2 ELECTRON PHOTOEMISSION IN SOLIDS

We observed the emission of correlated electron pairs from the valence band of solids following the absorption of single photons in the vacu um ultraviolet range. Applying a time-of-flight technique, we measured the energy distributions of correlated electron pairs emitted from cl ean Cu(001) and Ni(001) crystals. Significant differences between both metals were found. For the interpretation we suggest a single Step tw o-electron-photoionization process and a competing two step mechanism involving a single photoionization and a subsequent electron-electron collision in the valence band.