A couple of elementary open problems in the electronic structure theory of solids
are the sources of extra background electrons in x-ray photoelectron spectroscopy
and the asymmetric shapes of some peaks.
We have made breakthroughs on understanding the mechanisms of both phenomena.
In the case of the background contribution, a many-body effect was proposed, by which
intensity borrowing from a deeper core polarization increases the total number of liberated electron.
This hypothesis is supported well by the experiments of our collaborators.
In addition coupled resonances have been proposed to explain peak asymmetries.
This hypothesis has been supported numerically and is likely due to d-shell rearrangments on a core-hole site.
We are now working on an approach to semi-quantitative (trend-predictive) simulation and understanding.
Project publications. Click sidebar to see all publications.
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The background in X-ray photoelectron spectroscopy data originates, partially, from inelastically scattered photoelectrons. In fact, the current theoretical methods for calculating the background intensity are based on electron energy losses. However, a critical part of the experimental signal, which is known as the Shirley background, cannot be described within the current formalisms. This suggests that the Shirley electrons are not associated with energy losses of photoelectrons and must originate from a different photoexcitation phenomenon with a cross section of its own. We propose a mechanism based on core channeling as the physical origin of the Shirley signal.
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This work introduces the Coupled-Resonances (CR) line shape as a theoretically robust model for analyzing asymmetric peaks in photoelectron spectra, broadly applicable across various materials. The CR line shape extends the conventional Lorentzian distribution by incorporating an interference term that contributes to the peak asymmetry. This new approach addresses limitations of the widely used Doniach-Šunjić (D&S) model, which is often applied beyond its intended scope of metals with high densities of states at the Fermi level due to a lack of viable alternatives. Unlike the DS line shape, the CR model is integrable, enabling its use in precise chemical composition calculations, and it consistently provides superior fits to experimental data. The CR model's versatility is evident in its ability to simplify to a Lorentzian for a single resonance. However, with multiple resonance states, the total line shape is no longer a simple summation of individual peak contributions. Instead, a significant interference term emerges, profoundly contributing to the observed peak asymmetry and shifting the maximum peak intensity. This highlights the critical need to consider interference terms in multiplet calculations of lineshapes. The CR line shape has been implemented in the freely available software, AAnalyzer. While most asymmetric peaks are accurately described by CR Type-II (two resonances), some require CR Type-III (three resonances) for optimal fitting, as demonstrated in the included examples. Ultimately, the CR model offers a more accurate and versatile approach to analyzing asymmetric lineshapes in photoemission spectroscopy, with broad applicability to a wide range of materials, including metals.
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The background signal in X-ray Photoelectron Spectroscopy (XPS) consists of extrinsic and intrinsic components. This study focuses on characterizing the intrinsic background, particularly its structure across an extended binding energy (BE) range beyond the vicinity of the main photoelectron peak. The structure of the intrinsic background in the extended region can be reproduced through a rich set of wide Gaussian peaks. In the near-peak region, the intrinsic background is successfully characterized using the empirical Shirley algorithm. Since the Shirley algorithm fails when applied to extended energy regions by overshooting the experimental signal, a modified version must be employed that decays beyond the near-peak region. We found that a functional form flat near the peak (as in the Shirley algorithm) decays in a Gaussian-like manner for higher binding energies, satisfactorily reproduces the experimental data, and allows for revealing the rest of the rich structure of the intrinsic background. For this reason, we termed it a narrow-Shirley (NS) background. The characterization of the structure of the extended region of the intrinsic background enables the qualitative exploration of its physical origin. Building on previous work linking the near-peak Shirley signal to Interchannel Coupling with Valence Band Losses (ICVBL), we propose and provide qualitative evidence that this ICVBL mechanism is responsible for the entire intrinsic background structure across the extended energy range; this mechanism involves the absorption of a photon by a participating core level. Two of the predictions of the ICVBL mechanism are tested by comparing the structure of the intrinsic background with Auger Electron Spectroscopy (AES) and X-ray Absorption Spectroscopy (XAS) data; a third prediction, the expected modulation of the intrinsic background with photon energies around the threshold of the participating core level, is tested through synchrotron experiments.
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