The authors show that for an infinite system of noninteracting electrons the local properties of excited states with finite excitation energies are the same as for the ground state, if all one-particle states are extended. An interpretation of the X-ray photoelectron spectrum in terms of the local properties of individual eigenstates in the presence of a core hole is therefore not appropriate for an infinite system. Experimentally one can in general only study wave packets of such eigenstates, and the authors show how the wave packets are constructed for a particular experiment, Auger-electron-photoelectron coincidence spectroscopy. Using wave packets they can also for infinite systems introduce the concept of 'screened' and 'unscreened' peaks, which has an obvious meaning for small clusters of atoms.
 

Physical Review B, 26 2765-71, 1982.