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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Double Ionization of He by Electron Impact, (e,3e)

While the understanding of single ionization has proceeded considerably in the recent years, for particle impact double ionization comparatively few kinematically complete experiments and corresponding theoretical calculations have been performed.

Since the exact theoretical treatment of this four-body system (e.g. three electrons and one ion) with inclusion of all mutual interactions is not possible, existing theories restrict themselves to identify the most important interactions for a given kinematical situation and to consider only these in the calculation.

E.g. for fast collisions (E0 >2 keV) the projectile can be treated as a small pertubation neglecting its influence before and after the collision. Then the description of the two ejected target electrons in the continuum including their mutual interaction and the interaction with the ion can be performed with the methods developed for low energy (e,2e) collisions. Therefore the reaction can be treated as an effective three-body system.

img/(e,3e)_2.jpg

For projectile energies E0 below about 500 eV the full complexity of the four body system appears where none of the mutual interactions can be neglected.

If the E0 approaches the double ionization threshold at 79 eV the three final state electrons carry only little kinetic energy and, therefore, the electron-electron repulsion determines their mutual emission angles. This is illustrated by the following sequence of cross sections for the case that the three electrons have equal energies of 9 eV. Each frame corresponds to a situation where the emission angles of two electrons are fixed as shown by the red arrows and the angular distribution of the third electron is plotted. Going from one diagram to the next, the emission angle of electron 2 is varied. It is obvious that electron emission of the third electron is suppressed strongly for relative angles to any other electron below about 90.

These are the first kinematically complete data for double ionization at low impact energy. Due to the small double ionization cross section the experiment was accumulating data for 6 weeks, 24 h a day.

img/(e,3e)_2.jpg
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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 this@mpi-hd.mpg.de



Xueguang Ren

Dr. Xueguang Ren

Electron impact ionization of molecules and atoms

Room: Bo. 325
Tel.: +49 (0) 6221 516 - 609
Email: xueguang.ren@please delete this@mpi-hd.mpg.de


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