In the context of X-ray photoelectron spectroscopy (XPS), inelastic scattering of electrons refers to the process where photoelectrons emitted from a sample lose energy as they traverse through the material before escaping its surface. This energy loss occurs due to interactions with other electrons, plasmons, and excitations within the solid, resulting in diminished kinetic energy for the affected electrons. Inelastic scattering significantly impacts the measured XPS spectrum by generating a background signal and limiting the effective information depth—only electrons that avoid significant energy loss (i.e., those experiencing minimal or no inelastic scattering) contribute to well-defined photoelectron peaks. Peak intensities and shapes are thus a function of both the electron’s escape depth and the inelastic mean free path (IMFP), which typically ranges from 1–3 nanometers in solids. The dominance of inelastic scattering in XPS means the technique is inherently surface-sensitive, as only electrons originating near the sample surface can be detected without losing the spectral information necessary for elemental and chemical analysis.

IMFP can be calculated through:

• First-principles and dielectric models, using material energy loss functions from theory or experiment.

• Empirical relations like TPP-2M for quick estimates.

• Direct measurement by analysing energy loss spectra of electrons exiting the material.