= X-ray photoelectron spectroscopy
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= XPS
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= Electron spectroscopy for chemical analysis
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= ESCA
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<X-ray photoelectron spectroscopy> uses <X-ray> excitation to investigate <Electron> removal energies, including those of <core electrons>. The common name <electron spectroscopy for chemical analysis> emphasizes its use in distinguishing chemical surroundings. The whole recorded spectrum can also contain <Electrons> produced by processes other than direct <photoionization>; a peak assignment must identify the emission process as well as the <chemical element>.
In a modern implementation, a <quartz> crystal selects <aluminum> characteristic <X-rays> according to <Bragg's law> and focuses them onto the sample. <Vacuum> reduces gas collisions and contamination; a separately pumped entry chamber limits exposure to air. Lenses collect <photoelectrons>; an electrostatic <electron energy analyzer> transmits a narrow <kinetic energy> band. Stepping that band and counting detector pulses produces the spectrum. An electron flood source can compensate positive <electric charge> on insulating samples. Ion cleaning can alter vulnerable samples.<X-Ray Photoelectron Spectroscopy Instrumentation Features by Thermo Fisher Scientific>[Sections Ultra-high vacuum system; XPS X-ray source; Electron analyzer; Ion sources; Charge neutralization]{ref}
For a scan with count $N_j$ and dwell <time> $t_j$ at setting $j$, compare rates $N_j/t_j$, rather than raw counts when dwell times differ. Under an independent <Poisson distribution> model, the count <standard deviation> is approximately $\sqrt{N_j}$ for a sufficiently populated bin. Longer acquisition reduces this relative counting <measurement uncertainty>; it does not automatically improve separation of overlapping peaks. <Photoelectron peak overlap and resolution> gives an example in which the same chemical separation produces either two maxima or one.
\Image[/photoelectron-analyzer-energy-reference.png]
{title=Photoelectron energy selection and the analyzer energy reference}
{id=image-photoelectron-analyzer-energy-reference}
{description=Original functional diagram for an unbiased <electrical conductor> in electrical contact with an <electron energy analyzer>. The lower panel uses a common <Fermi level>; the two <vacuum> levels differ because the <work functions> differ. Vertical distances are schematic. The outgoing <Electron>'s total <energy> is measured above the analyzer's <vacuum> level to obtain its <kinetic energy>. This is an explanatory model, not a reconstruction of one historical instrument.}
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