Mapping the electron correlation in two-electron photoemission

Electronic correlations are manifested in many-body effects like superconductivity and magnetism. Established theoretical concepts show that the Coulomb and exchange interaction result in a tendency of two electrons to avoid each other, leading to an exchange-correlation (xc) hole. We will report on double photoemission (DPE) experiments using a time-of-flight setup consisting of a small central collector surrounded by a resistive anode. The first allows detection only within a narrow solid angle, therefore fixing the momentum. The resistive anode covers a solid angle of similar to 1 sr, the determination of the impact position results in momentum resolution. As a pulsed light source we used synchrotron radiation and we studied a NaCl(100) surface upon excitation with 34 eV photons. The very existence of coincidences is already a manifestation of the correlation. The onset of pair emission occurs when energy conservation allows the ejection of two electrons from the highest occupied level. We have made two key observations. If E-1 and E-2 are fixed such that a pair emission from the top of the valence band is possible, a zone of reduced intensity with a diameter of similar to 1.1 angstrom(-1) is visible. Recent calculations on DPE from a Cu(100) surface display exactly such a feature due to the xc hole. Hence we prove experimentally the very existence of the xc hole in double photoemission. The zone of reduced intensity disappears whenever emission below the top of the valence band becomes possible, indicating the sensitivity of the xc hole to inelastic scattering.