Correlated positron-electron pair emission from surfaces

Pair emission from surfaces - Fig.6
Fig. 1: A primary positron beam is focused onto a surface at normal incidence. The optical axis of each of the transfer lenses are symmetrically positioned in the same plane as the positron beam such that positrons or electrons were detected with a mean emission angle of 45° with respect to the surface normal.

The mutual influence of electrons within an electronic system is mediated by the Pauli-principle and the Coulomb interaction. In order to get a better microscopic understanding it would be highly desirable to disentangle the respective contributions to the electron correlation. One approach is to use a positron instead of an electron as primary particle. Due to the fact that positron and electron are distinguishable the constraint of the Pauli principle is removed. Hence, the emitted positron-electron pair will reveal the influence of the Coulomb interaction only, from which it should be possible to infer the effect of the Coulomb interaction among electrons. Before, it is necessary to determine as to whether correlated positron-electron pair emission takes place. This is by no means obvious since it is well-known that when matter and antimatter is brought together the particles will eventually annihilate.  

Pair emission from surfaces - Fig. 7
Fig. 2: 2D-energy distribution of emitted position-electron pairs upon primary positron excitation with Ep=47 eV. The sample is a NiO/Ag(100) film. The solid diagonal line marks the maximum energy the pair can have.

In a pioneering experiment at the research reactor FRMII we proved that positrons “survive” for a long enough period to cause the emission of a positron-electron pair [1]. The key aspect of the experimental set-up is presented in Fig. 1. A normal incident primary positron excites the sample. Two hemispherical spectrometers detect the emitted positron-electron pair in coincidence.

For systematic studies we have recently commissioned an in-house positron source. In Fig.2 we display the 2D-energy distribution obtained from a NiO film on Ag(100).   Positrons and electrons are distinguishable particles and a consequence is a 2D-energy distribution which is asymmetric.  By this is meant that that there is a preference of the positron to take a larger fraction of the energy compared to the electron.