Probing Sulfur Chemical and Electronic Structure with Experimental Observation and Quantitative Theoretical Prediction of Kα and Valence-to-Core Kβ X-ray Emission Spectroscopy

Abstract

An extensive experimental and theoretical study of the Kα and Kβ high-resolution X-ray emission spectroscopy (XES) of sulfur-bearing systems is presented. This study encompasses a wide range of organic and inorganic compounds, including numerous experimental spectra from both prior published work and new measurements. Employing a linear-response time-dependent density functional theory (LR-TDDFT) approach, strong quantitative agreement is found in the calculation of energy shifts of the core-to-core Kα as well as the full range of spectral features in the valence-to-core Kβ spectrum. The ability to accurately calculate the sulfur Kα energy shift supports the use of sulfur Kα XES as a bulk-sensitive tool for assessing sulfur speciation. The fine structure of the sulfur Kβ spectrum, in conjunction with the theoretical results, is shown to be sensitive to the local electronic structure including effects of symmetry, ligand type and number, and, in the case of organosulfur compounds, to the nature of the bonded organic moiety. This agreement between theory and experiment, augmented by the potential for high-access XES measurements with the latest generation of laboratory-based spectrometers, demonstrates the possibility of broad analytical use of XES for sulfur and nearby third-row elements. The effective solution of the forward problem, i.e., successful prediction of detailed spectra from known molecular structure, also suggests future use of supervised machine learning approaches to experimental inference, as has seen recent interest for interpretation of X-ray absorption near-edge structure (XANES).

An exploration of benchtop X‐ray emission spectroscopy for precise characterization of the sulfur redox state in cementitious materials

“…An average Bragg angle of 58.9 degrees ensures that the correct energy range is projected onto the detector plane. This spectrometer has been previously used for studies of oxidation state and more complex electronic structure for sulfur and phosphorus compounds showing excellent agreement with synchrotron results, including for the reference compounds used here and cited below 11,21,44,45 …”

“…Unlike XANES, which requires synchrotron access when the species of interest is dilute, nonresonant XES can be performed with lab‐based equipment even for relatively dilute systems 11,33,34 . We find that the sulfur content in several cementitious materials relevant for CWFs is sufficient for relatively rapid in‐house measurement of the S Kα XES, and that the resulting spectra can be fit with high precision to reference standards when allowing an additional fit parameter for small shifts in the nominal Kα 1 location due to chemical effects within the same nominal oxidation state 21,22 …”

“…More recently WD‐XRF has been used to look at cementitious materials and sulfate attack of cements, 18,19 and in these studies that pursued the question of oxidation state the photon energy resolution was unable to separate the Kα 1 and Kα 2 peaks. Consequently, estimations for oxidation state were somewhat sensitive to assumptions about relevant reference compounds because of the weak dispersion of fluorescence line energies even within a nominal classical oxidation state, as has been demonstrated with higher‐resolution S WD‐XRF studies over several decades 20–32 …”

Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering

“…4d / 2p transitions are calculated for the relaxed 2p- To analyze the character of the spectral features, we utilize a previously described weighting scheme. 73 Aer identifying the most dominant transitions within a specic energy range (i.e. the transitions of peaks a, b, c), we decompose their valence molecular orbitals contributions into weighted sums of atomic orbitals.…”

Phosphorus Kβ X-ray emission spectroscopy detects non-covalent interactions of phosphate biomolecules in situ

“…In recent years, Kβ X-ray emission spectroscopy (XES) has become a more frequently used technique, with a focus on 3d transition metals. [40][41][42][43][44][45][46] Some work has also been performed on 4d elements, [47][48][49] and sulfur Kβ X-ray emission has been somewhat explored, [50][51][52][53][54] including a study of the protonation state of aqueous sulfate, 55 but very few Kβ XES studies of phosphorus have been reported. 50,[56][57][58] Phosphorus Kβ X-ray emission spectroscopy (XES) has great potential as a probe of phosphorus in both molecular and biological systems, promising rich electronic structural information and compatibility with diverse sample preparations that would augment existing techniques.…”