Electrons that are bound in atoms, molecules and solids each occupy distinct states of well-defined binding energies. When light quanta deliver more than this amount of energy to an individual electron, the electron may be emitted into free space with excess (kinetic) energy that is h ν ν higher than the electron's binding energy. The distribution of kinetic energies thus reflects the distribution of the binding energies of the electrons in the atomic, molecular or crystalline system: an electron emitted from the state at binding energy E B is found at kinetic energy E k = h ν ν − − E B . This distribution is one of the main characteristics of the quantum system, and can be used for further studies in quantum chemistry and quantum physics.