Britannica Sources used in this answer

How does X-ray photoelectron spectroscopy identify elements on a material surface?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Moon rock crystals — surface analysis A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.

X-ray photoelectron spectroscopy (XPS) identifies elements on a material surface by measuring the kinetic energy of electrons ejected from the sample through photoionization.1

When a material is exposed to X-rays, incident photons cause the emission of electrons. Because the binding energy of these electrons is discrete and characteristic of specific elements, the resulting electron beam allows for precise elemental identification. The kinetic energy of an ejected electron is determined by the difference between the energy of the incident photon and the electron's binding energy.21

XPS is highly effective for qualitative analysis and can detect all elements except hydrogen and helium. Furthermore, because the chemical environment of an element can cause small, measurable "chemical shifts" in binding energy, XPS can also determine the oxidation state of elements, such as distinguishing between different forms of molybdenum. While it is a powerful tool for surface analysis, it offers limited lateral resolution due to the size of the X-ray beam.2