Video narration
How can X-rays reveal what a surface is made of-do they somehow recognize the material, like a fingerprint? They don't. XPS-X-ray photoelectron spectroscopy-shines X-rays onto a surface. The X-rays knock electrons out, and these emitted electrons are called photoelectrons. That's the key: the X-rays do not identify the material directly. The photoelectrons carry the signal. An analyzer measures each electron's kinetic energy. From that, XPS infers binding energy: how strongly the electron was held in the material. Different elements have characteristic binding-energy values, and small shifts can reveal different chemical states. Because electrons escape only from near the surface, XPS does not read the whole object. It reads the outermost layers. So how do X-rays identify a surface? By measuring the energies of the photoelectrons they eject, XPS identifies the elements and chemical states in those outermost layers.