The Photoelectron Spectroscopy Background #
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.
QUASES-IMFP-TPP2M is a valuable tool for quickly calculating IMFPs for a given material and photoelectron kinetic energy.
Following a photoelectron peak, electrons may undergo inelastic processes and produce intensity at higher binding energies that can either contribute to the background, or as distinct peak shapes. Non-background processes include interactions with plasmons, molecular orbitals, and band structures. The resultant peaks from these non-background processes should be included in quantification processes.
The higher we go, in terms of binding energy, the more photoemission peaks are contributing to the overall background, AND the lower the kinetic energy of the photoelectrons being released from our sample (meaning they are more easily scattered). Both of these points mean that as we get to very high BE, our background becomes much higher too.
In order to correctly process photoelectron peaks, we must account for this inelastic background. There exist a number of background models that enable us to account for this intensity and start to add peak models.
