The double photo emission is a very important tool to probe electron correlation in atomic systems as the excitation and ejection of more than one electron in a direct photo adsorption event is forbidden in a single particle picture.
Double photo emission has been studied intensely on gas targets so far but only very few experiments on this process exist for solid targets . Nevertheless solids are supposed to be very interesting targets as electron correlation plays a very important role in solid state physics e.g. superconductivity.
The experimental problem in investigating the ejection of more than one electron is to detect the electrons in coincidence with a reasonable event rate. The use of e.g. hemispherical analyzers for sequential scanning of the solid angle is not feasible. Here the the event rate depends on the acceptance angle of the spectrometer and is extremly low for multihit detection.
To avoid this problem we have used a spectrometer of the "COLTRIMS"-type with a multihit capable detector to cover the whole 2pi solid angle above the target. By this we are able to measure all combinations of ejectred momenta at once.
First results on double photo emission from a Ag(100) surface induced by 55eV photons:
A first sign of correlated emission of two electrons excited by one photon is a maximum ejection energy defined by the photon energy visible as a triangle shape in an Energy1 vs. Energy2 plot.
The emission characteristic of the two electrons is dominated by the direction of the polarization axis of the photon. The figures below show energy sharing at a constant sum energy for different angular combiation of the emitted electrons which confirm the scaling of the cross section with the skalar product e*(k1+k2) (e : polarization axis, k1,2 : ejected momenta), i.e. the direction of the observed sum momenta is determined by the polarization axis .
Further experiments with more extensive investigations and different targets are in preparation.
 R.Herrmann, S.Samrin, H.Schwabe, J.Kirschner, PRL 81 (1998) 2148
 J.Berakdar, PRB 58 (1998) 9808