Other factors can influence the peak width of photoelectron emissions, such as final-state vibrational broadening/Franck-Condon broadening (Gaussian – broadening due to  differences in the nuclear geometry between initial and final state), phonon broadening (Gaussian – excitation of phonons following core-hole formation), crystallinity (Gaussian – poorly crystalline or amorphous materials tend to produce broader peaks), temperature, molecular conformity, or instrument factors such as X-ray source linewidth and instrument performance factors (both Gaussian). These other factors should not discriminate between spin states during the photoemission process, and as such cannot explain the difference between the peak widths we see for selected doublets – so we must consider core-hole lifetimes only.

While there are a number of factors that affect the recorded peak width in XPS,  the atomically intrinsic property is the core-hole lifetime (τ), which dictates the ‘natural linewidth’ (Γ) of the photoemission peak. The natural photoemission spectrum of a released photoelectron is best represented by a Lorentzian distribution, where I the intensity (I) at energy E is given by:

Effectively, this means that the shorter lived a core-hole resulting from a photoemission, the broader the resulting peak shape. Metals, for example, will give much sharper peaks that insulators, since they can better stabilise the resulting core-hole.