Editing X-ray photoelectron spectroscopy (section)

==Chemical states and chemical shift==
[[File:hires.jpg|thumb|350px|High-resolution spectrum of an oxidized silicon wafer in the energy range of the Si 2p signal. The raw data spectrum (red) is fitted with five components or chemical states, A through E. The more oxidized forms of Si (SiO''<sub>x</sub>'', ''x'' = 1-2) appear at higher binding energies in the broad feature centered at 103.67 eV. The so-called metallic form of silicon, which resides below an upper layer of oxidized silicon, exhibits a set of doublet peaks at 100.30 eV (Si 2''p''<sub>1/2</sub>) and 99.69 eV (Si 2''p''<sub>3/2</sub>). The fact that the metallic silicon signal can be seen "through" the overlayer of oxidized Si indicates that the silicon oxide layer is relatively thin (2-3 nm). Attenuation of XPS signals from deeper layers by overlayers is often used in XPS to estimate layer thicknesses and depths.|alt=]]The ability to produce chemical state information, i.e. the local bonding environment of an atomic species in question from the topmost few nanometers of the sample makes XPS a unique and valuable tool for understanding the chemistry of the surface. The local bonding environment is affected by the formal oxidation state, the identity of its nearest-neighbor atoms, and its bonding hybridization to the nearest-neighbor or next-nearest-neighbor atoms. For example, while the nominal binding energy of the C<sub>1''s''</sub> electron is 284.6 eV, subtle but reproducible shifts in the actual binding energy, the so-called ''chemical shift'' (analogous to [[NMR spectroscopy]]), provide the chemical state information.{{citation needed|date=July 2019}}

Chemical-state analysis is widely used for carbon. It reveals the presence or absence of the chemical states of carbon, in approximate order of increasing binding energy, as: carbide (-'''C'''<sup>2−</sup>), silane (-Si-'''C'''H<sub>3</sub>), methylene/methyl/hydrocarbon (-'''C'''H<sub>2</sub>-'''C'''H<sub>2</sub>-,  '''C'''H<sub>3</sub>-CH<sub>2</sub>-, and -'''C'''H='''C'''H-), amine (-'''C'''H<sub>2</sub>-NH<sub>2</sub>), alcohol (-'''C'''-OH), ketone (-'''C'''=O), organic ester (-'''C'''OOR), carbonate (-'''C'''O<sub>3</sub><sup>2−</sup>), monofluoro-hydrocarbon (-'''C'''FH-CH<sub>2</sub>-), difluoro-hydrocarbon (-'''C'''F<sub>2</sub>-CH<sub>2</sub>-), and trifluorocarbon (-CH<sub>2</sub>-'''C'''F<sub>3</sub>), to name but a few.{{citation needed|date=July 2019}}

Chemical state analysis of the surface of a silicon wafer reveals chemical shifts due to different formal oxidation states, such as: n-doped silicon and p-doped silicon (metallic silicon), silicon suboxide (Si<sub>2</sub>O), silicon monoxide (SiO), Si<sub>2</sub>O<sub>3</sub>, and silicon dioxide (SiO<sub>2</sub>). An example of this is seen in the figure "High-resolution spectrum of an oxidized silicon wafer in the energy range of the Si 2''p'' signal".