Max-Planck-Institut für Kernphysik Heidelberg

Collisions of electrons and positrons with atoms and molecules

Priv.-Doz. Dr. Alexander Dorn and Dr. Claus Dieter Schröter

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Polarized electron source and surface physics

Until today, the understanding of the fragmentation dynamics of atomic and molecular many-body systems is still a major challenge for theory and experiment. The break-up of atoms and molecules by electron impact is characterized best by fully differential cross sections, which become accessible in kinematically complete studies and which provide the most stringent test for theory.

Using a Reaction Microscope we obtain complete pictures in momentum space of many-body processes, such as multiple ionization, simultaneous excitation -ionization, molecular fragmentation or ionization in the presence of strong laser fields [1-3]. The principle of functionality and latest technical developments of this novel multi-coincidence technique is described in literature (for a recent review see [4]).

Of particular interest are experiments at low incident electron energies in the order of a few 100 eV down to energies near ionization threshold. Furthermore experiments on the dissociative electron attachment (DEA) at subexcitation energies (only a few eV!) are aimed for. These experiments are difficult to perform due to the small cross sections, but they are feasible using this new imaging technique. Its application demands, however, an inherently cold and bunched electron beam of high quality.

Optimum conditions require an electron pulse width shorter than 500ps, a beam diameter at the target of less than 100μm and a pulse-repetition rate of up to a few MHz. For experiments near threshold or DEA experiments even cold electron beams (~10meV energy spread) are needed. All these demands can be fulfilled by a laser-driven photoelectron source, where the photocathode consists of a suitably chosen semiconductor heterostructure crystal activated to negative electron affinity (NEA)[5].

In the recent past, such a short-pulsed electron source using GaAs/AlGaAs-hetero-structure crystals has been set up [6] to meet the beam specifications being called for by our Reaction Microscope experiments. A quantum efficiency of 3% and a cathode lifetime of 25 days have been achieved. Planned future collision measurements require intense beams of cold and well-collimated electrons at low energy (<100eV). Design changes have been made to achieve this goal.


[1] A.Dorn, A.Kheifets, C.D.Schröter, B.Najjari, C.Höhr, R.Moshammerand J.Ullrich, Phys. Rev. Lett. 86, 3755 (2001)
[2] C.Höhr, A.Dorn, B.Najjari, D.Fischer, C.D.Schröter, and J.Ullrich, Phys. Rev. Lett. 94, 153201 (2005)
[3] G.Sakhelashvili, A.Dorn, C.Höhr, and J.Ullrich, A.S.Kheifetsand J.Lower, K.Bartschat, Phys. Rev. Lett. 95, 033201 (2005)
[3] G.Sakhelashvili, A.Dorn, C.Höhr, and J.Ullrich, A.S.Kheifetsand J.Lower, K.Bartschat, Phys. Rev. Lett. 95, 033201 (2005)
[4] J.Ullrich, R.Moshammer, A.Dorn, R.Dörner, L.P.H.Schmidt and H.Schmidt- Böcking, Rep. Prog. Phys. 66, 1463 (2003)
[5] R.L.Bell, Negative Electron Affinity Devices, Oxford Clarendon Press (1973)
[6] C.D.Schröter, A.Rudenko, A.Dorn, R.Moshammer, and J.Ullrich, Nucl. Instr. Meth. A 536, 312 (2005)

Photograph of the Reaction Microscope


Photograph of the photoemission source

Alexander Dorn

Priv.-Doz. Dr. Alexander Dorn

Electron collisions and Photoionization of Lithium

Room: Bo. 261b
Tel.: +49 (0) 6221 516 - 513
Email: alexander.dorn@please delete

Claus Dieter Schroeter

Dr. Claus Dieter Schroeter

Room: Bo. 335
Tel.: +49 (0) 6221 516 - 244
Email: claus.schroeter@please delete

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