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

  Research topics:  |  Ions in Traps  |  Electrons in Collisions  |  Lasers in Time  |  
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Operating Spectrometers

For our investigations we use three different multi-particle imaging spectrometers, so-called reaction microscopes:

  • The advanced reaction microscope(aRM) with a supersonic gas beam target is used for electron and positron impact ionization experiments.
  • The MOT-reaction microscope (MOTREMI), a reaction microscope with a magneto-optically trapped lithium target is used for laser ionization and electron impact ionization experiments.
  • Presently a compact reaction microscope for experiments at external beam sources is commissioned. It will enable experiments at synchrotron radiation facilities, free electron lasers and positron beam sources. It also comprises a high resolution electron projectile beam produced using a photocathode.

The Advanced Reaction Microscope (aRM)

For the electron collision experiments we use a modified reaction microscope. As in the standart version a pulsed projectile beam is crossed with a cold gas jet produced by supersonic expansion. The target fragments, ions and electrons are extracted to opposite sides by means of a weak axial electric field (E) generated by biased electrodes above and below the collision volume. An additional homogeneous axial magnetic field (B) confines the transversal motion of the electrons. Thus, all fragments emerging from a collsion are registered with two position and time resolving detectors. From the times-of-flight and the positions the initial momentum vectors of the particles are obtained.

The decisive feature of the present set up making it an unique and universal tool to study atomic and molecular collisions the collinear alignment of the projectile beam and the magnetic field. Therefore, the electron projectile beam is guided by the magnetic field into the target enabling a large range of primary beam energies from a few electron volts (eV) up to several thousand eV (keV). Furthermore projectiles undergoing a collision with a target electron and scattered out of the primary beam direction impinge on the electron detector and are detected in addition to the target fragments. This results in a strongly improved momentum resolution of the spectrometer even for hot or heavy target species where the ion momentum resolution is poor.

An essential prerequisite for this configuration is a central bore in the electron detector allowing to pass the intense primary beam and preventing the sensitive detector plates from beeing damaged.

Schematic view of the reaction microscope modified for the detection of energetic charged molecular fragments.
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The electron detector with a central bore to allow for the passage of the non-scattered projectile electrons.

Electron-Molecule Collisions

Presently we extend our studies to electron-molecule collisions. In particular we are interested in collisions where the molecule subsequently fragments. The detection of ionic fragments in many cases allows the reconstruction of the molecular axis in space during the collision. This opens the way to investigate how the reaction depends on the orientation of a molecule.

In order to adapt our set up to the detection of ionic molecular fragments which typically have energies from meV up to several eV we implemented a new ion detector with large diameter (80 mm) close to the target region (110 mm) and a central bore for passage of the projectile beam.

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Schematic view of the multi-fragment imaging spectrometer (reaction microscope).
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Side view of the new ion detector with the delay-line anode used for position read-out.
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Photograph of the experimental set-up. Right side: the vaccum chamber containing the target preparation (top) and the spectrometer (center). The pair of yellowish rings are the Helmholtz-magnetic field coils . Left side: Power supplies and data acquisition electronics.

The MOT-Reaction Microscope (MOTREMI)

In the recent years we have set up a new apparatus in which for the first time a magneto-optical trap was combined with a recoil-ion and electron momentum spectrometer (MOTREMI). In the figure below the experimental apparatus is shown schematically (left). Lithium atoms are trapped and cooled (T < 1 mK) in the crossing point of six pairwise counter-propagating laser beams (red) and ionized by an intense pulsed photon beam (blue) or by any other projectile beam. Charged fragments are momentum analyzed by the reaction microscope. The photograph shows a cloud of stored lithium atoms inside the apparatus. The atoms emit red fluorescence light after being excited by the trapping lasers

img/ecollMOT 1.jpg img/ecollMOT 2.jpg
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