This article focuses on X-ray emission spectroscopy (XES), its applications, limitations, and recent studies involving the technique.
What is XES?
XES is an element-specific method primarily used to analyze the partially occupied electronic structure of materials. The technique is one of the photon-in-photon-out spectroscopies in which an incident X-ray photon is used to excite a core electron, which leads to the transition of the electron from the ground state to the excited state, and then the excited state of the electron decays with the emission of an X-ray photon in order to fill the core hole.
The emitted photon energy represents the energy difference between the electronic levels involved during the excitation. The XES analyzes the energy dependence of the emitted X-ray photons by probing the decay process. These X-ray emission spectra represent partial and local electron density of states due to the electric dipole selection rule.
The Kα emission line that corresponds to a dipole-allowed 2p–1s transition is commonly observed in X-ray emission spectra. XES is also used to perform a quantitative and qualitative analysis of substances. Specific elements, such as fluorine or boron, can be analyzed using this technique.
XES acts as a complementary technique to X-ray absorption spectroscopy (XAS) by providing crucial information about the nature and electronic structure, including local spin- and charge-density of the bound ligands. Specifically, the high-resolution fluorescence detection technique helps in overcoming a number of major limitations associated with conventional XAS.
The technique measures the X-ray absorption spectrum by observing the intensity of a fluorescence line that corresponds to the decay process of a specific excited state using a narrow energy resolution. A crystal analyzer is used to select a narrow energy band from the emission line of the sample.
Applications of XES
Valence-to-Core Emission (V2C) Spectroscopy
Emission lines that are close to the Fermi energy level can be measured through this technique in order to identify the type of ligand for three-dimensional (3D) transition materials.
Site Selective XAS
This technique can help in observing differences in the photo-absorbing atom chemical states in terms of the spectral shape and positions of the emission bands. In samples where photo-absorbing atomic species are present in an extensive range of oxidation states, high-resolution X-ray fluorescence can be employed to separate the spectral contributions from various sites.
Spin Resolved XAS
Multiple studies related to the Kß emission line of metals using spin-resolved XAS demonstrated a sensitivity of the emission lines to the spin states of the photo-absorbing atoms. For instance, the primary emission line provides information about one spin state, while the satellite emission line provides information about the other spin state.
The X-ray absorption near edge structure (XANES) spectra recorded through the satellite or main emission line were dependent on the spin. Thus, the extended X-ray absorption fine structure (EXAFS) can be effectively recorded using these methods.
This technique is applied in the multi-edge spectral regions in samples where the absorption threshold interferes with the data collection in specific spectral regions, such as in manganite systems, where the L edge of rare earth elements and the K edge of manganese often overlap.
The intrinsic resolution of the absorption spectrum is significantly improved when the spectrum is recorded by measuring the emission line using an energy resolution with a higher value compared to the natural width of the emission line. Thus, the technique can be potentially used to resolve structures that are invisible in a conventional XAS spectrum. High-resolution XAS was already applied successfully in XANES studies of LIII edges of the third-row transition metals such as gold and platinum.
Limitations of XES
The lack of spectral sensitivity to small changes, such as changes due to a large number of spectral features in the X-ray emission signal and 1s core-hole lifetime broadening, and lack of required efficiency in the X-ray optics to capture a small fraction of the 4p solid angle of emitted photons, are the major limitations of XES.
Recent Studies on XES
Recently, systematic studies using XES were performed on a series of low-molecular-weight ferric and ferrous complexes. Results from these studies demonstrated that the Kβ main-line spectra were dominated by spin-state contributions, while the valence-to-core spectra displayed a higher sensitivity to the chemical environment. Additionally, the intensity and energy distributions were impacted by the coordination environment, oxidation state, and spin state. Thus, XES provided detailed information on the local electronic and geometric structure.
Studies were also performed on different absorption systems using XES coupled with DFT calculations in order to obtain an overview of various interactions of absorbates on surfaces. Additionally, XES was used to probe the occupied electronic states in boron-doped diamond materials, including nanodiamonds.
In other studies, XES provides both integral information about the electronic structure, such as effective charges on atoms, and differential information, such as relative energies of molecular orbitals and characteristics of their atomic orbital components.
To summarize, XES plays a critical role in the structural characterization of different substances, specifically in cases where other spectral methods are ineffective in revealing the identity of a substance.
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References and Further Reading
D. L. (n.d.). X-ray Emission Spectroscopy. - - Diamond Light Source. https://www.diamond.ac.uk/Instruments/Spectroscopy/Techniques/XES.html#
X-Ray Emission Spectroscopy - an overview |. (n.d.). ScienceDirect Topics. https://www.sciencedirect.com/topics/materials-science/x-ray-emission-spectroscopy
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