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However, these techniques do not directly identify the atomic numbers of the atoms neighboring the absorbing atom in multielement samples. In normal photoemission from a partially occupied energy level, sweeping the incoming photon energy causes a smooth variation in the photoelectron intensity as long as one is not near an absorption edge. The incoming photon excites a resonant transition from the lower to the upper of the two levels, followed immediately by a reverse transition back to the lower level, and the energy released in the downward transition excites a photoelectron from the upper level. Since the resonance occurs at a photon energy that is characteristic of the neighbor atom, the identity of the neighbor is established. In MnO, the researchers measured photoelectron spectra from the oxygen 1s core level.