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A chemical shift in photoelectron spectroscopy is a difference in binding energy for the same kind of core electron in different chemical surroundings, using comparable energy references. Identifying a chemical element assigns its family of lines; resolving chemical shifts within that family asks how its chemical atoms are bonded. For example, distinguishing a metal from its chemically modified surface requires more than detecting that element in both places.

The removal energy compares the initial electronic state with the state left after emission. Redistribution of the remaining electrons changes that final state's energy; a measured shift therefore includes relaxation as well as initial bonding effects.^[ref]

This makes chemical shift in photoelectron spectroscopy useful but conditional. Compare the same electron shell, the same photoelectron binding energy reference, and compatible sample conditions. Agreement with a chemically appropriate reference supports an assignment; a line position by itself need not uniquely determine an oxidation state. An unresolved sum of two chemical components can even have a centroid where neither component lies. The following Gaussian mixture distribution shows exactly how that happens without claiming a material identification.

Kai Siegbahn developed electron spectroscopy as a quantitative probe of chemical and electronic structure. In X-ray photoelectron spectroscopy, incident X-rays eject electrons whose measured kinetic energies reveal their binding energies after the instrument's reference offset is accounted for. Small changes of a core-level binding energy reflect the atom's chemical environment. Resolving and calibrating these shifts makes the spectrum sensitive to electronic surroundings as well as elemental identity, linking the spectrometer's design with a physical inference.^[ref]

Physics.^[ref]

Photoelectron spectroscopy measures orbital-dependent binding energies from which an effective charge can be inferred using a stated model. Ultraviolet photoelectron spectroscopy accesses outer-shell ionizations; X-ray photoelectron spectroscopy accesses deeper core states. For an isolated gas-phase atom and a calibrated threshold, measure emitted-electron kinetic energy and use $I=h\nu -E_{\mathrm{kin}}$, with the appropriate energy reference. Optical atomic term spectra and series limits also constrain valence ionization energies.

A hydrogenic inference would set $I\simeq R_{\infty }{Z}_{\mathrm{eff}}^2/n_{\mathrm{eff}}^2$, hence $Z_{\mathrm{eff}}\simeq n_{\mathrm{eff}}\sqrt{I/R_{\infty }}$, with $R_{\infty }$ here an energy of approximately $13.6\mathrm{eV}$ and $n_{\mathrm{eff}}$ a chosen effective shell parameter. The measured binding energies are experimental; the extracted $Z_{\mathrm{eff}}$ values remain model dependent. They need not equal Slater screening parameters fitted to radial functions.

Unlike the equal $ns,np$ values in Slater's rules, spectroscopic values distinguish s from p because their penetration and shielding differ; the more tightly bound s states usually imply larger binding-energy-derived effective charges at a common shell parameter. Subshell, atomic-term and, especially for heavier atoms, spin-orbit coupling structure give further distinctions. Electron relaxation following ionization and electron correlation also matter. Thus real binding energies vary across the heavier group-15 atoms even where the simple Slater outer-shell estimates plateau; no universal constant correction to the Slater values follows.^[ref]

Kai Siegbahn's early instrument used iron-free magnetic double focusing and a thin-window Geiger-Müller counter. His later instrument, compatible with ultra-high vacuum, used a spherical electrostatic electron energy analyzer and monochromatic aluminum X-rays. Argon mixed with methane supplied simultaneous $2p$ reference lines.^[ref]

Two sulfur lines in sodium thiosulfate distinguished chemical environments within one compound. A separate gold comparison showed a fingerprint obscuring substrate lines.^[ref]

For an unknown sample, first document its preparation and transfer, then compare survey and detailed scans under reproducible illumination, contact and acquisition conditions. Repeat a region to test change during exposure. If a cleaning treatment is used, compare before and after rather than assuming that cleaning preserves the original chemical bonds. Report the photoelectron binding energy reference, reference lines, charge-compensation conditions and instrumental settings with the inferred chemical shift in photoelectron spectroscopy. A chemical interpretation should survive these checks as well as explain the line separation.