Multi-particle imaging techniques for studying electron and positron impact ionization

Reactions of energetic free electrons with matter are of fundamental importance for a range of areas from plasma physics to radiation damage in living tissue. The best approach in order to get insight in such phenomena and for testing theoretical models is to study individual collisions and to fix their kinematics. This was done for a long time with kinematically complete (e,2e) experiments using electrostatic electron spectrometers to investigate electron impact single ionization of atoms and molecules. As the next step, we use a more efficient multi-electron and -ion imaging spectrometer which determines the momentum vectors of all charged reaction products by projecting them onto time and position sensitive detectors. Objects of study can be all evaporable atomic and molecular species and also clustered targets, i.e., small aggregates of atoms and molecules which are produced in supersonic gas jets.

An introduction of the experimental technique will be followed by a few examples of current experimental and theoretical results on single molecule ionization, where ion detection can reveal the molecular fragmentation channels or the spatial alignment of the molecule in the collision. In dimers and larger clusters energy and charge transfer processes can occur which can be identified most easily if two constituent atoms or molecules are ionized. This results in a Coulomb explosion with two ions emerging and three outgoing electrons, which ideally are detected in coincidence to get detailed information on the underlying reactions.

Positron beam sources normally are weak and the highly efficient multi-particle imaging technique should be ideally suited to study positron impact ionization. Accordingly, we have demonstrated that kinematically complete measurements for this reaction are feasible but beamtimes have not been sufficient to accumulate sufficient data to extract fully differential cross sections.