X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) both read surface chemistry — not just which elements are present, but what chemical state those elements are in — by measuring electrons ejected from the sample's outermost few nanometers, which is exactly the depth range that matters for surface contamination, oxidation, and interface chemistry questions.
AES reaches much finer spatial resolution than XPS, because an electron beam can be focused to a far smaller spot than an X-ray beam practically can. XPS's X-ray excitation source typically illuminates a spot tens to hundreds of micrometers across, giving it good chemical-state sensitivity but limited ability to probe one specific small feature; AES excites the same kind of surface-electron emission using a focused electron beam instead, which can be concentrated down to a sub-micron spot — letting AES pinpoint surface chemistry at one specific small feature in a way XPS's broader excitation spot simply can't match.
XPS generally produces more reliable chemical-state information because its X-ray excitation disturbs the sample's own electronic structure less than AES's electron-beam excitation does. AES's probing electron beam can itself alter the very chemical states the technique is trying to measure, particularly in sensitive or insulating materials, introducing a risk of beam-induced artifacts; XPS's X-ray excitation is generally gentler in this regard, which is part of why XPS remains the preferred technique for careful chemical-state analysis despite its coarser spatial resolution, while AES is reserved for situations where pinpointing one small feature matters more than achieving the cleanest possible chemical-state signal.
XPS and AES both answer the same fundamental question — what chemical state is this surface actually in, not just what elements are present — and the choice between them comes down to whether the analysis needs AES's finer spatial targeting or XPS's gentler, more chemically reliable excitation.