In an isolated atom, the electrons occupy certain energy levels. In a solid, the levels involved in bonding 'the valence states', become blurred, and electrons can occupy a range of energies called bands. The energy of an electron in the solid depends on its momentum. Hence, by detecting photoelectrons (see UPS) emitted from a surface at different emission angles, the energy of the electrons as a function of the momentum vector may be determined. This process is known as "band mapping" and is a powerful probe of the electronic structure of crystalline materials. The measurements can usually be compared with theoretical predictions.