Two electron photoemission processes in the valence band of solids

We present experimental evidence on photo double ionization processes in th e valence band of solids. Applying a 'Time-of-Flight' electron spectroscopy technique, we measured the energy distribution of pairs of correlated elec trons emitted from clean Cu(001)- and Ni(001)- crystals upon absorption of linearly polarized photons of 45eV energy. Data were taken for normal and g razing orientation of the light wave vector with respect to the crystal sur faces. The energy sharing distributions of electron pairs emitted from the vicinity of the Fermi-level show significant differences for Cu and Ni. For interpretation of our data, we consider a direct double ionization mechani sm in competition with a two step process involving single photoexcitation and a subsequent collision of the photoelectron with an additional band ele ctron. A theoretical approach by J. Berakdar including both mechanisms is i n good agreement with our data. In this theory the differences between Cu a nd Ni are traced back to distinct relative contributions of both mechanisms in the two metals due to the different average electron density.