  This work provides a unified theoretical treatment of the single and
correlated double-electron emission from a general electronic system. Using
Feshbach projection method, the states of interest are selected by the
projection operator; the Feshbach-Schur map determines the effective
Hamiltonian and the optical potential for the emitted electrons. On the other
hand, the nonequilibrium Green's functions method is demonstrated to be a
complementary approach and an explicit correspondence between both methods is
established. For a self-contained exposition some results on single electron
emission are re-derived using both formalisms. New insights and results are
obtained for the correlated electron-pair emission: This includes the effective
two-electron Hamiltonian, the explicit form of the Feshbach self-energy in
terms of the many-body self-energies, and the diagrammatic expansion of the
two-particle current. As an illustration of the diagrammatic technique the
process of the two-particle emission assisted by the excitation of plasmons is
explicitly worked out.
