Sensing the electron-electron correlation in solids via double photoemission

We discuss the electron pair emission from surfaces upon the absorption of a single photon, also called double photoemission (DPE). This experiment is particular sensitive to the electron-electron interaction, because for independent electrons the DPE intensity is zero. We outline the experimental development of this technique over the past decade. Going beyond the mere detection of pairs we advanced the instrumentation. Now we are able to measure the kinetic energies and emission angles of a wide angular acceptance. We will show how the available energy is distributed among the electrons and how the angular distributions took like. The latter enabled us to make contact to an important concept of modem solid state theory, namely the exchange-correlation hole. We demonstrate that the exchange-correlation hole manifests itself in a depletion zone of the coincidence intensity around the fixed emission direction of one electron. The experiments were performed at the synchrotron facilities BESSY I and BESSY II. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim