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X-ray photoelectron spectra of solid Cu₂O, CuCl, CuO, CuS, and CuSO₄ were measured. In all cases, satellites having various degrees of complexity are seen adjacent to the (Cu) 2p photolines. A multiple-excitation mechanism is proposed for the explanation of these satellites. The satellites are formed when the emission of a 2p photoelectron takes place in parallel with a valence-band-conduction-band excitation. These photolines are thus emitted with kinetic energies lowered by the amounts of allowed interband transitions.
T. Novakov (1971) studied this question.