Electron pair escape in C60 via collective modes
In the few-electron systems the emission of two electrons, following the absorption of a single photon, is dominated by the Coulomb interaction. The two electrons exchange momentum and interact mutually and with the residual ion up to infinite distance. The detection in coincidence of the two electrons allows a complete investigation of the different active mechanisms depending on the total energy and energy sharing between the two electrons. A detailed understanding of double photoemission has been achieved in two-electron systems like the He atom and H2 molecule.
In this work the double photoemission of C60 has been investigated at the Gas Phase beamline of Elettra at a few photon energies (about 10, 20 and 30 eV excess energy) in equal energy sharing conditions. Auger decay following C1s ionization populates the same dication states as direct double photoemission. Thus, for sake of comparison the Auger spectrum and photoelectron-Auger electron coincidence spectrum have been also measured. The experimental results have been rationalized via an ab-initio model which uses the non-equilibrium Green’s function approach to take into account collective excitations. In this way different plasmonic modes can be selectively included in the calculation. The emerging physical picture  is a three-step model where: (i) photoabsorption promotes a valence electron to a high-energy state in the continuum; (ii) this photoelectron inelastically excites charge-density fluctuations (plasmon creation) that (iii) decay emitting a second valence electron whose energy and angular correlations with the first one is measured in the coincidence set-up, revealing how charge-density fluctuations mediate electron –electron interaction. The agreement between theory and experiments proves that the correlated two-electron photoemission is a powerful tool to access electronic correlation also in complex systems.
 M. Schüler , Y. Pavlyuth, P. Bolognesi, L. Avaldi and J. Berakdar Scientific Reports 6 24396 (2016)