Electron pair emission from surfaces excited with photons and electrons

It has been shown theoretically that the double photoemission (DPE) intensity from an electronic system consisting of independent electrons vanishes. A non-zero DPE intensity requires electron-electron interaction. However, apart from the simultaneous ejection of two electrons upon photon absorption, another pathway of pair emission exists. It is possible that the photon is absorbed by a single electron. This electron eventually scatters from another electron, which can also lead to pair emission. The question arises to what extent the total coincidence intensity comes from a DPE process. We have addressed this issue by studying the pair emission from a Cu(111) surface excited by 50 eV photons as well as primary electron of 45 eV, which means that the maximum sum energy of the emitted pair is in both instances 40 eV. The experiments were performed with a time-of-flight spectrometer using two channelplate detectors with delay line anodes. As pulsed light source we used the BESSY storage ring in the single-bunch mode. For in-house experiments we employed a pulsed electron gun. We find that the 2D-energy distributions for photon excitation are markedly different from those obtained from electron excitation. Qualitatively this can be understood by the selection rule for double photoemission. Hence, the DPE intensity comes from the absorption of a photon followed by the ejection of an electron pair