The inelastic mean free path, or IMFP (λ), is defined as the average distance an electron with a given energy travels through a solid before losing energy by inelastic collision. Approximately 68 % of all photoelectrons will come from within a depth of λ, whereas the majority (99.7 %) of photoelectrons will come from a depth of 3λ.
For most core electrons excited by Al Kα X-rays, this depth is a few nanometers (nm). Elements or compounds with a greater density will have shallower IMFPs as will core electrons with higher binding energies (i.e. lower kinetic energies).
By application of the equations defined in references [1,2] the IMFP can be used to calculate the thickness of oxide films and thin overlayers. Tilting the sample with respect to the analyzer (Angle Resolved XPS) one can reduce the effective IMFP, and it is useful for gaining an understanding of thin surface layers. Such analysis can also be achieved using synchrotron radiation through changing the incident photon energy.
Many great tools exist to help you determine and calculate IMFPs, some of which may be found below:
 T.A. Carlson, G.E. McGuire, Study of the x-ray photoelectron spectrum of tungsten—tungsten oxide as a function of thickness of the surface oxide layer, J. Electron Spectrosc. Relat. Phenom. 1 (1972/73) 161. https://doi.org/10.1016/0368-2048(72)80029-X
 B.R. Strohmeier, An ESCA method for determining the oxide thickness on aluminum alloys, Surf. Interface Anal. 15 (1990) 51. https://doi.org/10.1002/sia.740150109