 The transition probability is then given in terms of the electric field as

In the sudden approximation, which assumes that an electron is instantaneously removed from the system of N electrons, the final and initial states of the system are taken as properly antisymmetrized products of the single particle states of the photoelectron formula_48, formula_49 and the states representing the remaining N-1 electron systems.The photoemission current of electrons of energy formula_50 and momentum formula_51 is then expressed as the products of


summed over all allowed initial and final states leading to the energy and momentum being observed.Here, "E" is measured with respect to the Fermi level "E"F and "E"k with respect to vacuum, so that formula_54 where formula_55, the so-called work function, is the energy difference between the two referent levels that is material, surface orientation, and surface condition dependent.