arXiv subjects

André Wählisch

Publications and source records attributed to André Wählisch.

4 recordsLinked to original sources

arXiv2022

Experimental determination of the gadolinium L subshells fluorescence yields and Coster-Kronig transition probabilities

The L subshell Coster Kronig (CK) transition factors of Gd have been experimentally determined by means of two different experimental approaches and compared to available literature data. On the one hand reference-free X-ray fluorescence (XRF) analysis using an energy-dispersive detector was applied. This method permitted in addition to determine the L subshell fluorescence yields since the absolute incident and emitted X-ray photon flux can be determined. On the other hand a wavelength dispersive spectrometer based on a modified von Hamos geometry using a full cylinder highly annealed pyrolithic graphite crystal for an experimental validation of the CK transition factors in an independent experiment. The use of this high energy resolution spectrometer allowed thanks to the capability to better discriminate the different X-ray emission lines to validate the reference-free XRF results. For both experiments the use of calibrated instrumentation enabled the provision a reliable uncertainty budget on the results obtained.

physics.atom-ph↗

arXiv2020

Speciation of FeS and FeS$_2$ by means of X-ray Emission Spectroscopy using a compact full-cylinder von Hamos spectrometer

We present Fe K$β$ X-ray emission (XES) and Fe K X-ray absorption spectra (XAS) of Iron(II)sulfide (FeS) and Iron(II)disulfide (FeS$_2$). While XES and XAS offer different discrimination capabilities for chemical speciation, depending on the valence states of the compounds probed, XES allows for using different excitation sources. The XES data was measured using polychromatic X-ray radiation with a full-cylinder von Hamos spectrometer being characterized by an energy window of up to 700 eV and a spectral resolving power of $E/ ΔE = 800$. The large energy window at a single position of the spectrometer components is made profit of to circumvent the instrumental sensitivity of wavelength-dispersive spectrometers to sample positioning. This results in a robust energy scale which is used to compare experimental data with ab initio valence-to-core calculations, which are carried out using the OCEAN package. To validate the reliability of the OCEAN package for the two sample systems, near edge X-ray absorption fine structure measurements of the Fe K absorption edge are compared to theory using the same input parameters as in the case of the X-ray emission calculations. Based on the example of iron sulfide compounds, the combination of XES experiments and OCEAN calculations allows unravelling the electronic structure of different transition metal sulfides and qualifying XES investigations for the speciation of different compounds.

cond-mat.mtrl-sci↗

arXiv2020

Validation of secondary fluorescence excitation in quantitative X-ray fluorescence analysis of thin alloy films

X-ray fluorescence (XRF) analysis is a widely applied technique for the quantitative analysis of thin films up to the $μ$m scale because of its non-destructive nature and because it is easily automated. When low uncertainties of the analytical results in the few percent range are required, the non-linear secondary fluorescence effect in multi-elemental samples may complicate an otherwise straightforward quantification, since it can easily exceed a relative contribution of 20%. The conventional solution, to rely on good performing reference samples, is hindered by their low availability, especially for thin film applications. To address this challenge, we demonstrate a flexible production method of multilayered, alloyed thin films with significant secondary fluorescence contributions. We use reference-free XRF analysis to validate the reliability of the physical model for secondary fluorescence, which includes a thorough uncertainty estimation. The investigated specimens are being qualified as calibration samples for XRF or other quantitative analyses.

cond-mat.mtrl-sci↗

arXiv2019

Experimental determination of line energies, line widths and relative transition probabilities of the Gadolinium L X-ray emission spectrum

In this work the complete L-emission spectrum of gadolinium with respect to line energies, natural line widths, and relative transition probabilities was investigated using monochromatized synchrotron radiation. The measurements were realized in the PTB laboratory at BESSY II by means of an in-house built von Hamos spectrometer based on up to two full-cylinder HAPG mosaic crystal. The von Hamos spectrometer is calibrated by means of elastically scattered photons from the employed synchrotron radiation beamline leading to a well-defined energy scale and an experimentally determined spectrometer response. A selective excitation of the gadolinium L subshells was carried out to ensure a robust deconvolution of neighboring emission lines of different L subshells. The experimental results are discussed in the context of existing data from common databases and published values since significant deviations, especially for the L$γ_2$ and L$γ_3$ emission lines, are observed. We further substantiate and discuss two satellite lines at the low-energy side of the L$β_{2,15}$ and L$γ_1$ emission lines arising from the N$_{4,5}$ subshell.

physics.atom-ph↗