X-ray photoemission spectroscopy, otherwise known as X-ray photoelectron spectroscopy (XPS), is a powerful spectroscopic technique for measuring the binding energies of core electrons. XPS as a technique is compatible with gases, liquids and solids and has become a staple analytical technique in a number of fields.1
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XPS is based on the same principle as all photoelectron spectroscopy methods. If a molecule or material is irradiated with light of a known energy above the ionization threshold, a photoelectron will be ejected. If the kinetic energy of the released photoelectron is measured and the energy of the incident radiation is well-characterized, the binding energy of the orbital from which the electron was ejected can be calculated.
Where XPS differs from the other photoelectron techniques is in the energy of the incident radiation used. By irradiating a sample with highly energetic X-rays the photoelectrons that are emitted can originate from ‘core’ orbitals. The core electrons in a molecule are the most tightly bound due to their close proximity to the positively charged nuclei. Even in large and complex molecular systems, the core orbitals often resemble those of isolated atoms.
The highly localized nature of the core orbitals means that each element tends to have a unique binding energy for its core electrons. Therefore, XPS is an excellent tool for the analysis of the elemental compositions of samples and surfaces.2 A similar analysis with lower photon energies to probe electrons from the valence orbitals that are often delocalized over several atomic sites in the molecule can be much more challenging to interpret, particularly in terms of the elemental composition or stoichiometry of the sample.
The ability of XPS to distinguish between atoms in different chemical environments earnt the technique the nickname ‘electron spectroscopy for chemical analysis’ (ESCA). The importance of ESCA as a tool that has now been adopted for routine analysis was partially recognized through the award of the 1981 Nobel Prize in physics to Kai Siegbahn for his contributions to high-resolution spectroscopy.
In materials and the analytical sciences, XPS is a routinely used technique for elemental and stoichiometric analysis of materials and surfaces. For many materials experiments, cathode-ray lamps can be used to generate sufficiently bright X-rays to record XPS in a laboratory setting.
For lower number density samples, such as liquids and gases, advanced light source facilities like synchrotrons and X-ray free-electron lasers are preferable due to the higher photon flux. Such light sources are also beneficial for complementary imaging and scattering experiments where high spatial coherence of the incident beam is required.
Advanced light sources are tunable in terms of photon energy and enable a much greater range of X-ray experiments. A great deal of XPS studies at such facilities has been focused on more fundamental science areas for understanding the behavior of electrons and exploring effects such as how the ionization process works and how this is affected by factors such as post-collisional interactions.3
Future of XPS
XPS is now a mature spectroscopy with routine use in a range of different scientific areas. A range of electron spectrometer types is available, from time-of-flight designs, magnetic bottles and hemispherical analyzers and measurements can be performed on a variety of sample types, including in operando measurements on catalysts.4
Hard X-ray photoelectron spectroscopy (HAXPES) has been one area of XPS that has been in recent developments, both for materials and gas phase science.5 Typically, the photon energies selected for XPS are optimized with a consideration of the kinetic energy resolution of the detected photoelectrons relative to the ionization threshold for the element being investigated while being sufficiently energetic to avoid post-collision effects.
Hard X-rays are typically used for either core-shell ionization of heavy elements, but low-energy X-rays are more commonly used for XPS due to the need for vacuum environments for electron detection. HAXPES is increasing the number of elements that can be studied with XPS methods and also looking at new regimes for the ionization processes in molecules and materials.
Another large development is the advent of free-electron lasers that can generate ultrashort X-ray pulses with high peak intensities. There are now over five operational X-ray free-electron lasers worldwide, with SHINE in Shanghai due to come online in the coming years. The UK is currently considering the conceptual design case for another such facility there.6
X-ray free-electron lasers have made it possible to perform time-resolved XPS experiments that can be used to recover dynamical processes in molecules and materials that unfold in response to some kind of external stimulus, like a light pulse. Time-resolved XPS measurements have made it possible to investigate short-lived transient states that play a key role in how photochemical processes occur in molecules and materials and are important tools in developing our understanding of the fundamental behavior of electrons and nuclei and their role in photoinduced reactivity.
References and Further Reading
Fadley, C. S. (2010). X-ray photoelectron spectroscopy : Progress and perspectives. Journal of Electron Spectroscopy and Related Phenomena, 178–179, pp. 2–32. https://doi.org/10.1016/j.elspec.2010.01.006
Watts, J. F., & Wolstenholme, J. (2019). An introduction to surface analysis by XPS and AES. John Wiley & Sons. doi.org/10.1002/0470867930.
Alagia, M., Richter, R., Stranges, S., Agåker, M., Ström, M., Söderström, J., ... & Rubensson, J. E. (2005). Core level ionization dynamics in small molecules studied by x-ray-emission threshold-electron coincidence spectroscopy. Physical Review A, 71(1), p. 012506. https://doi.org/10.1103/PhysRevA.71.012506
Benayad, A., Santini, C. C., & Bouchet, R. (2021). Operando XPS : A Novel Approach for Probing the Lithium / Electrolyte Interphase Dynamic Evolution. The Journal of Physical Chemistry A, 125, pp. 1069–1081. https://doi.org/10.1021/acs.jpca.0c09047
Weiland, C., Rumaiz, A. K., Pianetta, P., & Woicik, J. C. (2020). Recent applications of hard x-ray photoelectron spectroscopy Recent applications of hard x-ray photoelectron spectroscopy. J. Vac. Sci. Technol., 34, p. 030801. https://doi.org/10.1116/1.4946046
STFC (2022) £3.2 million in funding announced. Available at: https://www.clf.stfc.ac.uk/Pages/%C2%A33.2m-in-Funding-Announced-for-UK-XFEL-Research-and-Development.aspx,