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X-ray emission (XES), otherwise known as X-ray fluorescence (XRF), is a powerful tool used for the identification of elements in compounds and for interrogating the electronic structure of molecules.
While emission-based techniques are commonplace with visible incident photons, the advantage of XES is that it is highly element selective. This is because high-energy X-ray photons cause removal or excitation of electrons from the so-called ‘core’ levels of an atom.
The core electrons are the most tightly bound electrons and are closest to the nucleus. Even in molecular systems, they are highly localized on the individual atoms, and they show more atomic-like behavior and characteristic energies than the valence electrons that are excited in a visible fluorescence experiment.
The energies of the valence electrons in a molecule are strongly altered by their bonding environments and may be delocalized over the whole molecular structure, whereas the core electron energies are much less perturbed. For example, the energy of one of the atomic emission lines for atomic iron is at 7.1 keV, but even when iron is incorporated into a molecule, this emission line shifts by typically < 10 eV.1
XES experiments can also be performed with other high energy particles, not just photons. Particularly in the 1980s for light elements, many XES studies involved bombarding the sample with high energy electrons, a process known as electron impact ionization.2, 3 Here, rather than removal of the core electron due to the absorption of energy from the photon, the transfer of the kinetic energy from the incident electron causes excitation of the core electron. This is sometimes called particle-induced X-ray emission (PIXE).
One of the main differences in the behavior of the electrons in XES depends on whether the incident photon energy is resonant with a specific transition, or non-resonant.
Non-Resonant X-Ray Emission
Non-resonant XES is probably the most widely used form of XES as it does not require a monochromatized beam. Typically, the excitation energy is chosen so that it is ‘post-edge’, or sufficiently far above the ionization threshold for the core level electrons. Once the core electrons are ionized, an electron in a higher energy level will re-fill the hole that has been left in the core.
To conserve energy, as the electron moves to the lower energy state, a photon is emitted, with an energy that is equal to the difference between the ionized energy level and the electron’s previous energy level. This is how non-resonant XES can be used to probe the energy levels of occupied atoms.
One of the challenges with XES is that emission and detection of emitted photons are not always efficient processes. The photos are emitted isotropically, so it can be challenging to design experiments with good collection efficiency, and once core ionization or excitation has occurred, XES is not the only process that can occur.4
Particularly for the light elements, where the core 1s levels can be excited in the soft X-ray regime (200 – 2000 eV), XES is a highly inefficient process. This is because, after initial excitation of the core electrons, a competitive process called Auger decay can occur.
In Auger decay, after the core hole is formed and the electron collapses to a lower energy level, an electron known as an Auger electron is emitted from the sample, as opposed to a photon. While XES and Auger decay are competitive processes for all elements, for the light elements such as carbon, nitrogen, and oxygen, Auger decay occurs ~ 99 % of the time following each excitation event. For heavy elements, the yield of Auger decay versus XES is more like 1:99.5
Resonant X-Ray Emission
As many XES experiments use synchrotron sources that are tunable in energy, it is also possible to trigger XES by promoting a core electron to an occupied orbital of the atom or molecule by tuning the incident energy resonant to that transition. This is called resonant XES and makes it possible to explore the electronic structure of the unoccupied states of the molecule.
- Groot, F. De. (2001). High-Resolution X-ray Emission and X-ray Absorption Spectroscopy. Chem. Rev., 101, 1779–1808. https://doi.org/10.1021/cr9900681
- Sodhi, R. N. S., & Brion, C. E. (1985). High resolution carbon 1s and valence shell electronic excitation spectra of trans-1,3-butadiene and allene studied by electron energy loss spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 37(1), 1–21. https://doi.org/10.1016/0368-2048(85)80079-7
- Hitchcock, A. P. (1990). Core Excitation and Ionization of Molecules. Physica Scripta, T31, 159–170. https://doi.org/10.1088/0031-8949/1990/T31/023
- Hoszowska, J., & Dousse, J. C. (2004). High-resolution XES and RIXS studies with a von Hamos Bragg crystal spectrometer. Journal of Electron Spectroscopy and Related Phenomena, 137–140(SPEC. ISS.), 687–690. https://doi.org/10.1016/j.elspec.2004.02.005
- Thompson, A. C., & Vaughan, D. (Eds.). (2001). X-ray data booklet (Vol. 8, No. 4). Berkeley, CA: Lawrence Berkeley National Laboratory, University of California.