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Abstract

We provide a comprehensive study of the electron pair emission from a Pb surface at room temperature. We excited the sample via a primary electron beam or laboratory light source. Besides the excitation of the 6s and 6p valence states the weakly bound 5d core levels are accessible. This allows us to investigate the Auger-photoelectron pairs in coincidence. The electron pair excitation spectra can be largely explained by the underlying electronic structure. Varying the primary energy changes the relative contribution of the 6s and 6p states. The measured double photoemission intensity is dominated by the emission of 5d photoelectrons and the resulting Auger electron. The Auger electron line shape has mainly contributions due the 6p electrons, because the 6s electrons can not lead to the emission of an Auger electron due to energy conservation. From the sum energy spectra we find that the effective Coulomb interaction U_eff is close to zero. The double photoemission intensity without participation of the 5d levels displays rather featureless spectra. Among the materials which display superconductivity is Pb. The explanation of this effect requires the introduction of Cooper pairs. It was theoretically predicted that double photoemission of Cooper pairs is possible. We discuss the experimental feasibility of such measurements.