Probing correlated electron-pair emission from a W(001) surface

An electronic system which is excited by a photon can respond by the simultaneous emission of an electron-pair also called Double Photoemission (DPE). It has been established theoretically that a non-zero DPE intensity requires electron-electron interaction. Besides the simultaneous ejection of two electrons upon photon absorption, another possible pathway of pair emission is that the photon is absorbed by a single electron which scatters from another electron. To study their relative contributions we have analysed the pair emission from a W(001) surface excited by 25 eV and 29.5 eV photons in comparison with 20 eV and 25 eV electrons. The experiments were performed with a coincidence time-of-flight setup with two channelplate detectors in back-reflection geometry. In the (γ,2e) experiments BESSY II synchrotron facility operated in single-bunch mode was used as pulsed light source, while for the (e,2e) experiments, a pulsed electron gun was employed. We find that the 2D-energy distributions for photon absorption or electron excitation are markedly different. Qualitatively this difference can be understood by the selection rule for DPE. Hence, the (γ,2e) intensity has a significant contribution due to the DPE process