Repository logo

Limitations and Advancements in Soft x-ray Spectroscopy



Journal Title

Journal ISSN

Volume Title




Degree Level



Soft x-ray absorption spectroscopy (XAS) is a widely used method for probing the electronic structure of materials, yet it suffers from many complications related to the reliable measurement of absorption spectra and the proper theoretical modelling of spectral features. Problems due to different experimental aspects, such as sample charging, surface contamination and saturation effects, often introduce artifacts or distortions into a measured absorption spectrum. Even when measured accurately, the interpretation of absorption spectra is complicated by the absence of rigorous theoretical methods that properly account for the effect of the core hole on the remaining electronic structure, leading to inaccurate spectral simulations. The limits of the one electron model of core excitation were explored through a high resolution XAS study of the linear polyacenes. When measured at high resolution, the linear polyacenes exhibited a fine structure that could be assigned to excitation of non-equivalent carbon atoms in the molecules. The energies and intensities of these transitions were extracted and compared to transition intensities calculated from density functional theory, where the multi-electronic effects were approximated using a full core hole and half core hole model. The full core hole model was found to better approximate the trends in the spectra for the smaller molecules, but neither model was able to properly calculate the changes in the absorption spectra of the larger molecules. These results demonstrated the deficiencies of the one electron picture of core hole excitation and highlighted the need for incorporating multi-electronic effects into XAS simulations. To extend the capabilities of XAS, the use of partial fluorescence yields (PFY) was explored. One of the primary experimental limitation of XAS is that absorption spectra are measured by monitoring the total yield of electrons or photons from the sample. In both of these methods, a direct relationship between the measured spectra and the linear attenuation coefficient of the sample is not guaranteed. To overcome this drawback, the partial fluorescence yield measurement technique, employing a silicon drift detector, was used. By measuring partial fluorescence yields (PFY), as opposed to total yields, the effects of background fluorescence could be avoided. The inverse of the partial fluorescence yield was also demonstrated to be an effective way of avoiding saturation effects in some samples. The utility of the inverse partial fluorescence yield (IPFY) method was demonstrated in a study of several iron oxide minerals that were not possible to measure using conventional total yield methods. IPFY was also demonstrated for liquid samples where differences between the Fe PFY and IPFY were noted and attributed to resonant scattering effects in the PFY. In both the iron oxide study and the polyacene study, the experimental limitations related to the measurement of XAS were evaluated by comparison to x-ray Raman spectroscopy (XRS). XRS is a hard x-ray based method that probes core excitation in low-Z elements by measuring the energy loss of the scattered photons. Both XRS and XAS involve the same electronic transitions allowing for a direct comparison of spectra measured using the two methods, but the limitations of XAS related to the short penetration depth of soft x-rays are not encountered in XRS. While XRS measurements do not suffer from saturation or surface effects, the experimental resolution and count rates of this technique are limited. This dissertation demonstrated that, while XAS is an established method, there are many aspects of the technique that require additional development. Emerging calculation methods that can incorporate the interaction between the electron and core hole will improve data interpretation capabilities. New detector systems that can measure partial yields with high resolution and can be placed in specific geometries will continue to improve measurement quality and allow for the development of novel techniques, like IPFY. A growing use of XAS and XRS in concert will also improve the general applicability of core level excitation spectroscopies and help to advance our understanding of the materials around us.



soft x-rays, absorption spectroscopy, partial fluorescence yields, synchrotron, core hole, inverse partial fluorescence yield



Doctor of Philosophy (Ph.D.)


Physics and Engineering Physics




Part Of