Double Photoionization Studied by a Novel Configuration for Multi-Coincidence Technique

A novel approach for multi-coincidence technique on solids hasbeen tested at the ALOISA end station of the ELETTRA synchrotronradiation source. It consists of time coincidence detectionbetween a frame of hemispherical analysers and a channeltronused as a time of flight (TOF) spectrometer. This con-figuration presents the relevant advantage to be multichannelin energy, thus producing a sizeable increase in the phase spacevolume sampled by the electron-electron coincidence experiment.The present approach has been used to perform a surveyfor double photoionization (DPI) events on the Cu(111) singlecrystal surface, which is characterised by an expected crosssection 104 times smaller than the “single” photoionizationone. The expected DPI cross-section, computed prior to theexperiment, has constituted guidance in selecting experimentalconditions. Compared to the only previous DPI experiment, inthis case a higher photon energy (h.=150 eV) has been used.The reported experiment is of a great physical relevance as itconstitutes a test of recently developed descriptions of DPI insolids, and more generally it establishes a relevant step-forwardin the description of electron correlation in solids. Our results,although limited in breath by the insu.cient time discriminationof the present TOF spectrometer, clearly show that twomain channels of similar relevance contribute to the total coincidencecounts, that is, the resonant Auger-photoelectron andthe non-resonant DPI. This evidences the presence of a sizeableamount of double photoionization events, a fact that mayopen the possibility to directly explore the electron correlation,once a better time-discriminating TOF spectrometer isimplemented.The authors are grateful to the ALOISA beamline sta.members for the valuable support provided during the experimentsperformed at the ELETTRA synchrotron radiation facility,and they are also indebted to INFM for financial supportprovided through the “Supporto ELETTRA” and RRELTSTEFprojects