PES works by irradiating the sample with monochromatic light, usually in the form of X-rays or ultraviolet light. The energy of the incident photons is transferred to the electrons in the material, which can then overcome the binding energy holding them in place and be emitted from the surface. The kinetic energy of these emitted electrons is measured, and since the energy of the incident photon is known, the binding energy of the electrons can be calculated.
