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Short answer
Photoelectron spectroscopy (PES), also called photoemission spectroscopy, shines photons on a material and measures the energy of the electrons that the photons knock out. Because those energies depend on the orbital each electron came from, they show which elements are present and whether their atoms are bonded. Ultraviolet photons give UPS; X-ray photons give XPS.
What is photoelectron spectroscopy?
Photoelectron spectroscopy uses a photon to interact with an electron. When the photon’s energy exceeds the binding potential of an atomic orbital, the electron is propelled out of that orbital. The spectrometer measures the energy of the emitted electron, and from it scientists infer the energy of the orbital and therefore properties of the material itself. This is the photoelectric effect applied as an analytical method.
Two orbital groups matter in practice. Core electrons sit close to the nucleus; valence electrons sit farther out and take part in chemical bonds. The photon type decides which group you can reach, which is why the technique splits into named variants (see below).
The idea is old and the instrument is relatively young. Heinrich Hertz documented the photoelectric effect in 1887 and Albert Einstein explained it with light quanta in 1905, but the technique was widely applied only from the 1960s, because it took scientists decades to build instruments that generate photons of a single wavelength and detectors that measure the emitted energies. In his 1967 book, Kai Siegbahn and his coauthors laid out electron spectroscopy for chemical analysis, known today as X-ray photoelectron spectroscopy; Siegbahn received a Nobel Prize for this work.
What is X-ray photoelectron spectroscopy?
X-ray photoelectron spectroscopy (XPS) uses X-ray photons to energize core electrons. It can reveal which elements make up a material and whether there are bonds between them. The energy needed to dislodge a core electron depends slightly on the electrons involved in the molecule’s bonds, which is how XPS carries bonding information; this effect appears in reference data as a “chemical shift”. Older papers often use the name ESCA (electron spectroscopy for chemical analysis) for the same family of measurements.
PNNL, a US Department of Energy national laboratory, lists applications from medical (confirming a sufficient amount of antibacterial silver in a wound dressing) to energy storage (understanding the degradation of solar cells) to basic chemistry (phase transitions and crystal nucleation).
XPS vs UPS: which electrons, which question
| Technique | Photons | Electrons probed | Typical use named by PNNL |
|---|---|---|---|
| XPS | X-ray | Core electrons | Which elements are present and whether they are bonded; widely used across materials, energy and chemistry work |
| UPS | Ultraviolet | Valence electrons (those in bonds) | Historically, confirming orbital energies predicted by quantum mechanics; today, electron density of states in solid-state and condensed matter physics, and gas-phase characterisation of molecules |
Two further variants are named by PNNL: angle-resolved photoemission spectroscopy, used in condensed matter physics to measure electron energies of crystalline solids, and extreme-ultraviolet photoelectron spectroscopy, used to measure the conductivity of materials. A third surface technique with a different signal, Auger electron spectroscopy, is covered in its own guide.
How it works in practice
The instrument needs three things: a source that produces photons of a single type (ultraviolet or X-ray), an electron analyzer that measures the energy of the dislodged electrons, and a vacuum environment, because photoelectron spectrometers must function in vacuum. PNNL notes that analyzer resolution decreases as electron energy increases unless extra components such as optical lenses are added, and that overall resolution depends on parts such as the analyzer and the monochromatic source.
Because the emitted electron has to travel from the material to the detector, and that becomes less likely the more material it must pass through, the method is best used to infer surface information.
The same event that frees a photoelectron can also set off other processes, including X-ray fluorescence and the Auger process, in which a higher-energy electron drops into the vacated orbital. The energy released adds further information, which is why photoelectron and Auger-electron lines appear together in the same reference data.
Reading the spectrum with NIST SRD 20
A spectrum is a set of peaks at specific energies, and assigning those peaks to elements and chemical states is done against reference data rather than by memory. The NIST X-ray Photoelectron Spectroscopy Database (Standard Reference Database 20, version 5.0) is the free, web-accessible reference for this. From NIST’s own introduction page for the database:
- It contains over 22,000 line positions, chemical shifts, doublet splittings and energy separations of photoelectron and Auger-electron lines, drawn from a critical evaluation of the published literature.
- You can search by element, line type, line energy and many other variables, and identify unknown measured lines by matching them to previous measurements.
- Records carry information about the specimen material, the measurement conditions and the methods used to analyze the data, so you can judge how much weight a reported value deserves.
- It includes data on surface core-level shifts, and Wagner plots are available for 61 elements as of version 4.1.
- Version 5.0 added a Digital Object Identifier so the database itself can be cited.
Binding energies are only comparable if the energy scale is. NIST states that versions 2.0 and 3.0 of the database were standardised to a binding-energy scale calibrated on Au 4f 7/2 = 84.00 eV, Ag 3d 5/2 = 368.27 eV, Cu 2p 3/2 = 932.67 eV and C 1s (hydrocarbon) = 284.8 eV, and that later work at the UK National Physical Laboratory revised the three metal values by roughly 0.05 eV. The line positions also vary slightly with the X-ray source because of differences in X-ray lineshape. When you compare your own peak positions with a reference table, check which calibration each uses before reading a small shift as chemistry.
Where it matters for labs and facilities
A short checklist for scoping a measurement request with an instrument facility:
- Is the question about the surface? The signal comes from the near-surface region only, so bulk composition is a different question.
- Is it about core or valence electrons? Elements and bonding of core levels point to XPS; bonding-level (valence) electronic structure points to UPS.
- Can the sample live in vacuum? The spectrometer operates in vacuum. PNNL describes a cryo-XPS system under development to probe highly volatile materials such as liquid battery electrolytes.
- Which reference will you assign against? Decide whether peak assignment will use NIST SRD 20 and which energy calibration your data and the reference share.
- Does the facility’s access model fit? Shared-instrument arrangements are covered in core facility equipment sharing agreements, and qualification of analytical instruments in USP general chapter 1058 instrument qualification.
For readers new to the wider field, what analytical chemistry is and what a mass spectrometer is place surface spectroscopy next to other instrument-based methods, and nanotechnology is one of the research areas where surface characterisation is routine.
Common misunderstandings
- “It analyzes the whole sample.” It does not. PNNL describes it as best used to infer surface information.
- “PES, XPS and UPS are three different things.” PES is the family; XPS and UPS are the X-ray and ultraviolet versions.
- “ESCA is a different technique.” Older papers use ESCA for what is known today as XPS.
- “Peaks alone name the compound.” Peak positions give orbital energies from which elements and bonding are inferred; they need to be matched against calibrated reference data.
Frequently asked questions
What is photoelectron spectroscopy?
A method that irradiates a material with ultraviolet or X-ray photons and measures the energy of the electrons that are ejected, to learn which elements are present and how they are bonded. It is also called photoemission spectroscopy.
What is X-ray photoelectron spectroscopy?
The version of photoelectron spectroscopy that uses X-ray photons to energize core electrons. It reveals the elements in a material and whether there are bonds between them, and it is best used for surface information.
Sources
- Pacific Northwest National Laboratory, “Photoelectron Spectroscopy” explainer article, https://www.pnnl.gov/explainer-articles/photoelectron-spectroscopy (checked 2026-10-29).
- NIST, X-ray Photoelectron Spectroscopy Database, Standard Reference Database 20, Version 5.0, introduction page, https://srdata.nist.gov/xps/intro (checked 2026-10-29).
