Surface sensitivity of X-ray photoelectron spectroscopy

We give an overview of four terms (the inelastic mean free path (IMFP), the effective attenuation length (EAL), the mean escape depth (MED), and the information depth (ID)) that are frequently used (and sometimes misused) in statements on the surface sensitivity of X-ray photoelectron spectroscopy. We give definitions of each term and show how each term is intended for different applications. Misunderstanding of these terms often arises when the complicating effects of elastic scattering of the signal electrons are overlooked. As a result, numerical values of the IMFP, EAL, and MED for a given material and electron energy will generally be different. Values of the EAL, MED, and ID will also depend on the elastic-scattering properties of the sample and the experimental configuration. We give information on sources of data for IMFPs, EALs, and MEDs, and present simple analytical expressions from which EALs, MEDs, and IDs can be determined.