Editing X-ray photoelectron spectroscopy (section)

==Instrumentation==
The main components of an XPS system are the source of X-rays, an [[ultra-high vacuum]] (UHV)  chamber with [[mu-metal]] magnetic shielding, an electron collection lens, an electron energy analyzer, an electron detector system, a sample introduction chamber, sample mounts, a sample stage with the ability to heat or cool the sample, and a set of stage manipulators.

The most prevalent electron spectrometer for XPS is the [[Hemispherical electron energy analyzer|hemispherical electron analyzer]]. They have high energy resolution and spatial selection of the emitted electrons. Sometimes, however, much simpler electron energy filters - the cylindrical mirror analyzers are used, most often for checking the elemental composition of the surface. They represent a trade-off between the need for high count rates and high angular/energy resolution. This type consists of two co-axial cylinders placed in front of the sample, the inner one being held at a positive potential, while the outer cylinder is held at a negative potential. Only the electrons with the right energy can pass through this setup and are detected at the end. The count rates are high but the resolution (both in energy and angle) is poor.

Electrons are detected using [[electron multiplier]]s: a single channeltron for single energy detection, or arrays of channeltrons and microchannel plates for parallel acquisition. These devices consists of a glass channel with a resistive coating on the inside. A high voltage is applied between the front and the end. An incoming electron is accelerated to the wall, where it removes more electrons,  in such a way that an electron avalanche is created, until a measurable current pulse is obtained.{{citation needed|date=October 2019}}

=== Laboratory based XPS ===
In laboratory systems, either 10–30&nbsp;mm beam diameter non-monochromatic Al K<sub>α</sub> or Mg K<sub>α</sub> anode radiation is used, or a focused 20-500 [[micrometre|micrometer]] diameter beam single wavelength Al K<sub>α</sub> monochromatised radiation. Monochromatic Al K<sub>α</sub> X-rays are normally produced by diffracting and focusing a beam of non-monochromatic X-rays off of a thin disc of natural, crystalline [[quartz]] with a [[Miller index|<1010> orientation]]. The resulting wavelength is 8.3386 angstroms (0.83386&nbsp;nm) corresponding to a 1486.7 eV photon energy. Aluminum K<sub>α</sub> X-rays have an intrinsic [[#Full Width at Half Maximum (FWHM)|full width at half maximum (FWHM)]] of 0.43 eV, centered at 1486.7 eV (''E''/Δ''E'' = 3457).{{citation needed|date=July 2019}} For a well–optimized monochromator, the energy width of the monochromated aluminum K<sub>α</sub> X-rays is 0.16 eV, but energy broadening in common electron energy analyzers (spectrometers) produces an ultimate energy resolution on the order of FWHM=0.25 eV which is the ultimate energy resolution of most commercial systems. Under practical conditions, high energy-resolution settings produce peak widths (FWHM) between 0.4 and 0.6 eV for various elements and some compounds. For example, in a spectrum obtained for one minute at 20 eV pass energy using monochromated aluminum K<sub>α</sub> X-rays, the Ag 3''d''<sub>5/2</sub> peak for a clean silver film or foil will typically have a FWHM of 0.45 eV.{{citation needed|date=July 2019}} Non-monochromatic magnesium X-rays have a wavelength of 9.89 angstroms (0.989&nbsp;nm) which corresponds to a photon energy of 1253 eV. The energy width of the non-monochromated X-ray is roughly 0.70 eV, which is the ultimate energy resolution of a system using non-monochromatic X-rays.{{citation needed|date=July 2019}} Non-monochromatic X-ray sources do not use any crystal to diffract the X-rays allowing all primary X-rays lines and the full range of high-energy [[Bremsstrahlung]] X-rays (1–12 keV) to reach the surface. The ultimate energy resolution (FWHM) when using a non-monochromatic Mg K<sub>α</sub> source is 0.9–1.0 eV, which includes some contribution from spectrometer-induced broadening.{{citation needed|date=July 2019}}

===Synchrotron based XPS===
A breakthrough has been brought about in the last decades by the development of large scale [[synchrotron]] radiation facilities. Here, bunches of relativistic electrons kept in orbit inside a storage ring are accelerated through bending magnets or insertion devices like [[Wiggler (synchrotron)|wigglers]] and [[undulator]]s to produce a high brilliance and high flux photon beam. The beam is  orders of magnitude more intense and better collimated than typically produced by anode-based sources. Synchrotron radiation is also tunable over a wide wavelength range, and can be made polarized in several distinct ways. This way, photon can be selected yielding optimum photoionization cross-sections for probing a particular core level. The high photon flux, in addition, makes it possible to perform  XPS experiments also from low density atomic species, such as molecular and atomic adsorbates.

One of the synchrotron facilities that allows XPS measurement is Max IV synchrotron in Lund, Sweden. The Hippie beam line of this facility also allows to perform in operando Ambient Pressure X-Ray Photoelectron Spectroscopy (AP-XPS9. This latter technique allows to measure samples in ambient conditions, rather than in vacuum.<ref>{{cite journal | doi=10.1021/acscatal.3c02423 | title=Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by ''Operando'' Ambient Pressure X-ray Photoelectron Spectroscopy | date=2023 | last1=Dahl | first1=Øystein | last2=Sunding | first2=Martin Fleissner | last3=Killi | first3=Veronica | last4=Svenum | first4=Ingeborg-Helene | last5=Grandcolas | first5=Mathieu | last6=Andreassen | first6=Magnus | last7=Nilsen | first7=Ola | last8=Thøgersen | first8=Annett | last9=Jensen | first9=Ingvild Julie Thue | last10=Chatzitakis | first10=Athanasios | journal=ACS Catalysis | volume=13 | issue=17 | pages=11762–11770 | doi-access=free | hdl=10852/107512 | hdl-access=free }}</ref>