 This makes ARPES one of the most powerful direct methods of studying the electronic structure of solids.The technique is a refinement of ordinary photoemission spectroscopy.Photons of energy formula_1 are used to stimulate the transition of electrons from occupied to unoccupied electronic states of the solid; if the photon's energy is greater than the binding energy formula_2 of an electron, the electron will eventually leave the solid and be observed with kinetic energy formula_3 at an angle formula_4 relative to the surface normal, characteristic of the studied material.