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Two-photon photoemission is observed from unoccupied and occupied states of atomic oxygen adsorbed on Cu(111). The unoccupied state is found as an intermediate in the two-photon process with an energy of 2.79±{}0.07 eV above the Fermi level. The occupied state is excited by two photons and has a binding energy of 2.13±{}0.04 eV below the Fermi level.
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Rieger et al. (1987) studied this question.
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