 Since the allowed initial states are only those that are occupied, photoemission signal will reflect the Fermi-Dirac distribution function formula_56 in the form of a temperature-dependent sigmoid-shaped drop of intensity in the vicinity of "E"F. In the simple case of a two-dimensional one-band electronic system the intensity relation further reduces to formula_57.The electronic states in crystals are organized in energy bands, which have associated energy band dispersions formula_58 that are energy eigenvalues for delocalized electrons according to Bloch's theorem.From the plane-wave factor formula_59 in Bloch's decomposition of the wave functions it follows that the only allowed transitions when no other particles are involved are between the states whose crystal momenta differ by the reciprocal lattice vectors formula_60, i.e.