The Photoelectic Effect
The fundamental principle behind the XPS process is the existence of the so-called ‘photoelectric effect’, which describes the interaction between matter and radiation pertaining to the electrons within the target atom. Essentially, when radiation with enough energy interacts with an atom, it may remove an electron (emitted electron is called a photoelectron) which will be released with a given kinetic energy. This kinetic energy is related to that of the incoming radiation, the binding energy of the electron within the atom and a small energy penalty for electron removal, which is called the work function of the spectrometer (Φsp).
Ek = hv – EB – Φsp
Since the properties of hv and Φsp are known, and that of Ek measured – modern spectrometers typically automatically derive the binding energy of a measurement and display the results according to this value.