XPS is built on the photoelectric effect. When a photon with sufficient energy strikes an atom, it can eject a core electron. The energy balance governing this process is described by Einstein’s photoelectric equation, adapted for XPS:

BE = hν − KE − φ

Where:

• hν is the energy of the incoming X-ray photon (known)
• KE is the kinetic energy of the ejected photoelectron (measured by the spectrometer)
• φ is the work function of the spectrometer (a constant that is calibrated out)
• BE is the binding energy (calculated)

The spectrometer measures how fast the photoelectron arrives at the detector — its kinetic energy. Since the photon energy is fixed and the work function is accounted for during calibration, the binding energy can be calculated precisely.

This is why XPS is a non-destructive, quantitative technique. You do not need to destroy the sample — you simply illuminate it with X-rays and measure the energies of the electrons that come out.

In XPS of solid samples, binding energies are measured relative to the Fermi level of the sample, not to vacuum. This is the standard convention for solids and is important when comparing spectra between instruments or samples.

The Fermi level reference is practical because electrical contact between the sample and the spectrometer aligns their Fermi levels. This makes the measurement reproducible and independent of the sample’s absolute work function — though charge correction is still needed for insulating samples.

When binding energies are quoted in databases and reference tables, they are always Fermi level referenced unless stated otherwise.

Every element has core levels at well-defined binding energies. A few examples:

These values are characteristic of the neutral element. In practice, the measured binding energy will differ from these reference values depending on the chemical state of the atom — which leads to one of the most important concepts in XPS.

The binding energy of a core level electron is not fixed. It shifts depending on the chemical environment of the atom. This shift is called the chemical shift, and it is what gives XPS most of its analytical power.

The physical reason is straightforward. The binding energy of a core electron depends on the effective nuclear charge experienced by that electron. When the valence electrons around an atom are withdrawn — for example, when the atom forms a bond with an electronegative element like oxygen or fluorine — the remaining core electrons experience a greater effective nuclear charge. This makes them harder to remove, increasing the binding energy.

Conversely, when an atom gains electron density — through donation from a neighbouring atom or reduction — core electrons are more shielded from the nucleus, and the binding energy decreases.

In practice:

• Oxidised metals have higher binding energies than the pure metal
• Reduced species have lower binding energies
• Higher oxidation states shift binding energy to higher values
• Carbon bonded to fluorine (C–F) shows a large positive shift compared to hydrocarbon carbon (C–H)

Carbon 1s

The C 1s peak is one of the most widely analysed in XPS. The binding energy shifts systematically with the oxidation state and bonding environment of carbon:

Each step represents increased withdrawal of electron density from carbon, each step raises the binding energy, and each step is resolvable with a modern XPS spectrometer.

Iron 2p

Iron metal (Fe⁰) has a 2p₃/₂ binding energy of approximately 707 eV. In iron(II) oxide (FeO) this rises to around 709 eV. In iron(III) oxide (Fe₂O₃) it rises further to approximately 711 eV. These shifts allow the oxidation state of iron to be identified directly from the spectrum.