In photoemission spectroscopy (PES), photons with an energy $hf$ eject electrons from some material and the energy of the ejected electrons is used to obtain information about that material. The measurement is based on the photoelectric effect which was observed by Heinrich Hertz in 1887 and explained by Albert Einstein in 1905. Ultraviolet photons are typically used to investigate the valence electrons of molecules or solids while x-ray photons are typically used to determine the chemical composition of a sample.
Ultraviolet Photoemission Spectroscopy (UPS)
When an ultraviolet photons are used for photoemission, the measurement is called Ultraviolet Photoemission Spectroscopy (UPS). The light source is sometimes the spectral lines of a gas discharge lamp and sometimes synchrotron sources. With a synchrotron it is possible to continously sweep the photon energy. Ultraviolet photons can bring electrons in occupied states 0 eV to 100 eV below the Fermi energy into the vacuum above the sample. It can be used to measure the part of the electron density of states below the Fermi energy.
Angle Resolved Photoemission Spectroscopy (ARPES)
In a photoemission experiment, the component of the electron $\vec{k}$ vector parallel to the surface is unchanged when the electron is ejected from the surface. By measuring the ejected electrons as a function of the angle, it is possible to determine the energy and the $\vec{k}$ vector parallel to the surface. If the electronic state that is being measured is two-dimensional, such as graphene or a topological material, ARPES measures the $E(\vec{k})$ dispersion relation near the Fermi energy of the material.
Inverse Photoemission Spectroscopy (IPES)
In inverse photoemission spectroscopy, electrons with a well-defined energy are shot into the unoccupied states of the material above the Fermi energy. These electrons can emit photons when they fall to a lower energy level. By detecting these photons it is possible to infer the electron density of states above the Fermi energy.
X-ray Photoemission Spectroscopy (XPS)
When x-ray photons are used in a photoemission experiment, the core electrons are also ejected. The ejected core electrons have characteristic energies for every atom. For instance, the 1s electrons of a carbon atom are ejected with an energy of 284.2 eV. Tables of the energies of the emitted electrons can be found in references [1] and [2]. XPS is primarily used to determine the which atoms are present on a surface. It is also possible to infer which bonds an atom has formed to other atoms. When a core electron is missing it is possible that the atom will eject an additional Auger electron. These Auger electrons also have characteristic energies that will be observed by the detector. For those who are going to perform XPS experiments, reference [3] is a good place to start learning about the technique.
Photoemission Electron Microscopy (PEEM)
Photoemission Electron Microscopy (PEEM) is a spatially resolved photoemissin experiment where the light source scans the sample.