X-ray emission spectroscopy (XES) is a form of X-ray spectroscopy in which a core electron is excited by an incident X-ray photon and then this excited state decays by emitting an X-ray photon to fill the core hole. The energy of the emitted photon is the energy difference between the involved electronic levels. The analysis of the energy dependence of the emitted photons is the aim of the X-ray emission spectroscopy. XES is also sometimes referred to as X-ray Fluorescence (XRF) spectroscopy, and while the terms can be used interchangeably, XES more often describes high energy resolution techniques while XRF studies a wider energy range at lower resolution. There are several types of XES and can be categorized as non-resonant XES (XES), which includes K β {\displaystyle K_{\beta }} -measurements, valence-to-core (VtC/V2C)-measurements, and ( K α {\displaystyle K_{\alpha }} )-measurements, or as resonant XES (RXES or RIXS), which includes XXAS+XES 2D-measurement, high-resolution XAS, 2p3d RIXS, and Mössbauer-XES-combined measurements. In addition, Soft X-ray emission spectroscopy (SXES) is used in determining the electronic structure of materials by studying transitions between electron shells that are closer to the valence level.
History The first XES experiments were published by Lindh and Lundquist in 1924 In these early studies, the authors utilized the electron beam of an X-ray tube to excite core electrons and obtain the K β {\displaystyle K_{\beta }} -line spectra of sulfur and other elements. Three years later, Coster and Druyvesteyn performed the first experiments using photon excitation. Their work demonstrated that the electron beams produce artifacts, thus motivating the use of X-ray photons for creating the core hole. Subsequent experiments were carried out with commercial X-ray spectrometers and high-resolution spectrometers. While these early studies provided fundamental insights into the electronic configuration of small molecules, XES only came into broader use with the availability of high-intensity X-ray beams at synchrotron radiation facilities, which enabled the measurement of (chemically) dilute samples. In addition to the experimental advances, there has been progress in quantum chemical computations, which makes XES an intriguing tool for studying the electronic structure of chemical compounds. Henry Moseley, a British physicist, was the first to discover a relation between the K α {\displaystyle K_{\alpha }} -lines and the atomic numbers of the probed elements. This was the birth hour of modern X-ray spectroscopy. Later, these lines could be used in elemental analysis to determine the contents of a sample. William Lawrence Bragg later found a relation between the energy of a photon and its diffraction within a crystal. The formula he established, n λ = 2 d sin ( θ ) {\displaystyle n\lambda =2d\,\sin(\theta )} , says that an X-ray photon with a specific energy bends at a precisely defined angle within a crystal.
Equipment
Analyzers A special monochromator is needed to diffract the radiation produced in X-ray sources. This is because X-rays have a refractive index n ≈ 1. Bragg came up with the equation that describes X-ray/neutron diffraction when those particles pass a crystal lattice.(X-ray diffraction) For this purpose, "perfect crystals" have been produced in many shapes, depending on the geometry and energy range of the instrument. Although they are called perfect, there are miscuts within the crystal structure, which leads to offsets of the Rowland plane. These offsets can be corrected by turning the crystal while looking at a specific energy (for example: K α 2 {\displaystyle K_{\alpha 2}} -line of copper at 8027.83 eV). When the signal intensity is maximized, the photons diffracted by the crystal hit the detector in the Rowland plane. The instrument's horizontal plane will now have a slight offset, which can be corrected by increasing or decreasing the detector angle.
In the Von Hamos geometry, a cylindrically bent crystal disperses the radiation along its flat surface's plane and focuses it along its axis of curvature onto a line-like feature. The spatially distributed signal is recorded with a position-sensitive detector at the crystal's focusing axis, providing the overall spectrum. Alternative wavelength dispersive concepts have been proposed and implemented based on Johansson geometry, having the source positioned inside the Rowland circle. In contrast, an instrument based on Johann geometry has its source placed on the Rowland circle.
X-ray sources X-ray sources are produced for many different purposes, yet not every X-ray source can be used for spectroscopy. Commonly used sources for medical applications generally generate very "noisy" source spectra because the used cathode material must not be very pure for these measurements. These lines must be eliminated as much as possible to get a good resolution in all used energy ranges. For this purpose, normal X-ray tubes with highly pure tungsten, molybdenum, palladium, etc., are made. Except for the copper they are embedded in, they produce a relatively "white" spectrum. Another way of producing X-rays are particle accelerators. They produce X-rays from vectorial changes in their direction through magnetic fields. Whenever a moving charge changes direction, it has to give off radiation with the corresponding energy. In X-ray tubes, this directional change is the electron hitting the metal target (anode). In synchrotrons, the outer magnetic field accelerates the electron into a circular path. There are many X-ray tubes, and operators must choose accurately depending on what should be measured.
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