  A unified diagrammatic treatment single and double electron photoemission
currents is presented. The irreducible lesser density-density response function
is the starting point of these derivations. Diagrams for higher order processes
in which several electrons are observed in coincidence can likewise be
obtained. For physically relevant situations in which the photoemission
cross-section can be written as the Fermi Golden rule, the diagrams from the
nonequilibrium Green's function approach can be put direct correspondence with
that of the scattering theory.
