Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods of studying the electronic structure of solids and is the only truly momentum-resolved probe, which is essential for the investigation of low dimensional and strongly anisotropic systems. By measuring momentum and kinetic energy of the electrons photoemitted from a sample illuminated with radiation of energy larger than the material work function, it is possible to gain information on both energy and the momentum of the electronic excitations inside the solid.

As the intensity measured in photoemission experiments is proportional to the single-particle spectral function A(k,w)=-(1/p)ImG(k,w), ARPES provides direct insights on the Green's function G(k,w) which describes the propagation of an electron in a many-body system. This is of vital importance in elucidating the connection between electronic, magnetic, and chemical structure of solids, in particular for those complex systems which cannot be described within the independent-particle picture.

The last decade witnessed significant progress in this technique and its applications, thus ushering in a new era in photoelectron spectroscopy. Today, ARPES experiments with 2 meV energy resolution and 0.2 degree angular resolution are a reality even for photoemission on solids, providing detailed information on band dispersion and Fermi surface, as well as on the strength and nature of those many-body correlations which may profoundly affect the one-electron excitation spectrum and, in turn, determine the macroscopic physical properties.