valence systems.

Sulphur Kβ emission spectra reveal protonation states of aqueous sulfuric acid

In this paper we report an X-ray emission study of bulk aqueous sulfuric acid. Throughout the range of molarities from 1 M to 18 M the sulfur Kβ emission spectra from H2SO4 (aq) depend on the molar fractions and related deprotonation of H2SO4. We compare the experimental results with results from emission spectrum calculations based on atomic structures of single molecules and structures from ab initio molecular dynamics simulations. We show that the S Kβ emission spectrum is a sensitive probe of the protonation state of the acid molecules. Using non-negative matrix factorization we are able to extract the fractions of different protonation states in the spectra, and the results are in good agreement with the simulation for the higher part of the concentration range.

HIGH RESOLUTION STUDY OF THE K X-RAY SPECTRA FROM LOW Z ELEMENTS

Abstract The K X-ray spectra induced by 1.5 MeV proton bombardment of Ca, Ti and Fe solid targets were recorded by means of a flat crystal spectrometer. The K β5 line intensity, the KL satellite intensities and the KMM radiative Auger emission intensities were extracted from the measured spectra. Because of self-absorption in the thick target the K absorption edge, which is situated slightly above the energy of the K β1.3 line, significantly influences the measured spectra. The importance of the absorption edge correction for accurate intensity determination of the weak lines situated near the K absorption edge is described. The relevance of including the whole K X-ray spectrum in the fitting process when determining weak line intensities is discussed.