Valence electrons occupy the outermost occupied orbitals of an atom. These are the electrons that form chemical bonds, determine oxidation states, drive reactivity, and define the electronic and optical properties of materials.

In XPS terms, valence electrons appear in the valence band region of the spectrum — typically between 0 and approximately 20–25 eV binding energy, depending on the material. This region includes bonding orbitals, lone pairs, and in solids, the conduction and valence bands that define electronic behaviour.

Valence electrons have relatively low binding energies because they are furthest from the nucleus and most effectively shielded by the core electrons beneath them.

Although valence electrons are chemically more significant, XPS is primarily used to measure core levels. There are several important reasons for this.

Elemental specificity. Each element has core levels at binding energies that are characteristic and well-separated from those of other elements. The C 1s sits near 285 eV, the O 1s near 530 eV, the Fe 2p₃/₂ near 707 eV. These energies are large, distinct, and serve as unambiguous elemental fingerprints. Valence band features, by contrast, overlap heavily between elements and are far harder to assign to specific atoms.

Sensitivity to chemical state. Although core electrons do not participate in bonding, their binding energies are sensitive to the valence electron environment around the atom. When valence electrons are withdrawn by an electronegative neighbour, the core electrons experience a greater effective nuclear charge and their binding energy increases. This is the origin of the chemical shift — one of XPS’s most powerful analytical features.

Spectral clarity. Core level peaks are sharp, well-defined, and amenable to quantitative peak fitting. Valence band spectra are broader, more complex, and require theoretical support — such as density of states calculations — for full interpretation.

The chemical shift discussed in binding energy articles originates entirely in the valence electron distribution. This is the critical link between valence electrons and core level XPS spectra.

Consider carbon bonded to different neighbours:

• In a C–C hydrocarbon, the valence electrons are shared roughly equally. The C 1s binding energy sits near 285.0 eV.
• In C–OH, oxygen withdraws electron density from carbon through its electronegativity. The C 1s shifts to approximately 286.5 eV.
• In O–C=O (carboxyl), two oxygen atoms withdraw valence electron density. The C 1s shifts further to approximately 289.0 eV.

In each case, it is the redistribution of valence electrons that alters the electrostatic environment experienced by the core electrons — and it is the core electrons that XPS actually measures.

This relationship means that even though XPS directly probes core levels, the information it delivers is fundamentally about the valence electron chemistry. The core level acts as a sensitive reporter of what the valence electrons are doing.

XPS can also directly measure the valence band, though this is done less routinely than core level acquisition. The valence band spectrum appears in the low binding energy region of the survey scan and can be acquired at higher resolution as a dedicated scan.

In the valence band region, XPS reveals:

• The density of states near the Fermi level
• Bonding and antibonding orbital contributions
• Hybridisation between atomic orbitals in a solid
• The presence and character of d or f electron states in transition metals

However, the valence band XPS cross-section — the probability of photoionisation — is much lower for valence electrons than for core electrons, especially with standard Al Kα radiation. This makes valence band XPS relatively insensitive and the spectra less well-resolved compared to core level spectra.

For detailed valence band and electronic structure analysis, UPS (Ultraviolet Photoelectron Spectroscopy) is the preferred technique.