We theoretically show that the oxygen 1s x-ray photoelectron spectrum (O 1s XPS) directly refelcts the unoccupied part of the electronic states of strongly correlated oxides. On the basis of numerically exact diagonalization calculations performed using a finite-size cluster model, we study the electron- or hole-doping dependence of the O 1s XPS of two-dimensional copper compounds in detail. The O 1s XPS has a weak charge-transfer (CT) satellite structure, which is located at about 10 eV above the main line. In this CT final state, an O 2p valence electron ejected by the O 1s core hole potential is transferred to the conduction states. Thus, the line shape of the CT satellite reflects the upper Hubbard band for electron-doped or undoped cuprates and also the Zhang–Rice singlet band for hole-doped cuprates.

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