Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy or spin measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies or magnetic properties of electrons in the substance. The term refers to various techniques, depending on whether the ionization energy is provided by X-ray,. Types (XPS) was developed by starting in 1957 and is used to study the energy levels of atomic core electrons, primarily in solids. Siegbahn referred to the technique as "electron s. The physics behind the PES technique is an application of the. The sample is exposed to a beam of UV or XUV light inducing photoelectric ionization. The energies of the emitted photoelectrons are charact. • (ARPES)• (IPS)•, including zero electron kinetic energy spectroscopy (ZEKE).