= Chemical shift in photoelectron spectroscopy
{title2=$\Delta E_B$}
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.<Electron Spectroscopy for Atoms, Molecules and Condensed Matter by Kai Manne Börje Siegbahn>[p. 69: frozen-orbital approximation and relaxation]{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.
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